ACETYLENE

THE PRINCIPLES OF ITS GENERATION AND USE

A PRACTICAL HANDBOOK ON THE PRODUCTION, PURIFICATION, AND SUBSEQUENT TREATMENT OF ACETYLENE FOR THE DEVELOPMENT OF LIGHT, HEAT, AND POWER

BY

F. H. LEEDS, F.I.C.

FOR SOME YEARS TECHNICAL EDITOR OF THE JOURNAL "ACETYLENE"

AND

W. J. ATKINSON BUTTERFIELD, M.A.

AUTHOR OF "THE CHEMISTRY OF GAS MANUFACTURE"

Second Edition

REVISED AND ENLARGED

PREFATORY NOTE TO THE FIRST EDITION

In compiling this work on the uses and application of acetylene, the special aim of the authors has been to explain the various physical and chemical phenomena:

(1) Accompanying the generation of acetylene from calcium carbide and water.

(2) Accompanying the combustion of the gas in luminous or incandescent burners, and

(3) Its employment for any purpose--(a) neat, (b) compressed into cylinders, (c) diluted, and (d) as an enriching material.

They have essayed a comparison between the value of acetylene and other illuminants on the basis of "illuminating effect" instead of on the misleading basis of pure "illuminating power," a distinction which they hope and believe will do much to clear up the misconceptions existing on the subject. Tables are included, for the first time (it is believed) in English publications, of the proper sizes of mains and service-pipes for delivering acetylene at different effective pressures, which, it is hoped, will prove of use to those concerned in the installation of acetylene lighting systems.

June 1903

NOTE TO THE SECOND EDITION

The revision of this work for a new edition was already far advanced when it was interrupted by the sudden death on April 30, 1908, of Mr. F. H. Leeds. The revision was thereafter continued single-handed, with the help of very full notes which Mr. Leeds had prepared, by the undersigned. It had been agreed prior to Mr. Leeds' death that it would add to the utility of the work if descriptions of a number of representative acetylene generators were given in an Appendix, such as that which now appears at the conclusion of this volume. Thanks are due to the numerous firms and individuals who have assisted by supplying information for use in this Appendix.

W. J. ATKINSON BUTTERFIELD

WESTMINSTER

August 1909

CONTENTS

[a]CHAPTER I]

INTRODUCTORY--THE COST AND ADVANTAGES OF ACETYLENE LIGHTING

Intrinsic advantages
Hygienic advantages
Acetylene and paraffin oil
Blackened ceilings
Cost of acetylene lighting
Cost of acetylene and coal-gas
Cost of acetylene and electric lighting
Cost of acetylene and paraffin oil
Cost of acetylene and air-gas
Cost of acetylene and candles
Tabular statement of costs (to face)
Illuminating power and effect

[a]CHAPTER II]

THE PHYSICS AND CHEMISTRY OF THE REACTION BETWEEN CARBIDE AND WATER

Nature of calcium carbide
Storage of calcium carbide
Fire risks of acetylene lighting
Purchase of carbide
Quality and sizes of carbide
Treated and scented carbide
Reaction between carbide and water
--chemical nature
--heat evolved
--difference between heat and temperature
--amount of heat evolved
--effect of heat on process of generation
Reaction:
--effects of heat
--effect of heat on the chemical reaction
--effects of heat on the acetylene
--effects of heat on the carbide
Colour of spent carbide
Maximum attainable temperatures
Soft solder in generators
Reactions at low temperatures
Reactions at high temperatures
Pressure in generators

[a]CHAPTER III]

THE GENERAL PRINCIPLES OF ACETYLENE GENERATION ACETYLENE GENERATING APPARATUS

Automatic and non-automatic generators
Control of the chemical reaction
Non-automatic carbide-to-water generators
Non-automatic water-to-carbide generators
Automatic devices
Displacement gasholders
Action of water-to-carbide generators
Action of carbide-to-water generators
Use of oil in generator
Rising gasholder
Deterioration of acetylene on storage
Freezing and its avoidance
Corrosion in apparatus
Isolation of holder from generator
Water-seals
Vent pipes and safety valve
Frothing in generator
Dry process of generation
Artificial lighting of generator sheds

[a]CHAPTER IV]

THE SELECTION OF AN ACETYLENE GENERATOR

Points to be observed
Recommendations of Home Office Committee
British and Foreign regulations for the construction and installation of acetylene generating plant

[a]CHAPTER V]

THE TREATMENT OF ACETYLENE AFTER GENERATION

Impurities in calcium carbide
Impurities of acetylene
Removal of moisture
Generator impurities in acetylene
Filters
Carbide impurities in acetylene
Washers
Reasons for purification
Necessary extent of purification
Quantity of impurities in acetylene
Purifying materials
Bleaching powder
Heratol, frankoline, acagine, and puratylene
Efficiency of purifying material
Minor reagent
Method of a gas purifier
Methods of determining exhaustion of purifying material
Regulations for purification
Drying
Position of purifier
Filtration
General arrangement of plans
Generator residues
Disposal of residue

[a]CHAPTER VI]

THE CHEMICAL AND PHYSICAL PROPERTIES OF ACETYLENE

Physical properties
Leakage
Heat of combustion
Explosive limits
Range of explosibility
Solubility in liquids
Toxicity
Endothermic nature
Polymerisation
Heats of formation and combustion
Colour of flame
Radiant efficiency
Chemical properties
Reactions with copper

[a]CHAPTER VII]

MAINS AND SERVICE-PIPES--SUBSIDIARY APPARATUS

Meters
Governors
Gasholder pressure
Pressure-gauges
Dimensions of mains and pipes
Velocity of flow in pipes
Service-pipes and mains
Leakage
Pipes and fittings
Laying mains
Expelling air from pipes
Tables of pipes and mains

[a]CHAPTER VIII]

COMBUSTION OF ACETYLENE IN LUMINOUS BURNERS--THEIR DISPOSITION

Nature of luminous flames
Illuminating power
Early burners
Injector and twin-flame burners
Illuminating power of self-luminous burners
Glassware for burners

[a]CHAPTER IX]

INCANDESCENT BURNERS--HEATING APPARATUS--MOTORS--AUTOGENOUS SOLDERING

Merits of incandescent lighting
Conditions for incandescent lighting
Illuminating power of incandescent burners
Durability of mantles
Typical incandescent burners
Acetylene for heating and cooking
Acetylene motors
Blowpipes
Autogenous soldering and welding

[a]CHAPTER X]

CARBURETTED ACETYLENE

Carburetted acetylene
Illuminating power of carburetted acetylene
Carburetted acetylene for "power"

[a]CHAPTER XI]

COMPRESSED AND DISSOLVED ACETYLENE--MIXTURES WITH OTHER GASES

Compression
Dissolved acetylene
Solution in acetone
Liquefied acetylene
Dilution with carbon dioxide
Dilution with air
Mixed carbides
Dilution with, methane and hydrogen
Self-inflammable acetylene
Enrichment with acetylene
Partial pressure
Acetylene-oil-gas

[a]CHAPTER XII]

SUNDRY USES

Destruction of noxious moths
Destruction of phylloxera and mildew
Manufacture of lampblack
Production of tetrachlorethane
Utilisation of residues
Sundry uses for the gas

[a]CHAPTER XIII]

PORTABLE ACETYLENE LAMPS AND PLANT

Table and vehicular lamps
Flare lamps
Cartridges of carbide
Cycle-lamp burners
Railway lighting

[a]CHAPTER XIV]

VALUATION AND ANALYSIS OF CARBIDE

Regulations of British Acetylene Association
Regulations o£ German Acetylene Association
Regulations of Austrian Acetylene Association
Sampling carbide
Yield of gas from small carbide
Correction of volumes for temperature and pressure
Estimation of impurities
Tabular numbers

[a]APPENDIX]

DESCRIPTIONS OP GENERATORS

America: Canada
America: United States
Austria-Hungary
Belgium
France
Germany
Great Britain and Ireland

[a]INDEX]

[a]INDEX TO APPENDIX]

ACETYLENE

[CHAPTER I]

INTRODUCTORY--THE COST AND ADVANTAGES OF ACETYLENE LIGHTING

Acetylene is a gas [Footnote: For this reason the expression, "acetylene gas," which is frequently met with, would be objectionable on the ground of tautology, even if it were not grammatically and technically incorrect. "Acetylene-gas" is perhaps somewhat more permissible, but it is equally redundant and unnecessary.] of which the most important application at the present time is for illuminating purposes, for which its properties render it specially well adapted. No other gas which can be produced on a commercial scale is capable of giving, volume for volume, so great a yield of light as acetylene. Hence, apart from the advantages accruing to it from its mode of production and the nature of the raw material from which it is produced, it possesses an inherent advantage over other illuminating gases in the smaller storage accommodation and smaller mains and service-pipes requisite for the maintenance of a given supply of artificial light. For instance, if a gasholder is required to contain sufficient gas for the lighting of an establishment or district for twenty-four hours, its capacity need not be nearly so great if acetylene is employed as if oil-gas, coal-gas, or other illuminating gas is used. Consequently, for an acetylene supply the gasholder can be erected on a smaller area and for considerably less outlay than for other gas supplies. In this respect acetylene has an unquestionable economical advantage as a competitor with other varieties of illuminating gas for supplies which have generally been regarded as lying peculiarly within their preserves. The extent of this advantage will be referred to later.

The advantages that accrue to acetylene from its mode of production, and the nature of the raw material from which it is obtained, are in reality of more importance. Acetylene is readily and quickly produced from a raw material--calcium carbide--which, relatively to the yield of light of the gaseous product, is less bulky than the raw materials of other gases. In comparison also with oils and candles, calcium carbide is capable of yielding, through the acetylene obtainable from it, more light per unit of space occupied by it. This higher light-yielding capacity of calcium carbide, ready to be developed through acetylene, gives the latter gas a great advantage over all other illuminants in respect of compactness for transport or storage. Hence, where facilities for transport or storage are bad or costly, acetylene may be the most convenient or cheapest illuminant, notwithstanding its relatively high cost in many other cases. For example, in a district to which coal and oil must be brought great distances, the freight on them may be so heavy that--regarding the question as simply one of obtaining light in the cheapest manner--it may be more economical to bring calcium carbide an equal or even greater distance and generate acetylene from it on the spot, than to use oil or make coal-gas for lighting purposes, notwithstanding that acetylene may not be able to compete on equal terms with oil--or coal-gas at the place from which the carbide is brought. Likewise where storage accommodation is limited, as in vehicles or in ships or lighthouses, calcium carbide may be preferable to oil or other illuminants as a source of light. Disregarding for the moment intrinsic advantages which the light obtainable from acetylene has over other lights, there are many cases where, owing to saving in cost of carriage, acetylene is the most economical illuminant; and many other cases where, owing to limited space for storage, acetylene far surpasses other illuminants in convenience, and is practically indispensable.

The light of the acetylene flame has, however, some intrinsic advantages over the light of other artificial illuminants. In the first place, the light more closely resembles sunlight in composition or "colour." It is more nearly a pure "white" light than is any other flame or incandescent body in general use for illuminating purposes. The nature or composition of the light of the acetylene flame will be dealt with more exhaustively later, and compared with that afforded by other illuminants; but, speaking generally, it may be said that the self-luminous acetylene light is superior in tint, to all other artificial lights, for which reason it is invaluable for colour-judging and shade-matching. In the second place, when the gas issues from a suitable self-luminous burner under proper pressure, the acetylene flame is perfectly steady; and in this respect it in preferable to most types of electric light, to all self- luminous coal-gas flames and candles, and to many varieties of oil-lamp. In steadiness and freedom from flicker it is fully equal to incandescent coal-gas light, but it in distinctly superior to the latter by virtue of its complete freedom from noise. The incandescent acetylene flame emits a slight roaring, but usually not more than that coming from an atmospheric coal-gas burner. With the exception of the electric arc, self-luminous acetylene yields a flame of unsurpassed intensity, and yet its light is agreeably soft. In the third place, where electricity is absent, a brilliancy of illumination which can readily be obtained from self-luminous acetylene can otherwise only be procured by the employment of the incandescent system applied either to coal-gas or to oil; and there are numerous situations, such as factories, workshops, and the like, where the vibration of the machinery or the prevalence of dust renders the use of mantles troublesome if not impossible. Anticipating what will be said later, in cases like these, the cost of lighting by self-luminous acetylene may fairly be compared with self-luminous coal- gas or oil only; although in other positions the economy of the Welsbach mantle must be borne in mind.

Acetylene lighting presents also certain important hygienic advantages over other forms of flame lighting, in that it exhausts, vitiates, and heats the air of a room to a less degree, for a given yield of light, than do either coal-gas, oils, or candles. This point in favour of acetylene is referred to here only in general terms; the evidence on which the foregoing statement is based will be recorded in a tabular comparison of the cost and qualities of different illuminants. Exhaustion of the air means, in this connexion, depletion of the oxygen normally present in it. One volume of acetylene requires 2-1/2 volumes of oxygen for its complete combustion, and since 21 volumes of oxygen are associated in atmospheric air with 79 volumes of inert gases--chiefly nitrogen--which do not actively participate in combustion, it follows that about 11.90 volumes of air are wholly exhausted, or deprived of oxygen, in the course of the combustion of one volume of acetylene. If the light which may be developed by the acetylene is brought into consideration, it will be found that, relatively to other illuminants, acetylene causes less exhaustion of the air than any other illuminating agent except electricity. For instance, coal-gas exhausts only about 6- 1/2 times its volume of air when it is burnt; but since, volume for volume, acetylene ordinarily yields from three to fifteen times as much light as coal-gas, it follows that the same illuminative value is obtainable from acetylene by considerably less exhaustion of the air than from coal-gas. The exact ratio depends on the degree of efficiency of the burners, or of the methods by which light is obtained from the gases, as will be realised by reference to the table which follows. Broadly speaking, however, no illuminant which evolves light by combustion (oxidation), and which therefore requires a supply of oxygen or air for its maintenance, affords light with so little exhaustion of the air as acetylene. Hence in confined, ill-ventilated, or crowded rooms, the air will suffer less exhaustion, and accordingly be better for breathing, if acetylene is chosen rather than any other illuminant, except electricity.

Next, in regard to vitiation of the air, by which is meant the alteration in its composition resulting from the admixture of products of combustion with it. Electric lighting is as superior to other modes of lighting in respect of direct vitiation as of exhaustion of the air, because it does not depend on combustion. Putting it aside, however, light is obtainable by means of acetylene with less attendant vitiation of the air than by means of any other gas or of oil or candles. The principal vitiating factor in all cases is the carbonic acid produced by the combustion. Now one volume of acetylene on combustion yields two volumes of carbonic acid, whereas one volume of coal-gas yields about 0.6 volume of carbonic acid. But even assuming that the incandescent system of lighting is applied in the case of coal-gas and not of acetylene, the ratio of the consumption of the two gases for the development of a given illuminative effect will be such that no more carbonic acid will be produced by the acetylene; and if the incandescent system is applied either in both cases or in neither, the ratio will be greatly in favour of acetylene. The other factors which determine the vitiation of the air of a room in which the gas is burning are likewise under ordinary conditions more in favour of acetylene. They are not, however, constant, since the so-called "impurities," which on combustion cause vitiation of the air, vary greatly in amount according to the extent to which the gases have been purified. London coal-gas, which was formerly purified to the highest degree practically attainable, used to contain on the average only 10 to 12 grains of sulphur per 100 cubic feet, and virtually no other impurity. But now coal-gas, in London and most provincial towns, contains 40 to 50 grains of sulphur per 100 cubic foot. At least 5 grains of ammonia per 100 cubic foot in also present in coal-gas in some towns. Crude acetylene also contains sulphur and ammonia, that coming from good quality calcium carbide at the present day including about 31 grains of the former and 25 grains of the latter per 100 cubic feet. But crude acetylene is also accompanied by a third impurity, viz., phosphoretted hydrogen or phosphine, which in unknown in coal-gas, and which is considerably more objectionable than either ammonia or sulphur. The formation, behaviour, and removal of those various impurities will be discussed in Chapter V.; but here it may be said that there is no reason why, if calcium carbide of a fair degree of purity has been used, and if the gas has been generated from it in a properly designed and smoothly working apparatus-- this being quite as important as, or even more important than, the purity of the original carbide--the gas should not be freed from phosphorus, sulphur, and ammonia to the utmost necessary or desirable extent, by processes which are neither complicated nor expensive. And if this is done, as it always should be whenever the acetylene is required for domestic lighting, the vitiation of the air of a room due to the "impurities" in the gas will become much less in the case of acetylene than in that of even well-purified coal-gas; taking equal illuminating effect as the basis for comparison.

Acetylene is similarly superior, speaking generally, to petroleum in respect of impurities, though the sulphur present in petroleum oils, such as are sold in this country for household use, though very variable, is often quite small in amount, and seldom is responsible for serious vitiation of the atmosphere.

Regarding somewhat more closely the relative convenience and safety of acetylene and paraffin for the illumination of country residences, it may be remarked that an extraordinarily great amount of care must be bestowed upon each separate lamp if the whole house is to be kept free from an odour which is very offensive to the nostrils; and the time occupied in this process, which of itself is a disagreeable one, reaches several hours every day. Habit has taught the country dweller to accept as inevitable this waste of time, and largely to ignore the odour of petroleum in his abode; but the use of acetylene entirely does away with the daily cleaning of lamps, and, if the pipe-fitting work has been done properly, yields light absolutely unaccompanied by smell. Again, unless most carefully managed, the lamp-room of a large house, with its store of combustible oil, and its collection of greasy rags, must unavoidably prove a sensible addition to the risk of fire. The analogue of the lamp- room when acetylene is employed is the generator-house, and this is a separate building at some distance from the residence proper. There need be no appreciable odour in the generator-house, except during the times of charging the apparatus; but if there is, it passes into the open air instead of percolating into the occupied apartments.

The amount of heat developed by the combustion of acetylene also is less for a given yield of light than that developed by most other illuminants. The gas, indeed, is a powerful heating gas, but owing to the amount consumed being so small in proportion to the light developed, the heat arising from acetylene lighting in a room is less than that from most other illuminating agents, if the latter are employed to the extent required to afford equally good illumination. The ratio of the heat developed in acetylene lighting to that developed in, e.g., lighting by ordinary coal-gas, varies considerably according to the degree of efficiency of the burners, or, in other words, of the methods by which light is obtained from the gases. Volume for volume, acetylene yields on combustion about three and a half times as much heat as coal- gas, yet, owing to its superior efficiency as an illuminant, any required light may be obtained through it with no greater evolution of heat than the best practicable (incandescent) burners for coal-gas produce. The heat evolved by acetylene burners adequate to yield a certain light is very much less than that evolved by ordinary flat-flame coal-gas burners or by oil-lamps giving the same light, and is not more than about three times as much as that from ordinary electric lamps used in numbers sufficient to give the same light. More exact figures for the ratio between the heat developed in acetylene lighting and that in other modes of lighting are given in the table already referred to.

In connexion with the smaller amount of heat developed per unit of light when acetylene is the illuminant, the frequently exaggerated claim that acetylene does not blacken ceilings at all may be studied. Except it be a carelessly manipulated petroleum-lamp, no form of artificial illuminant employed nowadays ever emits black smoke, soot, or carbon, in spite of the fact that all luminous flames commercially capable of utilisation do contain free carbon in the elemental state. The black mark on a ceiling over a source of light is caused by a rising current of hot air and combustion products set up by the heat accompanying the light, which current of hot gas carries with it the dust and dirt always present in the atmosphere of an inhabited room. As this current of air and burnt gas travels in a fairly concentrated vertical stream, and as the ceiling is comparatively cool and exhibits a rough surface, that dust and dirt are deposited on the ceiling above the flame, but the stain is seldom or never composed of soot from the illuminant itself. Proof of this statement may be found in the circumstance that a black mark is eventually produced over an electric glow-lamp and above a pipe delivering hot water. Clearly, therefore, the depth and extent of the mark will depend on the volume and temperature of the hot gaseous current; and since per unit of light acetylene emits a far smaller quantity of combustion products and a far smaller amount of heat than any other flame illuminant except incandescent coal-gas, the inevitable black mark over its flame takes very much longer to appear. Quite roughly speaking, as may be deduced from what has already been said on this subject, the luminous flame of acetylene "blackens" a ceiling at about the same rate as a coal-gas burner of the best Welsbach type.

There is one respect in which acetylene and other flame illuminants are superior to electric lighting, viz., that they sterilise a larger volume of air. All the air which is needed to support combustion, as well as the excess of air which actually passes through the burner tube and flame in incandescent burners, is obviously sterilised; but so also is the much larger volume of air which, by virtue of the up-current due to the heat of the flame, is brought into anything like close proximity with the light. The electric glow-lamp, and the most popular and economical modern enclosed electric arc-lamp, sterilise only the much smaller volume of air which is brought into direct contact with their glass bulbs. Moreover, when large numbers of persons are congregated in insufficiently ventilated buildings--and many public rooms are insufficiently ventilated--the air becomes nauseous to inspire and positively detrimental to the health of delicate people, by reason of the human effluvia which arise from soiled raiment and uncleansed or unhealthy bodies, long before the proportion of carbonic acid by itself is high enough to be objectionable. Thus a certain proportion of carbonic acid coming from human lungs and skin is more harmful than the same proportion of carbonic acid derived from the combustion of gas or oil. Hence acetylene and flame illuminants generally have the valuable hygienic advantages over electric lighting, not only of killing a far larger number of the micro-organisms that may be present in the air, but, by virtue of their naked flames, of burning up and destroying a considerable quantity of the aforesaid odoriferous matter, thus relieving the nose and materially assisting in the prevention of that lassitude and anæmia occasionally follow the constant inspiration of air rendered foul by human exhalations.

The more important advantages of acetylene as an illuminant have now been indicated, and it remains to discuss the cost of acetylene lighting in comparison with other modes of procuring artificial light. At the outset it may be stated that a very much greater reduction in the price of calcium carbide--from which acetylene is produced--than is likely to ensue under the present methods and conditions of manufacture will be required to make acetylene lighting as cheap as ordinary gas lighting in towns in this country, provided incandescent burners are used for the gas. On the score of cheapness (and of convenience, unless the acetylene were delivered to the premises from some central generating station) acetylene cannot compete as an illuminant with coal-gas where the latter costs, say, not more than 5s. per 1000 cubic feet, if only reasonable attention is given to the gas-burners, and at least a quarter of them are on the incandescent system. If, on the other hand, coal-gas is misused and wasted through the employment only of interior or worn-out flat-flame burners, while the best types of burner are used for acetylene, the latter gas may prove as cheap for lighting as coal-gas at, say, 2s. 6d. per 1000 cubic feet (and be far better hygienically); whereas, contrariwise, if coal-gas is used only with good and properly maintained incandescent burners, it may cost over 10s. per 1000 cubic feet, and be cheaper than acetylene burned in good burners (and as good from the hygienic standpoint). More precise figures on the relative costs of coal-gas lighting and acetylene lighting are given in the tabular statement at the close of this chapter.

With regard to electric lighting it is somewhat difficult to lay down a fair basis of comparison, owing to the wide variations in the cost of current, and in the efficiency of lamps, and to the undoubted hygienic and aesthetic claims of electric lighting to precedence. But in towns in this country where there is a public electricity supply, electric lighting will be used rather than acetylene for the same reasons that it is preferred to coal-gas. Cost is only a secondary consideration in such cases, and where coal-gas is reasonably cheap, and nevertheless gives place to electric lighting, acetylene clearly cannot hope to supplant the latter. [Footnote: Where, however, as is frequently the case with small public electricity-supply works, the voltage of the supply varies greatly, the fluctuations in the light of the lamps, and the frequent destruction of fuses and lamps, are such manifest inconveniences that acetylene is in fact now being generally preferred to electric lighting in such circumstances.] But where current cannot be had from an electricity-supply undertaking, and it is a question, in the event of electric lighting being adopted, of generating current by driving a dynamo, either by means of a gas-engine supplied from public gas-mains, by means of a special boiler installation, or by means of an oil-engine or of a power gas-plant and gas-engine, the claims of acetylene to preference are very strong. An important factor in the estimation of the relative advantages of electricity and acetylene in such cases is the cost of labour in looking after the generating plant. Where a gas-engine supplied from public gas-mains is used for driving the dynamo, electric lighting can be had at a relatively small expenditure for attendance on the generating plant. But the cost of the gas consumed will be high, and actually light could be obtained directly from the gas by means of incandescent mantles at far loss cost than by consuming the gas in a motor for the indirect production of light by means of electric current. Therefore electric lighting, if adopted under these conditions, must be preferred to gas lighting from considerations which are deemed to outweigh those of a much higher cost, and acetylene does not present so great advantages over coal-gas as to affect the choice of electric lighting. But in the cases where there is no public gas-supply, and current must be generated from coal or coke or oil consumed on the spot, the cost of the skilled labour required to look after either a boiler, steam-engine and dynamo, or a power gas-plant and gas-engine or oil- engine and dynamo, will be so heavy that unless the capacity of the installation is very great, acetylene will almost certainly prove a cheaper and more convenient method of obtaining light. The attention required by an acetylene installation, such as a country house of upwards of thirty rooms would want, is limited to one or two hours' labour per diem at any convenient time during daylight. Moreover, the attendant need not be highly paid, as he will not have required an engineman's training, as will the attendant on an electric lighting plant. The latter, too, must be present throughout the hours when light is wanted unless a heavy expenditure has been incurred on accumulators. Furthermore, the capital outlay on generating plant will be very much less for acetylene than for electric lighting. General considerations such as these lead to the conclusion that in almost all country districts in this country a house or institution could be lighted more cheaply by means of acetylene than by electricity. In the tabular statement of comparative costs of different modes of lighting, electric lighting has been included only on the basis of a fixed cost per unit, as owing to the very varied cost of generating current by small installations in different parts of the country it would be futile to attempt to give the cost of electric lighting on any other basis, such as the prime cost of coal or coke in a particular district. Where current is supplied by a public electricity- supply undertaking, the cost per unit is known, and the comparative costs of electric light and acetylene can be arrived at with tolerable precision. It has not been thought necessary to include in the tabular statement electric arc-lamps, as they are only suitable for the lighting of large spaces, where the steadiness and uniformity of the illumination are of secondary importance. Under such conditions, it may be stated parenthetically, the electric arc-light is much less costly than acetylene lighting would be, but it is now in many places being superseded by high-pressure gas or oil incandescent lights, which are steady and generally more economical than the arc light.

The illuminant which acetylene is best fitted to supersede on the score of convenience, cleanliness, and hygienic advantages is oil. By oil is meant, in this connection, the ordinary burning petroleum, kerosene, or paraffin oil, obtained by distilling and refining various natural oils and shales, found in many countries, of which the United States (principally Pennsylvania), Russia (the Caucasus chiefly), and Scotland are practically the only ones which supply considerable quantities for use in Great Britain. Attempts are often made to claim superiority for particular grades of these oils, but it may be at once stated that so for as actual yield of light is concerned, the same weight of any of the commercial oils will give practically the same result. Hence in the comparative statement of the cost of different methods of lighting, oil will be taken at the cheapest rate at which it could ordinarily be obtained, including delivery charges, at a country house, when bought by the barrel. This rate at the present time is about ninepence per gallon. A higher price may be paid for grades of mineral oil reputed to be safer or to give a "brighter" or "clearer" light; but as the quantity of light depends mainly upon the care and attention bestowed on the burner and glass fittings of the lamp, and partly upon the employment of a suitable wick, while the safety of each lamp depends at least as much upon the design of that lamp, and the accuracy with which the wick fits the burner tube, as upon the temperature at which the oil "flashes," the extra expense involved in burning fancy-priced oils will not be considered here.

The efficiency (i.e., the light yielded per pint or other unit volume consumed) of oil-lamps varies greatly, and, speaking broadly, increases with the power of the lamp. But as large or high-power lamps are not needed throughout a house, it is fairer to assume that the light obtainable from oil in ordinary household use is the mean of that afforded by large and that afforded by small lamps. A large oil-lamp as commonly used in country houses will give a light of about 20 candle- power, while a convenient small lamp will give a light of not more than about 5 candle-power. The large lamp will burn about 55 hours for every gallon of oil consumed, or give an illuminating duty of about 1100 candle-hours (i.e., the product of candle-power by burning-hours) per gallon. The small lamp, on the other hand, will burn about 140 hours for every gallon of oil consumed, or give an illuminating duty of about 700 candle-hours per gallon. Actually large lamps would in most country houses be used only in the entrance hall, living-rooms, and kitchen, while passages and minor rooms on the lower floors would be lighted by small lamps. Hence, making due allowance for the lower rate of consumption of the small lamps, it will be seen that, given equal numbers of large and small lamps in use, the mean illuminating duty of a gallon of oil as burnt in country houses will be 987, or, in round figures, 990 candle-hours. Usually candles are used in the bedrooms of country houses where the lower floors are lighted by means of petroleum lamps; but when acetylene is installed in such a house it will frequently be adopted in the principal bed- and dressing-rooms as well as in the living-rooms, as, unless candles are employed very lavishly, they are really totally inadequate to meet the reasonable demands for light of, e.g., a lady dressing for dinner. Where acetylene displaces candles as well as lamps in a country house, it is necessary, in comparing the cost of the new illuminant with that of the candles and oil, to bear in mind the superior degree of illumination which is secured in all rooms, at least where candles were formerly used.

In regard to exhaustion and vitiation of the air, and to heat evolved, self-luminous petroleum lamps stand on much the same footing as coal-gas when the latter is burned in flat-flame burners, if the comparison is based on a given yield of light. A large lamp, owing to its higher illuminating efficiency, is better in this respect than a small one-- light for light, it is more hygienic than ordinary flat-flame coal-gas burners, while a small lamp is less hygienic. It will therefore be understood at once, from what has already been said about the superiority on hygienic grounds of acetylene to flat-flame coal-gas lighting, that acetylene is in this respect far superior to petroleum lamps. The degree of its superiority is indicated more precisely by the figures quoted in the tabular statement which concludes this chapter.

Before giving the tabular statement, however, it is necessary to say a few words in regard to one method of lighting which, may possibly develop into a more serious competitor with acetylene for the lighting of the better class of country house than any of the illuminating agents and modes of lighting so far referred to. The method in question is lighting by so-called air-gas used for raising mantles to incandescence in upturned or inverted burners of the Welsbach-Kern type. "Air-gas" is ordinary atmospheric air, more or less completely saturated with the vapour of some highly volatile hydrocarbon. The hydrocarbons practically applied have so far been only "petroleum spirit" or "carburine," and "benzol." "Petroleum spirit" or "carburine" consists of the more highly volatile portion of petroleum, which is removed by distillation before the kerosene or burning oil is recovered from the crude oil. Several grades of this highly volatile petroleum distillate are distinguished in commerce; they differ in the temperature at which they begin to distil and the range of temperature covered by their distillation, and, speaking more generally, in their degree of volatility, uniformity, and density. If the petroleum distillate is sufficiently volatile and fairly uniform in character, good air-gas may be produced merely by allowing air to pass over an extended surface of the liquid. The vapour of the petroleum spirit is of greater density than air, and hence, if the course of the air-gas is downward from the apparatus at which it is produced, the flow of air into the apparatus and over the surface of the spirit will be automatically maintained by the "pull" of the descending air-gas when once the flow has been started until the outlet for the air-gas is stopped or the spirit in the apparatus is exhausted. Hence, if the apparatus for saturating air with the vapour of the light petroleum is placed well above all the points at which the air-gas is to be burnt-- e.g., on the roof of the house--the production of the air-gas may by simple devices become automatic, and the only attention the apparatus will require will be the replenishing of its reservoir from time to time with light petroleum. But a number of precautions are required to make this simple process operate without interruption or difficulty. For instance, the evaporation of the spirit must not be so rapid relatively to its total bulk as to lower its temperature, and thereby that of the overflowing air, too much; the reservoir must be protected from extreme cold and extreme heat; and the risk of fire from the presence of a highly volatile and highly inflammable liquid on or near the roof of the house must be met. This risk is one to which fire insurance companies take exception.

More commonly, however, air-gas is made non-automatically, or more or less automatically by the employment of some mechanical means. The light petroleum, benzol, or other suitable volatile hydrocarbon is volatilised, where necessary, by the application of gentle heat, while air is driven over or through it by means of a small motor, which in some cases is a hot-air engine operated by heat supplied by a flame of the air-gas produced. These air-gas producers, or at least the reservoir of volatile hydrocarbon, may be placed in an outbuilding, so that the risk of fire in the house itself is minimised. They require, however, as much attention as an acetylene generator, usually more. It is difficult to give reliable data as to the cost of air-gas, inclusive of the expenses of production. It varies considerably with the description of hydrocarbon employed, and its market price. Air-gas is only slightly inferior hygienically to acetylene, and the colour of its light is that of the incandescent light as produced by coal-gas or acetylene. Air-gas of a certain grade may be used for lighting by flat-flame burners, but it has been available thus for very many years, and has failed to achieve even moderate success. But the advent of the incandescent burner has completely changed its position relatively to most other illuminants, and under certain conditions it seems likely to be the most formidable competitor with acetylene. Since air-gas, and the numerous chemically identical products offered under different proprietary names, is simply atmospheric air more or less loaded with the vapour of a volatile hydrocarbon which is normally liquid, it possesses no definite chemical constitution, but varies in composition according to the design of the generating plant, the atmospheric temperature at the time of preparation, the original degree of volatility of the hydrocarbon, the remaining degree of volatility after the more volatile portions have been vaporised, and the speed at which the air is passed through the carburettor. The illuminating power and the calorific value of air-gas, unless the manufacture is very precisely controlled, are apt to be variable, and the amount of light, emitted, either in self-luminous or in incandescent burners, is somewhat indeterminate. The generating plant must be so constructed that the air cannot at any time be mixed with as much hydrocarbon vapour as constitutes an explosive mixture with it, otherwise the pipes and apparatus will contain a gas which will forthwith explode if it is ignited, i.e., if an attempt is made to consume it otherwise than in burners with specially small orifices. The safely permissible mixtures are (1) air with less hydrocarbon vapour than constitutes an explosive mixture, and (2) air with more hydrocarbon vapour than constitutes an explosive mixture. The first of these two mixtures is available for illuminating purposes only with incandescent mantles, and to ensure a reasonable margin of safety the mixing apparatus must be so devised that the proportion of hydrocarbon vapour in the air-gas can never exceed 2 per cent. From Chapter VI. it will be evident that a little more than 2 per cent. of benzene, pentane or benzoline vapour in air forms an explosive mixture. What is the lowest proportion of such vapours in admixture with air which will serve on combustion to maintain a mantle in a state of incandescence, or even to afford a flame at all, does not appear to have been precisely determined, but it cannot be much below 1- 1/2 per cent. Hence the apparatus for producing air-gas of this first class must be provided with controlling or governing devices of such nicety that the proportion of hydrocarbon vapour in the air-gas is maintained between about 1-1/2 and 2 per cent. It is fair to say that in normal working conditions a number of devices appear to fulfil this requirement satisfactorily. The second of the two mixtures referred to above, viz., air with more hydrocarbon vapour than constitutes an explosive mixture, is primarily suitable for combustion in self-luminous burners, but may also be consumed in properly designed incandescent burners. But the generating apparatus for such air-gas must be equipped with some governing or controlling device which will ensure the proportion of hydrocarbon vapour in the mixture never falling below, say, 7 per cent. On the other hand, if saturation of the air with the vapour is practically attained, should the temperature of the gas fall before it arrives at the point of combustion, part of the spirit will condense out, and the product will thus lose part of its illuminating or calorific intensity, besides partially filling the pipes with liquid products of condensation. The loss of intensity in the gas during cold weather may or may not be inconvenient according to circumstances; but the removal of part of the combustible material brings the residual air-gas nearer to its limit of explosibility--for it is simply a mixture of combustible vapour with air, which, normally, is not explosive because the proportion of spirit is too high--and thus, when led into an atmospheric burner, the extra amount of air introduced at the injector jets may cause the mixture to be an explosive mixture of air and spirit, so that it will take fire within the burner tube instead of burning quietly at the proper orifice. This matter will be made clearer on studying what is said about explosive limits in Chapter VI., and what is stated about incandescent acetylene (carburetted or not) in Chapters IX. and X. Clearly, however, high-grade air-gas is only suitable for preparation at the immediate spot where it is to be consumed; it cannot be supplied to a complete district unless it is intentionally made of such lower intensity that the proportion of spirit is too small ever to allow of partial deposition in the mains during the winter.

It is perhaps necessary to refer to the more extended use of candles for lighting in some few houses in which lamps are disliked on aesthetic, or, in some cases, ostensibly on hygienic grounds. Candle lighting, speaking broadly, is either very inadequate so far as ordinary living-rooms are concerned, or, if adequate, is very costly. Tests specially carried out by one of the authors to determine some of the figures required in the ensuing table show that ordinary paraffin or "wax" candles usually emit about 20 per cent. more light than that given by the standard spermaceti candle, whose luminosity is the unit by which the intensity of other lights is reckoned in Great Britain; and also that the light so emitted by domestic candles is practically unaffected by the sizes--"sixes," "eights," or "twelves"--burnt. In the sizes examined the light evolved has varied between 1.145 and 1.298 "candles," perhaps tending to increase slightly with the diameter of the candle tested. Hence, to obtain illumination in a room equal on the average to that afforded by 100 standard candles, or some other light or lights aggregating 100 candle- power, would require the use of only 80 to 85 ordinary paraffin, ozokerite, or wax candles. But actually the essential objects in a room could be equally well illuminated by, say, 30 candles well distributed, as by two or three incandescent gas-burners, or four or five large oil- lamps. Lights of high intensity, such as powerful gas-burners or oil- lamps, must give a higher degree of illumination in their immediate vicinity than is really necessary, if they are to illuminate adequately the more distant objects. The dissemination and diffusion of their light can be greatly aided by suitable colouring of ceilings, walls and drapings; but unless the illumination by means of lights of relatively high intensity is made almost wholly indirect, candles or other lights of low intensity, such as small electric glow-lamps, can, by proper distribution, be made to give more uniform or more suitably apportioned illumination. In this respect candles have an economical and, in some measure, a material advantage over acetylene also. (But when the method of lighting is by flames--candle or other--the multiplication of the number of units which is involved when they are of low intensity, seriously increases the risk of fire through accidental contact of inflammable material with any one of the flames. This risk is much greater with naked flames, such as candles, than with, say, inverted incandescent gas flames, which are to all intents and purposes fully protected by a closed glass globe.) Hence, in the tabular statement which follows of the comparative cost, &c., of different illuminants, it will be assumed that 30 good candles would in practice be equally efficient in regard to the illumination of a room as large oil-lamps, acetylene flames, or incandescent gas-burners aggregating 100 candle-power.

For the same reason it will be assumed that electric glow-lamps of low intensity (nominally of 8 candle-power or less), aggregating 70-80 candle-power, will practically serve, if suitably distributed, equally as well as 100 candle-power obtained from more powerful sources of light. Electric glow-lamps of a nominal intensity of 16 candles or thereabouts, and good flat-flame gas-burners, aggregating 90-95 candle-power, will similarly be taken as equivalent, if suitably distributed, to 100 candle- power from more powerful sources of light. Of the latter it will be assumed that each source has an intensity between 20 and 30 candle-power, such as is afforded by a large oil-lamp, a No. 1 Welsbach-Kern upturned, or a "Bijou" inverted incandescent gas-burner, or a 0.70-cubic-foot-per- hour acetylene burner. Either of these sources of light, when used in sufficient numbers, so that with proper distribution they light a room adequately, will be taken in the tabular statement which follows as affording, per candle-power evolved, the standard illuminating effect required in that room. The same illuminating effect will be regarded as attainable by means of candles aggregating only 35 per cent., or small electric glow-lamps aggregating 77 per cent., or large electric glow- lamps and flat-flame gas-burners aggregating 90 to 95 per cent. of this candle-power; while if sources of light of higher intensity are used, such as Osram or Tantalum electric lamps, or the larger incandescent gas- burners (the Welsbach "C" or "York," or the Nos. 3 or 4 Welsbach-Kern upturned, or the No. 1 or larger size inverted burners) or incandescent acetylene burners, it will be assumed that their aggregate candle-power must be in excess by about 15 per cent., in order to compensate for the impossibility of obtaining equally well distributed illumination. These assumptions are based on general considerations and data as to the effect of sources of light of different intensities in giving practically the same degree of illumination in a room; it would occupy too much space here to discuss more fully the grounds on which they have been made. It must suffice to say that they have been adopted with the object of being perfectly fair to each means of illumination.

COST PER HOUR AND HYGIENIC EFFECT OF LIGHTING BY DIFFERENT MEANS

The data (except in the column headed "cost per 100 candle-hours") refer to the illumination afforded by medium-sized (0.5 to 0.7 cubic foot per hour) acetylene burners yielding together a light of about 100 candle- power, and to the approximately equivalent illumination as afforded by other means of illumination, when the lighting-units or sources of light are rationally distributed.

Interest and depreciation charges on the outlay on piping or wiring a house, on brackets, fittings, lamps, candelabra, and storage accommodation (for carbide and oil) have been taken as equivalent for all modes of lighting, and omitted in computing the total cost. The cost of labour for attendance on acetylene plant, oil lamps, and candles is an uncertain and variable item--approximately equal for all these modes of lighting, but saved in coal-gas and electric lighting from public supply mains.

______________________________________________________________________
| | | | | | |
| | |Candle- | Number |Aggregate| Cost |
| | |Power of| of | Candle- | per |
| | Description of | each |Lighting | Power | 100 |
|Illuminant. | Burner or Lamp. |Lighting| Units |Afforded.|Candle-|
| | | Unit. |Required.|(About.) |Hours. |
| | |(About.)| | |Pence. |
|____________|____________________|________|_________|_________|_______|
| | | | | | |
| |Self-luminous; 0.5 | | | | |
| | cubic foot per hour| 18 | 5 | 90 | 1.11 |
| |Self-luminous; 0.7 | | | | |
| Acetylene | cubic foot per hour| 27 | 4 | 108 | 1.02 |
| |Self-luminous; 1.0 | | | | |
| | cubic foot per hour| 45.5 | 3 | 136 | 0.85 |
| |Incandescent; 0.5 | | | | |
| | cubic foot per hour| 50 | 3 | 150 | 0.49 |
|____________|____________________|________|_________|_________|_______|
| | | | | | |
| Petroleum | Large lamp . . . . | 20 | 5 | 100 | 0.84 |
| (paraffin | | | | | |
| oil) | Small lamp . . . . | 5 | 14 | 70 | 1.31 |
|____________|____________________|________|_________|_________|_______|
| | | | | | |
| |Flat flame (bad) 5 | | | | |
| | cubic feet per hour| 8 | 10 | 80 | 3.75 |
| |Flat flame (good) 6 | | | | |
| Coal Gas | cubic feet per hour| 16 | 6 | 96 | 2.25 |
| |Incandescent (No. 1 | | | | |
| | Kern or Bijou In- | 25 | 4 | 100 | 0.38 |
| | verted); 1-1/2 | | | | |
| | cubic feet per hour| | | | |
|____________|____________________|________|_________|_________|_______|
| | | | | | |
| Candles |"Wax" (so-called) . | 1.2 | 30 | 35 | 6.14 |
|____________|____________________|________|_________|_________|_______|
| | | | | | |
| | Small glow . . . . | 7 | 11 | 77 | 2.81 |
| | Large glow . . . . | 13 | 7 | 91 | 2.90 |
| Electricity| | | | | |
| | Tantalum . . . . . | 19 | 5 | 95 | 1.52 |
| | Osram . . . . . . | 14 | 7 | 98 | 1.00 |
|____________|____________________|________|_________|_________|_______|

_______________________________________________________________________
| | | | | | |
| | |Inci- | Exhaus- |Vitiation | Heat |
| | | den- | tion of | of Air. |Produced.|
| | Description of | tal |Air.Cubic|Cubic Feet|Number of|
|Illuminant. | Burner or Lamp. |Expen-|Feet Dep-| of Car- |Units of |
| | | ces. |rived of |bonic Acid| Heat. |
| | | | Oxygen. | Formed. |Calories.|
|____________|____________________|______|_________|__________|_________|
| | | | | | |
| |Self-luminous; 0.5 | | | | |
| | cubic foot per hour| [1] | 29.8 | 5.0 | 900 |
| |Self-luminous; 0.7 | | | | |
| Acetylene | cubic foot per hour| | 33.3 | 5.6 | 1010 |
| |Self-luminous; 1.0 | | | | |
| | cubic foot per hour| | 35.7 | 6.0 | 1000 |
| |Incandescent; 0.5 | | | | |
| | cubic foot per hour| [2] | 17.9 | 3.0 | 545 |
|____________|____________________|______|_________|__________|_________|
| | | | | | |
| Petroleum | Large lamp . . . . | | 140.0 | 19.6 | 3630 |
| (paraffin | | [3] | | | |
| oil) | Small lamp . . . . | | 154.0 | 21.6 | 4000 |
|____________|____________________|______|_________|__________|_________|
| | | | | | |
| |Flat flame (bad) 5 | | | | |
| | cubic feet per hour| Nil | 270.0 | 27.0 | 7750 |
| |Flat flame (good) 6 | | | | |
| Coal Gas | cubic feet per hour| Nil | 195.0 | 19.5 | 5580 |
| |Incandescent (No. 1 | | | | |
| | Kern or Bijou In- | [4] | 27.0 | 2.7 | 775 |
| | verted); 1-1/2 | | | | |
| | cubic feet per hour| | | | |
|____________|____________________|______|_________|__________|_________|
| | | | | | |
| Candles |"Wax" (so-called) . | Nil | 100.5 | 13.7 | 2700 |
|____________|____________________|______|_________|__________|_________|
| | | | | | |
| | Small glow . . . . |2s.6d.| Nil | Nil | 285 |
| | Large glow . . . . |2s.6d.| " | " | 360 |
| Electricity| | [5] | | | |
| | Tantalum . . . . . |7s.6d.| " | " | 172 |
| | Osram . . . . . . | 6s. | " | " | 96 |
|____________|____________________|______|_________|__________|_________|

[Footnote 1: Interest and depreciation charges on generating and purifying plant = 0.15 penny. Purifying material and burner renewals = 0.05 penny.]

[Footnote 2: Mantle renewals as for coal-gas.]

[Footnote 3: Renewals of wicks and chimneys = 0.02 penny.]

[Footnote 4: Renewals and mantles (and chimneys) at contract rate of 3s. per burner per annum.]

[Footnote 5: Renewals of lamps and fuses, at price indicated per lamp per annum.]

The conventional method of making pecuniary comparisons between different sources of artificial light consists in simply calculating the cost of developing a certain number of candle-hours of light--i.e., a certain amount of standard candle-power for a given number of hours--on the assumption that as many separate sources of light are employed as may be required to bring the combined illuminating power up to the total amount wanted. In view of the facts as to dissemination and diffusion, or the difference between sheer illuminating power and useful illuminating effect, which have just been elaborated, and in view of the different intensities of the different unit sources of light (which range from the single candle to a powerful large incandescent gas-burner or a metallic filament electric lamp), such a method of calculation is wholly illusory. The plan adopted in the following table may also appear unnecessarily complicated; but it is not so to the reader if he remembers that the apparently various amount of illumination is corrected by the different numbers of illuminating units until the amount of simple candle-power developed, whatever illuminant be employed, suffices to light a room having an area of about 300 square feet (i.e., a room, 17-1/2 feet square, or one 20 feet long by 15 feet wide), so that ordinary print may be read comfortably in any part of the room, and the titles of books, engravings, &c., in any position on the walls up to a height of 8 feet from the ground may be distinguished with ease. The difference in cost, &c., of a greater or less degree of illumination, or of lighting a larger or smaller room by acetylene or any other of the illuminants named, will be almost directly proportional to the cost given for the stated conditions. Nevertheless, it should be recollected that when the conventional system is retained--useful illuminating effect being sacrificed to absolute illuminating power--acetylene is made to appear cheaper in comparison with all weaker unit sources of light, and dearer in comparison with all stronger unit sources of light than the accompanying table indicates it to be. In using the comparative figures given in the table, it should be borne in mind that they refer to more general and more brilliant illumination of a room than is commonly in vogue where the lighting is by means of electric light, candles, or oil- lamps. The standard of illumination adopted for the table is one which is only gaining general recognition where incandescent gas or acetylene lighting is available, though in exceptional cases it has doubtless been attained by means of oil-lamps or flat-flame gas-burners, but very rarely if ever by means of carbon-filament electric glow-lamps, or candles. It assumes that the occupants of a room do not wish to be troubled to bring work or book "to the light," but wish to be able to work or read wheresoever in the room they will, without consideration of the whereabouts of the light or lights.

It should, perhaps, be added that so high a price as 5s. per 1000 cubic feet for coal-gas rarely prevails in Great Britain, except in small outlying towns, whereas the price of 6d. per Board of Trade unit for electricity is not uncommonly exceeded in the few similar country places in which there is a public electricity supply.

[CHAPTER II]

THE PHYSICS AND CHEMISTRY OF THE REACTION BETWEEN CARBIDE AND WATER

THE NATURE OF CALCIUM CARBIDE.--The raw material from which, by interaction with water, acetylene is obtained, is a solid body called calcium carbide or carbide of calcium. Inasmuch as this substance can at present only be made on a commercial scale in the electric furnace--and so far as may be foreseen will never be made on a large scale except by means of electricity--inasmuch as an electric furnace can only be worked remuneratively in large factories supplied with cheap coal or water power; and inasmuch as there is no possibility of the ordinary consumer of acetylene ever being able to prepare his own carbide, all descriptions of this latter substance, all methods of winning it, and all its properties except those which concern the acetylene-generator builder or the gas consumer have been omitted from the present book. Hitherto calcium carbide has found but few applications beyond that of evolving acetylene on treatment with water or some aqueous liquid, hygroscopic solid, or salt containing water of crystallisation; but it has possibilities of further employment, should its price become suitable, and a few words will be devoted to this branch of the subject in Chapter XII. Setting these minor uses aside, calcium carbide has no intrinsic value except as a producer of acetylene, and therefore all its characteristics which interest the consumer of acetylene are developed incidentally throughout this volume as the necessity for dealing with them arises.

It is desirable, however, now to discuss one point connected with solid carbide about which some misconception prevails. Calcium carbide is a body which evolves an inflammable, or on occasion an explosive, gas when treated with water; and therefore its presence in a building has been said to cause a sensible increase in the fire risk because attempts to extinguish a fire in the ordinary manner with water may cause evolution of acetylene which should determine a further production of flame and heat. In the absence of water, calcium carbide is absolutely inert as regards fire; and on several occasions drums of it have been recovered uninjured from the basement of a house which has been totally destroyed by fire. With the exception of small 1-lb. tins of carbide, used only by cyclists, &c., the material is always put into drums of stout sheet-iron with riveted or folded seams. Provided the original lid has not been removed, the drums are air- and water-tight, so that the fireman's hose may be directed upon them with impunity. When a drum has once been opened, and not all of its contents have been put into the generator, ordinary caution--not merely as regards fire, but as regards the deterioration of carbide when exposed to the atmosphere--suggests either that the lid must be made air-tight again (not by soldering it), [Footnote: Carbide drums are not uncommonly fitted with self-sealing or lever-top lids, which are readily replaced hermetically tight after opening and partial removal of the contents of the drum.] or preferably that the rest of the carbide shall be transferred to some convenient receptacle which can be perfectly closed. [Footnote: It would be a refinement of caution, though hardly necessary in practice, to fit such a receptacle with a safety-valve. If then the vessel were subjected to sudden or severe heating, the expansion of the air and acetylene in it could not possibly exert a disruptive effect upon the walls of the receptacle, which, in the absence of the safety-valve, is imaginable.] Now, assuming this done, the drums are not dependent upon soft solder to keep them sound, and so they cannot open with heat. Fire and water, accordingly, cannot affect them, and only two risks remain: if stored in the basement of a tall building, falling girders, beams or brickwork may burst them; or if stored on an upper floor, they may fall into the basement and be burst with the shock--in either event water then having free access to the contents. But drums of carbide would never be stored in such positions: a single one would be kept in the generator-house; several would be stored in a separate room therein, or in some similar isolated shed. The generator-house or shed would be of one story only; the drums could neither fall nor have heavy weights fall on them during a fire; and therefore there is no reason why, if a fire should occur, the firemen should not be permitted to use their hose in the ordinary fashion. Very similar remarks apply to an active acetylene generator. Well built, such plant will stand much heat and fire without failure; if it is non-automatic, and of combustible materials contains nothing but gas in the holder, the worst that could happen in times of fire would be the unsealing of the bell or its fracture, and this would be followed, not at all by any explosion, but by a fairly quiet burning of the escaping gas, which would be over in a very short time, and would not add to the severity of the conflagration unless the generator-house were so close to the residence that the large flame of burning gas could ignite part of the main building. Even if the heat were so great near the holder that the gas dissociated, it is scarcely conceivable that a dangerous explosion should arise. But it is well to remember, that if the generator-house is properly isolated from the residence, if it is constructed of non-inflammable materials, if the attendant obeys instructions and refrains from taking a naked light into the neighbourhood of the plant, and if the plant itself is properly designed and constructed, a fire at or near an acetylene generator is extremely unlikely to occur. At the same time, before the erection of plant to supply any insured premises is undertaken, the policy or the company should be consulted to ascertain whether the adoption of acetylene lighting is possibly still regarded by the insurers as adding an extra risk or even as vitiating the whole insurance.

REGULATIONS FOR THE STORAGE OF CARBIDE: BRITISH.--There are also certain regulations imposed by many local authorities respecting the storage of carbide, and usually a licence for storage has to be obtained if more than 5 lb. is kept at a time. The idea of the rule is perfectly justifiable, and it is generally enforced in a sensible spirit. As the rules may vary in different localities, the intending consumer of acetylene must make the necessary inquiries, for failure to comply with the regulations may obviously be followed by unpleasantness.

Having regard to the fact that, in virtue of an Order in Council dated July 7, 1897, carbide may be stored without a licence only in separate substantial hermetically closed metal vessels containing not more than 1 lb. apiece and in quantities not exceeding 5 lb. in the aggregate, and having regard also to the fact that regulations are issued by local authorities, the Fire Offices' Committee of the United Kingdom has not up to the present deemed it necessary to issue special rules with reference to the storage of carbide of calcium.

The following is a copy of the rules issued by the National Board of Fire Underwriters of the UNITED STATES OF AMERICA for the storage of calcium carbide on insured premises:

RULES FOR THE STORAGE OF CALCIUM CARBIDE.

(a) Calcium carbide in quantities not to exceed six hundred (600) pounds may be stored, when contained in approved metal packages not to exceed one hundred (100) pounds each, inside insured property, provided that the place of storage be dry, waterproof and well ventilated, and also provided that all but one of the packages in any one building shall be sealed and the seals shall not be broken so long as there is carbide in excess of one (1) pound in any other unsealed package in the building.

(b) Calcium carbide in quantities in excess of six hundred (600) pounds must be stored above ground in detached buildings, used exclusively for the storage of calcium carbide, in approved metal packages, and such buildings shall be constructed to be dry, waterproof and well ventilated.

(c) Packages to be approved must be made of metal of sufficient strength to insure handling the package without rupture, and be provided with a screwed top or its equivalent.

They must be constructed so as to be water- and air-tight without the use of solder, and conspicuously marked "CALCIUM CARBIDE--DANGEROUS IF NOT KEPT DRY."

The following is a summary of the AUSTRIAN GOVERNMENT rules relating to the storage and handling of carbide:

(1) It must be sold and stored only in closed water-tight vessels, which, if the contents exceed 10 kilos., must be marked in plain letters "CALCIUM CARBIDE--TO BE KEPT CLOSED AND DRY." They must not be of copper and if soldered must be opened by mechanical means and not by unsoldering. They must be stored out of the reach of water.

(2) Quantities not exceeding 300 kilos. may be stored in occupied houses, provided the single drums do not exceed 100 kilos. nominal capacity. The storage-place must be dry and not underground.

(3) The limits specified in Rule 2 apply also to generator-rooms, with the proviso also that in general the amount stored shall not exceed five days' consumption.

(4) Quantities ranging from 300 to 1000 kilos. must be stored in special well-ventilated uninhabited non-basement rooms in which lights and smoking are not allowed.

(5) Quantities exceeding 1000 kilos. must be stored in isolated fireproof magazines with light water-tight roofs. The floors must be at least 8 inches above ground-level.

(6) Carbide in water-tight drums may be stored in the open in a fenced enclosure at least 30 feet from buildings, adjoining property, or inflammable materials. The drums must be protected from wet by a light roof.

(7) The breaking of carbide must be done by men provided with respirators and goggles, and care taken to avoid the formation of dust.

(8) Local or other authorities will issue from time to time special regulations in regard to carbide trade premises.

The ITALIAN GOVERNMENT rules relating to the storage and transport of carbide follow in the main those of the Austrian Government, but for quantities between 300 and 2000 kilos sanction is required from the local authorities, and for larger quantities from superior authorities. The storage of quantities ranging from 300 to 2000 kilos is forbidden in dwelling-houses and above the latter quantity the storage-place must be isolated and specially selected. No special permit is required for the storage of quantities not exceeding 300 kilos. Workmen exposed to carbide dust arising from the breaking of carbide or otherwise must have their eyes and respiratory organs suitably protected.

THE PURCHASE OF CARBIDE.--Since calcium carbide is only useful as a means of preparing acetylene, it should be bought under a guarantee (1) that it contains less impurities than suffice to render the crude gas dangerous in respect of spontaneous inflammability, or objectionable in a manner to be explained later on, when consumed; and (2) that it is capable of evolving a fixed minimum quantity of acetylene when decomposed by water. Such determination, however, cannot be carried out by the ordinary consumer for himself. A generator which is perfectly satisfactory in general behaviour, and which evolves a sufficient proportion of the possible total make of gas to be economical, does not of necessity decompose the carbide quantitatively; nor is it constructed in a fashion to render an exact measurement of the gas liberated at standard temperature and pressure easy to obtain. For obvious reasons the careful consumer of acetylene will keep a record of the carbide decomposed and of the acetylene generated--the latter perhaps only in terms of burner- hours, or the like; but in the event of serious dispute as to the gas- making capacity of his raw material, he must have a proper analysis made by a qualified chemist.

Calcium carbide is crushed by the makers into several different sizes, in each of which all the lumps exceed a certain size and are smaller than another size. It is necessary to find out by experiment, or from the maker, what particular size suits the generator best, for different types of apparatus require different sizes of carbide. Carbide cannot well be crushed by the consumer of acetylene. It is a difficult operation, and fraught with the production of dust which is harmful to the eyes and throat, and if done in open vessels the carbide deteriorates in gas- making power by its exposure to the moisture of the atmosphere. True dust in carbide is objectionable, and practically useless for the generation of acetylene in any form of apparatus, but carbide exceeding 1 inch in mesh is usually sold to satisfy the suggestions of the British Acetylene Association, which prescribes 5 per cent, of dust as the maximum. Some grades of carbide are softer than others, and therefore tend to yield more dust if exposed to a long journey with frequent unloadings.

There are certain varieties of ordinary carbide known as "treated carbide," the value of which is more particularly discussed in Chapter III. The treatment is of two kinds, or of a combination of both. In one process the lumps are coated with a strong solution of glucose, with the object of assisting in the removal of spent lime from their surface when the carbide is immersed in water. Lime is comparatively much more soluble in solutions of sugar (to which class of substances glucose belongs) than in plain water; so that carbide treated with glucose is not so likely to be covered with a closely adherent skin of spent lime when decomposed by the addition of water to it. In the other process, the carbide is coated with or immersed in some oil or grease to protect it from premature decomposition. The latter idea, at least, fulfils its promises, and does keep the carbide to a large extent unchanged if the lumps are exposed to damp air, while solving certain troubles otherwise met with in some generators (cf. Chapter III.); but both operations involve additional expense, and since ordinary carbide can be used satisfactorily in a good fixed generator, and can be preserved without serious deterioration by the exercise of reasonable care, treated carbide is only to be recommended for employment in holderless generators, of which table-lamps are the most conspicuous forms. A third variant of plain carbide is occasionally heard of, which is termed "scented" carbide. It is difficult to regard this material seriously. In all probability calcium carbide is odourless, but as it begins to evolve traces of gas immediately atmospheric moisture reaches it, a lump of carbide has always the unpleasant smell of crude acetylene. As the material is not to be stored in occupied rooms, and as all odour is lost to the senses directly the carbide is put into the generator, scented carbide may be said to be devoid of all utility.

THE REACTION BETWEEN CARBIDE AND WATER.--The reaction which occurs when calcium carbide and water are brought into contact belongs to the class that chemists usually term double decompositions. Calcium carbide is a chemical compound of the metal calcium with carbon, containing one chemical "part," or atomic weight, of the former united to two chemical parts, or atomic weights, of the latter; its composition expressed in symbols being CaC_2. Similarly, water is a compound of two chemical parts of hydrogen with one of oxygen, its formula being H_2O. When those two substances are mixed together the hydrogen of the water leaves its original partner, oxygen, and the carbon of the calcium carbide leaves the calcium, uniting together to form that particular compound of hydrogen and carbon, or hydrocarbon, which is known as acetylene, whose formula is C_2H_2; while the residual calcium and oxygen join together to produce calcium oxide or lime, CaO. Put into the usual form of an equation, the reaction proceeds thus--

(1) CaC_2 + H_2O = C_2H_2 + CaO.

This equation not only means that calcium carbide and water combine to yield acetylene and lime, it also means that one chemical part of carbide reacts with one chemical part of water to produce one chemical part of acetylene and one of lime. But these four chemical parts, or molecules, which are all equal chemically, are not equal in weight; although, according to a common law of chemistry, they each bear a fixed proportion to one another. Reference to the table of "Atomic Weights" contained in any text-book of chemistry will show that while the symbol Ca is used, for convenience, as a contraction or sign for the element calcium simply, it bears a more important quantitative significance, for to it will be found assigned the number 40. Against carbon will be seen the number 12; against oxygen, 16; and against hydrogen, 1. These numbers indicate that if the smallest weight of hydrogen ever found in a chemical compound is called 1 as a unit of comparison, the smallest weights of calcium, carbon, and oxygen, similarly taking part in chemical reactions are 40, 12, and 16 respectively. Thus the symbol CaC_2, comes to convoy three separate ideas: (a) that the substance referred to is a compound of calcium and carbon only, and that it is therefore a carbide of calcium; (b) that it is composed of one chemical part or atom of calcium and two atoms of carbon; and (c) that it contains 40 parts by weight of calcium combined with twice twelve, or 24, parts of carbon. It follows from (c) that the weight of one chemical part, now termed a molecule as the substance is a compound, of calcium carbide is (40 + 2 x 12) = 64. By identical methods of calculation it will be found that the weight of one molecule of water is 18; that of acetylene, 26; and that of lime, 56. The general equation (1) given above, therefore, states in chemical shorthand that 64 parts by weight of calcium carbide react with 18 parts of water to give 26 parts by weight of acetylene and 56 parts of lime; and it is very important to observe that just as there are the same number of chemical parts, viz., 2, on each side, so there are the same number of parts by weight, for 64 + 18 = 56 + 26 = 82. Put into other words equation (1) shows that if 64 grammes, lb., or cwts. of calcium carbide are treated with 18 grammes, lb., or cwts. of water, the whole mass will be converted into acetylene and lime, and the residue will not contain any unaltered calcium carbide or any water; whence it may be inferred, as is the fact, that if the weights of carbide and water originally taken do not stand to one another in the ratio 64 : 18, both substances cannot be entirely decomposed, but a certain quantity of the one which was in excess will be left unattacked, and that quantity will be in exact accordance with the amount of the said excess--indifferently whether the superabundant substance be carbide or water.

Hitherto, for the sake of simplicity, the by-product in the preparation of acetylene has been described as calcium oxide or quicklime. It is, however, one of the leading characteristics of this body to be hygroscopic, or greedy of moisture; so that if it is brought into the presence of water, either in the form of liquid or as vapour, it immediately combines therewith to yield calcium hydroxide, or slaked lime, whose chemical formula is Ca(OH)_2. Accordingly, in actual practice, when calcium carbide is mixed with an excess of water, a secondary reaction takes place over and above that indicated by equation (1), the quicklime produced combining with one chemical part or molecule of water, thus--

CaO + H_2O = Ca(OH)_2.

As these two actions occur simultaneously, it is more usual, and more in agreement with the phenomena of an acetylene generator, to represent the decomposition of calcium carbide by the combined equation--

(2) CaC_2 + 2H_2O = C_2H_2 + Ca(OH)_2.

By the aid of calculations analogous to those employed in the preceding paragraph, it will be noticed that equation (2) states that 1 molecule of calcium carbide, or 64 parts by weight, combines with 2 molecules of water, or 36 parts by weight, to yield 1 molecule, or 26 parts by weight of acetylene, and 1 molecule, or 74 parts by weight of calcium hydroxide (slaked lime). Here again, if more than 36 parts of water are taken for every 64 parts of calcium carbide, the excess of water over those 36 parts is left undecomposed; and in the same fashion, if less than 36 parts of water are taken for every 64 parts of calcium carbide, some of the latter must remain unattacked, whilst, obviously, the amount of acetylene liberated cannot exceed that which corresponds with the quantity of substance suffering complete decomposition. If, for example, the quantity of water present in a generator is more than chemically sufficient to attack all the carbide added, however largo or small that excess may be, no more, and, theoretically speaking, no less, acetylene can ever be evolved than 26 parts by weight of gas for every 64 parts by weight of calcium carbide consumed. It is, however, not correct to invert the proposition, and to say that if the carbide is in excess of the water added, no more, and, theoretically speaking, no less, acetylene can ever be evolved than 26 parts by weight of gas for every 36 parts of water consumed, as might be gathered from equation (2); because equation (1) shows that 26 parts of acetylene may, on occasion, be produced by the decomposition of 18 parts by weight of water. From the purely chemical point of view this apparent anomaly is explained by the circumstance that of the 36 parts of water present on the left-hand aide of equation (2), only one-half, i.e., 18 parts by weight, are actually decomposed into hydrogen and oxygen, the other 18 parts remaining unattacked, and merely attaching themselves as "water of hydration" to the 56 parts of calcium oxide in equation (1) so as to produce the 74 parts of calcium hydroxide appearing on the right-hand side of equation (2). The matter is perhaps rendered more intelligible by employing the old name for calcium hydroxide or slaked lime, viz., hydrated oxide of calcium, and by writing its formula in the corresponding form, when equation (2) becomes

CaC_2 + 2H_2O = C_2H_2 + CaO.H_2O.

It is, therefore, absolutely correct to state that if the amount of calcium carbide present in an acetylene generator is more than chemically sufficient to decompose all the water introduced, no more, and theoretically speaking no less, acetylene can ever be liberated than 26 parts by weight of gas for every 18 parts by weight of water attacked. This, it must be distinctly understood, is the condition of affairs obtaining in the ideal acetylene generator only; since, for reasons which will be immediately explained, when the output of gas is measured in terms of the water decomposed, in no commercial apparatus, and indeed in no generator which can be imagined fit for actual employment, does that output of gas ever approach the quantitative amount; but the volume of water used, if not actually disappearing, is always vastly in excess of the requirements of equation (2). On the contrary, when the make of gas is measured in terms of the calcium carbide consumed, the said make may, and frequently does, reach 80, 90, or even 99 per cent. of what is theoretically possible. Inasmuch as calcium carbide is the one costly ingredient in the manufacture of acetylene, so long as it is not wasted-- so long, that is to say, as nearly the theoretical yield of gas is obtained from it--an acetylene generator is satisfactory or efficient in this particular; and except for the matter of solubility discussed in the following chapter, the quantity of water consumed is of no importance whatever.

HEAT EVOLVED IN THE REACTION.--The chemical reaction between calcium carbide and water is accompanied by a large evolution of heat, which, unless due precautions are taken to prevent it, raises the temperature of the substances employed, and of the apparatus containing them, to a serious and often inconvenient extent. This phenomenon is the most important of all in connexion with acetylene manufacture; for upon a proper recognition of it, and upon the character of the precautions taken to avoid its numerous evil effects, depend the actual value and capacity for smooth working of any acetylene generator. Just as, by an immutable law of chemistry, a given weight of calcium carbide yields a given weight of acetylene, and by no amount of ingenuity can be made to produce either more or less; so, by an equally immutable law of physics, the decomposition of a given weight of calcium carbide by water, or the decomposition of a given weight of water by calcium carbide, yields a perfectly definite quantity of heat--a quantity of heat which cannot be reduced or increased by any artifice whatever. The result of a production of heat is usually to raise the temperature of the material in which it is produced; but this is not always the case, and indeed there is no necessary connexion or ratio between the quantity of heat liberated in any form of chemical reaction--of which ordinary combustion is the commonest type--and the temperature attained by the substances concerned. This matter has so weighty a bearing upon acetylene generation, and appears to be so frequently misunderstood, that a couple of illustrations may with advantage be studied. If a vessel full of cold water, and containing also a thermometer, is placed over a lighted gas-burner, at first the temperature of the liquid rises steadily, and there is clearly a ratio between the size of the flame and the speed at which the mercury mounts up the scale. Finally, however, the thermometer indicates a certain point, viz., 100° C, and the water begins to boil; yet although the burner is untouched, and consequently, although heat must be passing into the vessel at the same rate as before, the mercury refuses to move as long as any liquid water is left. By the use of a gas meter it might be shown that the same volume of gas is always consumed (a) in raising the temperature of a given quantity of cold water to the boiling- point, and another equally constant volume of gas is always consumed (b) in causing the boiling water to disappear as steam. Hence, as coal-gas is assumed for the present purpose to possess invariably the same heating power, it appears that the same quantity of heat is always needed to convert a given amount of cold water at a certain temperature into steam; but inasmuch as reference to the meter would show that about 5 times the volume of gas is consumed in changing the boiling water into steam as is used in heating the cold water to the boiling-point, it will be evident that the temperature of the mass is raised as high by the heat evolved during the combustion of one part of gas as it is by that liberated on the combustion of 6 times that amount.

A further example of the difference between quantity of heat and sensible temperature may be seen in the combustion of coal, for (say) one hundredweight of that fuel might be consumed in a very few minutes in a furnace fitted with a powerful blast of air, the operation might be spread over a considerable number of hours in a domestic grate, or the coal might be allowed to oxidise by exposure to warm air for a year or more. In the last case the temperature might not attain that of boiling water, in the second it would be about that of dull redness, and in the first it would be that of dazzling whiteness; but in all three cases the total quantity of heat produced by the time the coal was entirely consumed would be absolutely identical. The former experiment with water and a gas-burner, too, might easily be modified to throw light upon another problem in acetylene generation, for it would be found that if almost any other liquid than water were taken, less gas (i.e., a smaller quantity of heat) would be required to raise a given weight of it from a certain low to a certain high temperature than in the case of water itself; while if it were possible similarly to treat the same weight of iron (of which acetylene generators are constructed), or of calcium carbide, the quantity of heat used to raise it through a given number of thermometric degrees would hardly exceed one-tenth or one- quarter of that needed by water itself. In technical language this difference is due to the different specific heats of the substances mentioned; the specific heat of a body being the relative quantity of heat consumed in raising a certain weight of it a certain number of degrees when the quantity of heat needed to produce the same effect on the same weight of water is called unity. Thus, the specific heat of water being termed 1.0, that of iron or steel is 0.1138, and that of calcium carbide 0.247, [Footnote: This is Carlson's figure. Morel has taken the value 0.103 in certain calculations.] both measured at temperatures where water is a liquid. Putting the foregoing facts in another shape, for a given rise in temperature that substance will absorb the most heat which has the highest specific heat, and therefore, in this respect, 1 part by weight of water will do the work of roughly 9 parts by weight of iron, and of about 4 parts by weight of calcium carbide.

From the practical aspect what has been said amounts to this: During the operation of an acetylene generator a large amount of heat is produced, the quantity of which is beyond human control. It is desirable, for various reasons, that the temperature shall be kept as low as possible. There are three substances present to which the heat may be compelled to transfer itself until it has opportunity to pass into the surrounding atmosphere: the material of which the apparatus is constructed, the gas which is in process of evolution, and whichever of the two bodies-- calcium carbide or water--is in excess in the generator. Of these, the specific heat at constant pressure of acetylene has unfortunately not yet been determined, but its relative capacity for absorbing heat is undoubtedly small; moreover the gas could not be permitted to become sufficiently hot to carry off the heat without grave disadvantages. The specific heat of calcium carbide is also comparatively small, and there are similar disadvantages in allowing it to become hot; moreover it is deficient in heat-conducting power, so that heat communicated to one portion of the mass does not extend rapidly throughout, but remains concentrated in one spot, causing the temperature to rise objectionably. Steel has a sufficient amount of heat-conducting power to prevent undue concentration in one place; but, as has been stated, its specific heat is only one-ninth that of water. Water is clearly, therefore, the proper substance to employ for the dissipation of the heat generated, although it is strictly speaking almost devoid of heat-conducting power; for not only is the specific heat of water much greater than that of any other material present, but it possesses in a high degree the faculty of absorbing heat throughout its mass, by virtue of the action known as convection, provided that heat is communicated to it at or near the bottom, and not too near its upper surface. Moreover, water is a much more valuable substance for dissipating heat than appears from the foregoing explanation; for reference to the experiment with the gas- burner will show that six and a quarter times as much heat can be absorbed by a given weight of water if it is permitted to change into steam, as if it is merely raised to the boiling-point; and since by no urging of the gas-burner can the temperature be raised above 100° C. as long as any liquid water remains unevaporated, if an excess of water is employed in an acetylene generator, the temperature inside can never-- except quite locally--exceed 100° C., however fast the carbide be decomposed. An indefinitely large consumption of water by evaporation in a generator matters nothing, for the liquid may be considered of no pecuniary value, and it can all be recovered by condensation in a subsequent portion of the plant.

It has been said that the quantity of heat liberated when a certain amount of carbide suffers decomposition is fixed; it remains now to consider what that quantity is. Quantities of heat are always measured in terms of the amount needed to raise a certain weight of water a certain number of degrees on the thermometric scale. There are several units in use, but the one which will be employed throughout this book is the "Large Calorie"; a large calorie being the amount of heat absorbed in raising 1 kilogramme of water 1° C. Referring for a moment to what has been said about specific heats, it will be apparent that if 1 large calorie is sufficient to heat 1 kilo, of water through 1° C. the same quantity will heat 1 kilo. of steel, whose specific heat is roughly 0.11, through (10/011) = 9° C., or, which comes to the same thing, will heat 9 kilos, of steel through 1° C.; and similarly, 1 large calorie will raise 4 kilos. of calcium carbide 1° C. in temperature, or 1 kilo. 4° C. The fact that a definite quantity of heat is manifested when a known weight of calcium carbide is decomposed by water is only typical; for in every chemical process some disturbance of heat, though not necessarily of sensible (or thermometric) character, occurs, heat being either absorbed or set free. Moreover, if when given weights of two or more substances unite to form a given weight of another substance, a certain quantity of heat is set free, precisely the same amount of heat is absorbed, or disappears, when the latter substance is decomposed to form the same quantities of the original substances; and, per contra, if the combination is attended by a disappearance of heat, exactly the same amount is liberated when the compound is broken up into its first constituents. Compounds are therefore of two kinds: those which absorb heat during their preparation, and consequently liberate heat when they are decomposed--such being termed endothermic; and those which evolve heat during their preparation, and consequently absorb heat when they are decomposed--such being called exothermic. If a substance absorbs heat during its formation, it cannot be produced unless that heat is supplied to it; and since heat, being a form of motion, is equally a form of energy, energy must be supplied, or work must be done, before that substance can be obtained. Conversely, if a substance evolves heat during its formation, its component parts evolve energy when the said substance is being produced; and therefore the mere act of combination is accompanied by a facility for doing work, which work may be applied in assisting some other reaction that requires heat, or may be usefully employed in any other fashion, or wasted if necessary. Seeing that there is a tendency in nature for the steady dissipation of energy, it follows that an exothermic substance is stable, for it tends to remain as it is unless heat is supplied to it, or work is done upon it; whereas, according to its degree of endothermicity, an endothermic substance is more or less unstable, for it is always ready to emit heat, or to do work, as soon as an opportunity is given to it to decompose. The theoretical and practical results of this circumstance will be elaborated in Chapter VI., when the endothermic nature of acetylene is more fully discussed.

A very simple experiment will show that a notable quantity of heat is set free when calcium carbide is brought into contact with water, and by arranging the details of the apparatus in a suitable manner, the quantity of heat manifested may be measured with considerable accuracy. A lengthy description of the method of performing this operation, however, scarcely comes within the province of the present book, and it must be sufficient to say that the heat is estimated by decomposing a known weight of carbide by means of water in a small vessel surrounded on all sides by a carefully jacketed receptacle full of water and provided with a sensitive thermometer. The quantity of water contained in the outer vessel being known, and its temperature having been noted before the reaction commences, an observation of the thermometer after the decomposition is finished, and when the mercury has reached its highest point, gives data which show that the reaction between water and a known weight of calcium carbide produces heat sufficient in amount to raise a known weight of water through a known thermometric distance; and from these figures the corresponding number of large calories may easily be calculated. A determination of this quantity of heat has been made experimentally by several investigators, including Lewes, who has found that the heat evolved on decomposing 1 gramme of ordinary commercial carbide with water is 0.406 large calorie. [Footnote: Lewes returns his result as 406 calories, because he employs the "small calorie." The small calorie is the quantity of heat needed to raise 1 gramme of water 1° C.; but as there are 1000 grammes in 1 kilogramme, the large calorie is equal to 1000 small calories. In many respects the former unit is to be preferred.] As the material operated upon contained only 91.3 per cent. of true calcium carbide, he estimates the heat corresponding with the decomposition of 1 gramme of pure carbide to be 0.4446 large calorie. As, however, it is better, and more in accordance with modern practice, to quote such data in terms of the atomic or molecular weight of the substance concerned, and as the molecular weight of calcium carbide is 64, it is preferable to multiply these figures by 64, stating that, according to Lewes' researches, the heat of decomposition of "1 gramme- molecule" (i.e., 64 grammes) of a calcium carbide having a purity of 91.3 per cent. is just under 26 calories, or that of 1 gramme-molecule of pure carbide 28.454 calories. It is customary now to omit the phrase "one gramme-molecule" in giving similar figures, physicists saying simply that the heat of decomposition of calcium carbide by water when calcium hydroxide is the by-product, is 28.454 large calories.

Assuming all the necessary data known, as happens to be the case in the present instance, it is also possible to calculate theoretically the heat which should be evolved on decomposing calcium carbide by means of water. Equation (2), given on page 24, shows that of the substances taking part in the reaction 1 molecular weight of calcium carbide is decomposed, and 1 molecular weight of acetylene is formed. Of the two molecules of water, only one is decomposed, the other passing to the calcium hydroxide unchanged; and the 1 molecule of calcium hydroxide is formed by the combination of 1 atom of free calcium, 1 atom of free oxygen, and 1 molecule of water already existing as such. Calcium hydroxide and water are both exothermic substances, absorbing heat when they are decomposed, liberating it when they are formed. Acetylene is endothermic, liberating heat when it is decomposed, absorbing it when it is produced. Unfortunately there is still some doubt about the heat of formation of calcium carbide, De Forcrand returning it as -0.65 calorie, and Gin as +3.9 calories. De Forcrand's figure means, as before explained, that 64 grammes of carbide should absorb 0.65 large calorie when they are produced by the combination of 40 grammes of calcium with 24 grammes of carbon; the minus sign calling attention to the belief that calcium carbide is endothermic, heat being liberated when it suffers decomposition. On the contrary, Gin's figure expresses the idea that calcium carbide is exothermic, liberating 3.9 calories when it is produced, and absorbing them when it is decomposed. In the absence of corroborative evidence one way or the other, Gin's determination will be accepted for the ensuing calculation. In equation (2), therefore, calcium carbide is decomposed and absorbs heat; water is decomposed and absorbs heat; acetylene is produced and absorbs heat; and calcium hydroxide is produced liberating heat. On consulting the tables of thermo-chemical data given in the various text-books on physical chemistry, all the other constants needed for the present purpose will be found; and it will appear that the heat of formation of water is +69 calories, that of acetylene -58.1 calories, and that of calcium hydroxide, when 1 atom of calcium, 1 atom of oxygen, and 1 molecule of water unite together, is +160.1 calories. [Footnote: When 1 atom of calcium, 2 atoms of oxygen, and 2 atoms of hydrogen unite to form solid calcium hydroxide, the heat of formation of the latter is 229.1 (cf. infra). This value is simply 160.1 + 69.0 = 229.1; 69.0 being the heat of formation of water.] Collecting the results into the form of a balance-sheet, the effect of decomposing calcium carbide with water is this:

Therefore when 64 grammes of calcium carbide are decomposed by water, or when 18 grammes of water are decomposed by calcium carbide (the by- product in each case being calcium hydroxide or slaked lime, for the formation of which a further 18 grammes of water must be present in the second instance), 29.1 large calories are set free. It is not possible yet to determine thermo-chemical data with extreme accuracy, especially on such a material as calcium carbide, which is hardly to be procured in a state of chemical purity; and so the value 28.454 calories experimentally found by Lewes agrees very satisfactorily, considering all things, with the calculated value 29.1 calories. It is to be noticed, however, that the above calculated value has been deduced on the assumption that the calcium hydroxide is obtained as a dry powder; but as slaked lime is somewhat soluble in water, and as it evolves 3 calories in so dissolving, if sufficient water is present to take up the calcium hydroxide entirely into the liquid form (i.e., that of a solution), the amount of heat set free will be greater by those 3 calories, i.e., 32.1 large calories altogether.

THE PROCESS OF GENERATION.--Taking 28 as the number of large calories developed when 64 grammes of ordinary commercial calcium carbide are decomposed with sufficient water to leave dry solid calcium hydroxide as the by-product in acetylene generation, this quantity of heat is capable of exerting any of the following effects. It is sufficient (1) to raise 1000 grammes of water through 28° C., say from 10° C. (50° F., which is roughly the temperature of ordinary cold water) to 38° C. It is sufficient (2) to raise 64 grammes of water (a weight equal to that of the carbide decomposed) through 438° C., if that were possible. It would raise (3) 311 grammes of water through 90° C., i.e., from 10° C. to the boiling-point. If, however, instead of remaining in the liquid state, the water were converted into vapour, the same quantity of heat would suffice (4) to change 44.7 grammes of water at 10° C. into steam at 100° C.; or (5) to change 46.7 grammes of water at 10° C. into vapour at the same temperature. It is an action of the last character which takes place in acetylene generators of the most modern and usual pattern, some of the surplus water being evaporated and carried away as vapour at a comparatively low temperature with the escaping gas; for it must be remembered that although steam, as such, condenses into liquid water immediately the surrounding temperature falls below 100° C., the vapour of water remains uncondensed, even at temperatures below the freezing- point, when that vapour is distributed among some permanent gas--the precise quantity of vapour so remaining being a function of the temperature and barometric height. Thus it appears that if the heat evolved during the decomposition of calcium carbide is not otherwise consumed, it is sufficient in amount to vaporise almost exactly 3 parts by weight of water for every 4 parts of carbide attacked; but if it were expended upon some substance such as acetylene, calcium carbide, or steel, which, unlike water, could not absorb an extra amount by changing its physical state (from solid to liquid, or from liquid to gas), the heat generated during the decomposition of a given weight of carbide would suffice to raise an equal weight of the particular substance under consideration to a temperature vastly exceeding 438° C. The temperature attained, indeed, measured in Centigrade degrees, would be 438 multiplied by the quotient obtained on dividing the specific heat of water by the specific heat of the substance considered: which quotient, obviously, is the "reciprocal" of the specific heat of the said substance.

The analogy to the combustion of coal mentioned on a previous page shows that although the quantity of heat evolved during a certain chemical reaction is strictly fixed, the temperature attained is dependent on the time over which the reaction is spread, being higher as the process is more rapid. This is due to the fact that throughout the whole period of reaction heat is escaping from the mass, and passing into the atmosphere at a fairly constant speed; so that, clearly, the more slowly heat is produced, the better opportunity has it to pass away, and the less of it is left to collect in the material under consideration. During the action of an acetylene generator, there is a current of gas constantly travelling away from the carbide, there is vapour of water constantly escaping with the gas, there are the walls of the generator itself constantly exposed to the cooling action of the atmosphere, and there is either a mass of calcium carbide or of water within the generator. It is essential for good working that the temperature of both the acetylene and the carbide shall be prevented from rising to any noteworthy extent; while the amount of heat capable of being dissipated into the air through the walls of the apparatus in a given time is narrowly limited, depending upon the size and shape of the generator, and the temperature of the surrounding air. If, then, a small, suitably designed generator is working quite slowly, the loss of heat through the external walls of the apparatus may easily be rapid enough to prevent the internal temperature from rising objectionably high; but the larger the generator, and the more rapidly it is evolving gas, the less does this become possible. Since of the substances in or about a generator water is the one which has by far the largest capacity for absorbing heat, and since it is the only substance to which any necessary quantity of heat can be safely or conveniently transmitted, it follows that the larger in size an acetylene generator is, or the more rapidly that generator is made to deliver gas, the more desirable is it to use water as the means for dissipating the surplus heat, and the more necessary is it to employ an apparatus in which water is in large chemical excess at the actual place of decomposition.

The argument is sometimes advanced that an acetylene generator containing carbide in excess will work satisfactorily without exhibiting an undesirable rise in internal temperature, if the vessel holding the carbide is merely surrounded by a large quantity of cold water. The idea is that the heat evolved in that particular portion of the charge which is suffering decomposition will be communicated with sufficient speed throughout the whole mass of calcium carbide present, whence it will pass through the walls of the containing vessel into the water all round. Provided the generator is quite small, provided the carbide container is so constructed as to possess the maximum of superficial area with the minimum of cubical capacity (a geometrical form to which the sphere, and in one direction the cylinder, are diametrically opposed), and provided the walls of the container do not become coated internally or externally with a coating of lime or water scale so as to diminish in heat- transmitting power, an apparatus designed in the manner indicated is undoubtedly free from grave objection; but immediately any of those provisions is neglected, trouble is likely to ensue, for the heat will not disappear from the place of actual reaction at the necessary speed. Apparent proof that heat is not accumulating unduly in a water-jacketed carbide container even when the generator is evolving gas at a fair speed is easy to obtain; for if, as usually happens, the end of the container through which the carbide is inserted is exposed to the air, the hand may be placed upon it, and it will be found to be only slightly warm to the touch. Such a test, however, is inconclusive, and frequently misleading, because if more than a pound or two of carbide is present as an undivided mass, and if water is allowed to attack one portion of it, that particular portion may attain a high temperature while the rest is comparatively cool: and if the bulk of the carbide is comparatively cool, naturally the walls of the containing vessel themselves remain practically unheated. Three causes work together to prevent this heat being dissipated through the walls of the carbide vessel with sufficient rapidity. In the first place, calcium carbide itself is a very bad conductor of heat. So deficient in heat-conducting power is it that a lump a few inches in diameter may be raised to redness in a gas flame at one spot, and kept hot for some minutes, while the rest of the mass remains sufficiently cool to be held comfortably in the fingers. In the second place, commercial carbide exists in masses of highly irregular shape, so that when they are packed into any vessel they only touch at their angles and edges; and accordingly, even if the material were a fairly good heat conductor of itself, the air or gas present between each lump would act as an insulator, protecting the second piece from the heat generated in the first. In the third place, the calcium hydroxide produced as the by-product when calcium carbide is decomposed by water occupies considerably more space than the original carbide--usually two or three times as much space, the exact figures depending upon the conditions in which it is formed--and therefore a carbide container cannot advisedly be charged with more than one-third the quantity of solid which it is apparently capable of holding. The remaining two-thirds of the space is naturally full of air when the container is first put into the generator, but the air is displaced by acetylene as soon as gas production begins. Whether that space, however, is occupied by air, by acetylene, or by a gradually growing loose mass of slaked lime, each separate lump of hot carbide is isolated from its neighbours by a material which is also a very bad heat conductor; and the heat has but little opportunity of distributing itself evenly. Moreover, although iron or steel is a notably better conductor of heat than any of the other substances present in the carbide vessel, it is, as a metal, only a poor conductor, being considerably inferior in this respect to copper. If heat dissipation were the only point to be studied in the construction of an acetylene apparatus, far better results might be obtained by the employment of copper for the walls of the carbide container; and possibly in that case a generator of considerable size, fitted with a water- jacketed decomposing vessel, might be free from the trouble of overheating. Nevertheless it will be seen in Chapter VI. that the use of copper is not permissible for such purposes, its advantages as a good conductor of heat being neutralised by its more important defects.

When suitable precautions are not taken to remove the heat liberated in an acetylene apparatus, the temperature of the calcium carbide occasionally rises to a remarkable degree. Investigating this point, Caro has studied the phenomena of heat production in a "dipping" generator-- i.e., an apparatus in which a cage of carbide is alternately immersed in and lifted out of a vessel containing water. Using a generator designed to supply five burners, he has found a maximum recording thermometer placed in the gas space of the apparatus to give readings generally between 60° and 100° C.; but in two tests out of ten he obtained temperatures of about 160° C. To determine the actual temperature of the calcium carbide itself, he scattered amongst the carbide charge fragments of different fusible metallic alloys which were known to melt or soften at certain different temperatures. In all his ten tests the alloys melting at 120° C. were fused completely; in two tests other alloys melting at 216° and 240° C. showed signs of fusion; and in one test an alloy melting at 280° C. began to soften. Working with an experimental apparatus constructed on the "dripping" principle-- i.e., a generator in which water is allowed to fall in single drops or as a fine stream upon a mass of carbide--with the deliberate object of ascertaining the highest temperatures capable of production when calcium carbide is decomposed in this particular fashion, and employing for the measurement of the heat a Le Chatelier thermo-couple, with its sensitive wires lying among the carbide lumps, Lewes has observed a maximum temperature of 674° C. to be reached in 19 minutes when water was dripped upon 227 grammes of carbide at a speed of about 8 grammes per minute. In other experiments he used a laboratory apparatus designed upon the "dipping" principle, and found maximum temperatures, in four different trials, of 703°, 734°, 754°, and 807° C., which were reached in periods of time ranging from 12 to 17 minutes. Even allowing for the greater delicacy of the instrument adopted by Lewes for measuring the temperature in comparison with the device employed by Caro, there still remains an astonishing difference between Caro's maximum of 280° and Lewes' maximum of 807° C. The explanation of this discrepancy is to be inferred from what has just been said. The generator used by Caro was properly made of metal, was quite small in size, was properly designed with some skill to prevent overheating as much as possible, and was worked at the speed for which it was intended--in a word, it was as good an apparatus as could be made of this particular type. Lewes' generator was simply a piece of glass and metal, in which provisions to avoid overheating were absent; and therefore the wide difference between the temperatures noted does not suggest any inaccuracy of observation or experiment, but shows what can be done to assist in the dissipation of heat by careful arrangement of parts. The difference in temperature between the acetylene and the carbide in Caro's test accentuates the difficulty of gauging the heat in a carbide vessel by mere external touch, and supplies experimental proof of the previous assertions as to the low heat-conducting power of calcium carbide and of the gases of the decomposing vessel. It must not be supposed that temperatures such as Lewes has found ever occur in any commercial generator of reasonably good design and careful construction; they must be regarded rather as indications of what may happen in an acetylene apparatus when the phenomena accompanying the evolution of gas are not understood by the maker, and when all the precautions which can easily be taken to avoid excessive heating have been omitted, either by building a generator with carbide in excess too large in size, or by working it too rapidly, or more generally by adopting a system of construction unsuited to the ends in view. The fact, however, that Lewes has noted the production of a temperature of 807° C. is important; because this figure is appreciably above the point 780° C., at which acetylene decomposes into its elements in the absence of air.

Nevertheless the production of a temperature somewhat exceeding 100° C. among the lumps of carbide actually undergoing decomposition can hardly be avoided in any practical generator. Based on a suggestion in the "Report of the Committee on Acetylene Generators" which was issued by the British Home Office in 1902, Fouché has proposed that 130° C., as measured with the aid of fusible metallic rods, [Footnote: An alloy made by melting together 55 parts by weight of commercial bismuth and 45 parts of lead fuses at 127° C., and should be useful in performing the tests.] should be considered the maximum permissible temperature in any part of a generator working at full speed for a prolonged period of time. Fouché adopts this figure on the ground that 130° C. sensibly corresponds with the temperature at which a yellow substance is formed in a generator by a process of polymerisation; and, referring to French conditions, states that few actual apparatus permit the development of so high a temperature. As a matter of fact, however, a fairly high temperature among the carbide is less important than in the gas, and perhaps it would be better to say that the temperature in any part of a generator occupied by acetylene should not exceed 100° C. Fraenkel has carried out some experiments upon the temperature of the acetylene immediately after evolution in a water-to-carbide apparatus containing the carbide in a subdivided receptacle, using an apparatus now frequently described as belonging to the "drawer" system of construction. When a quantity of about 7 lb. of carbide was distributed between 7 different cells of the receptacle, each cell of which had a capacity of 25 fluid oz., and the apparatus was caused to develop acetylene at the rate of 7 cubic feet per hour, maximum thermometers placed immediately over the carbide in the different cells gave readings of from 70° to 90° C., the average maximum temperature being about 80° C. Hence the Austrian code of rules issued in 1905 governing the construction of acetylene apparatus contains a clause to the effect that the temperature in the gas space of a generator must never exceed 80° C.; whereas the corresponding Italian code contains a similar stipulation, but quotes the maximum temperature as 100° C. (vide Chapter IV.).

It is now necessary to see why the production of an excessively high temperature in an acetylene generator has to be avoided. It must be avoided, because whenever the temperature in the immediate neighbourhood of a mass of calcium carbide which is evolving acetylene under the attack of water rises materially above the boiling-point of water, one or more of three several objectionable effects is produced--(a) upon the gas generated, (b) upon the carbide decomposed, and (c) upon the general chemical reaction taking place.

It has been stated above that in moat generators when the action between the carbide and the water is proceeding smoothly, it occurs according to equation (2)--

(2) CaC_2 + 2H_2O = C_2H_2 + Ca(OH)_2

rather than in accordance with equation (1)--

(1) CaC_2 + H_2O = C_2H_2 + CaO.

This is because calcium oxide, or quicklime, the by-product in (1), has considerable affinity for water, evolving a noteworthy quantity of heat when it combines with one molecule of water to form one molecule of calcium hydroxide, or slaked lime, the by-product in (2). If, then, a small amount of water is added to a large amount of calcium carbide, the corresponding quantity of acetylene may be liberated on the lines of equation (1), and there will remain behind a mixture of unaltered calcium carbide, together with a certain amount of calcium oxide. Inasmuch as both these substances possess an affinity for water (setting heat free when they combine with it), when a further limited amount of water is introduced into the mixture some of it will probably be attracted to the oxide instead of to the carbide present. It is well known that at ordinary temperatures quicklime absorbs moisture, or combines with water, to produce slaked lime; but it is equally well known that in a furnace, at about a red heat, slaked lime gives up water and changes into quicklime. The reaction, in fact, between calcium oxide and water is reversible, and whether those substances combine or dissociate is simply a question of temperature. In other words, as the temperature rises, the heat of hydration of calcium oxide diminishes, and calcium hydroxide becomes constantly a less stable material. If now it should happen that the affinity between calcium carbide and water should not diminish, or should diminish in a lower ratio than the affinity between calcium oxide and water as the temperature of the mass rises from one cause or other, it is conceivable that at a certain temperature calcium carbide might be capable of withdrawing the water of hydration from the molecule of slaked lime, converting the latter into quicklime, and liberating one molecule of acetylene, thus--

(3) CaC_2 + Ca_2(OH) = C_2H_2 + 2CaO.

It has been proved that a reaction of this character does occur, the temperature necessary to determine it being given by Lewes as from 420° to 430° C., which is not much more than half that which he found in a generator having carbide in excess, albeit one of extremely bad design. Treating this reaction in the manner previously adopted, the thermo- chemical phenomena of equation (3) are:

[1 Footnote: Into its elements, Ca, O_2, and H_2; cf. footnote, p: 31.]

Or, since the calcium hydroxide is only dehydrated without being entirely decomposed, and only one molecule of water is broken up, it may be written:

which comes to the same thing. Putting the matter in another shape, it may be said that the reaction between calcium carbide and water is exothermic, evolving either 14.0 or 29.1 calories according as the byproduct is calcium oxide or solid calcium hydroxide; and therefore either reaction proceeds without external assistance in the cold. The reaction between carbide and slaked lime, however, is endothermic, absorbing 1.1 calories; and therefore it requires external assistance (presence of an elevated temperature) to start it, or continuous introduction of heat (as from the reaction between the rest of the carbide present and the water) to cause it to proceed. Of itself, and were it not for the disadvantages attending the production of a temperature remotely approaching 400° C. in an acetylene generator, which disadvantages will be explained in the following paragraphs, there is no particular reason why reaction (3) should not be permitted to occur, for it involves (theoretically) no loss of acetylene, and no waste of calcium carbide. Only one specific feature of the reaction has to be remembered, and due practical allowance made for it. The reaction represented by equation (2) proceeds almost instantaneously when the calcium carbide is of ordinarily good quality, and the acetylene resulting therefrom is wholly generated within a very few minutes. Equation (3), on the contrary, consumes much time for its completion, and the gas corresponding with it is evolved at a gradually diminishing speed which may cause the reaction to continue for hours--a circumstance that may be highly inconvenient or quite immaterial according to the design of the apparatus. When, however, it is desired to construct an automatic acetylene generator, i.e., an apparatus in which the quantity of gas liberated has to be controlled to suit the requirements of any indefinite number of burners in use on different occasions, equation (3) becomes a very important factor in the case. To determine the normal reaction (No. 2) of an acetylene generator, 64 parts by weight of calcium carbide must react with 36 parts of water to yield 26 parts by weight of acetylene, and apparently both carbide and water are entirely consumed; but if opportunity is given for the occurrence of reaction (3), another 64 parts by weight of carbide may be attacked, without the addition of any more water, producing, inevitably, another 26 parts of acetylene. If, then, water is in chemical excess in the generator, all the calcium carbide present will be decomposed according to equation (2), and the action will take place without delay; after a few minutes' interval the whole of the acetylene capable of liberation will have been evolved, and nothing further can possibly happen until another charge of carbide is inserted in the apparatus. If, on the other hand, calcium carbide is in chemical excess in the generator, all the water run in will be consumed according to equation (2), and this action will again take place without delay; but unless the temperature of the residual carbide has been kept well below 400° C., a further evolution of gas will occur which will not cease for an indeterminate period of time, and which, by strict theory, given the necessary conditions, might continue until a second volume of acetylene equal to that liberated at first had been set free. In practice this phenomenon of a secondary production of gas, which is known as "after-generation," is regularly met with in all generators where the carbide is in excess of the water added; but the amount of acetylene so evolved rarely exceeds one-quarter or one-third of the main make. The actual amount evolved and the rate of evolution depend, not merely upon the quantity of undecomposed carbide still remaining in contact with the damp lime, but also upon the rapidity with which carbide naturally decomposes in presence of liquid water, and the size of the lumps. Where "after-generation" is caused by the ascent of water vapour round lumps of carbide, the volume of gas produced in a given interval of time is largely governed by the temperature prevailing and the shape of the apparatus. It is evident that even copious "after-generation" is a matter of no consequence in any generator provided with a holder to store the gas, assuming that by some trustworthy device the addition of water is stopped by the time that the holder is two-thirds or three-quarters full. In the absence of a holder, or if the holder fitted is too small to serve its proper purpose, "aftergeneration" is extremely troublesome and sometimes dangerous, but a full discussion of this subject must be postponed to the next chapter.

EFFECT OF HEAT ON ACETYLENE.--The effect of excessive retention of heat in an acetylene generator upon the gas itself is very marked, as acetylene begins spontaneously to suffer change, and to be converted into other compounds at elevated temperatures. Being a purely chemical phenomenon, the behaviour of acetylene when exposed to heat will be fully discussed in Chapter VI. when the properties of the gas are being systematically dealt with. Here it will be sufficient to assume that the character of the changes taking place is understood, and only the practical results of those changes as affecting the various components of an acetylene installation have to be studied. According to Lewes, acetylene commences to "polymerise" at a temperature of about 600° C., when it is converted into other hydrocarbons having the same percentage composition, but containing more atoms of carbon and hydrogen in their molecules. The formula of acetylene is C_2H_2 which means that 2 atoms of carbon and 2 atoms of hydrogen unite to form 1 molecule of acetylene, a body evidently containing roughly 92.3 per cent. by weight of carbon and 7.7 per cent. by weight of hydrogen. One of the most noteworthy substances produced by the polymerisation of acetylene is benzene, the formula of which is C_6H_6, and this is formed in the manner indicated by the equation--

(4) 3C_2H_2 = C_6H_6.

Now benzene also contains 92.3 per cent. of carbon and 7.7 per cent. by weight of hydrogen in its composition, but its molecule contains 6 atoms of each element. When the chemical formula representing a compound body indicates a substance which is, or can be obtained as, a gas or vapour, it convoys another idea over and above those mentioned on a previous page. The formula "C_2H_2," for example, means 1 molecule, or 26 parts by weight of acetylene, just as "H_2" means 1 molecule, or 2 parts by weight of hydrogen; but both formulæ also mean equal parts by volume of the respective substances, and since H_2 must mean 2 volumes, being twice H, which is manifestly 1, C_2H_2 must mean 2 volumes of acetylene as well. Thus equation (4) states that 6 volumes of acetylene, or 3 x 26 parts by weight, unite to form 2 volumes of benzene, or 78 parts by weight. If these hydrocarbons are burnt in air, both are indifferently converted into carbon dioxide (carbonic acid gas) and water vapour; and, neglecting for the sake of simplicity the nitrogen of the atmosphere, the processes may be shown thus:

(5) 2C_2H_2 + 5O_2 = 4CO_2 + 2H_2O.

(6) 2C_6H_6 + 15O_2 = 12CO_2 + 6H_2O.

Equation (5) shows that 4 volumes of acetylene combine with 10 volumes of oxygen to produce 8 volumes of carbon dioxide and 4 of water vapour; while equation (6) indicates that 4 volumes of benzene combine with 30 volumes of oxygen to yield 24 volumes of carbon dioxide and 12 of water vapour. Two parts by volume of acetylene therefore require 5 parts by volume of oxygen for perfect combustion, whereas two parts by volume of benzene need 15--i.e., exactly three times as much. In order to work satisfactorily, and to develop the maximum of illuminating power from any kind of gas consumed, a gas-burner has to be designed with considerable skill so as to attract to the base of the flame precisely that volume of air which contains the quantity of oxygen necessary to insure complete combustion, for an excess of air in a flame is only less objectionable than a deficiency thereof. If, then, an acetylene burner is properly constructed, as most modern ones are, it draws into the flame air corresponding with two and a half volumes of oxygen for every one volume of acetylene passing from the jets; whereas if it were intended for the combustion of benzene vapour it would have to attract three times that quantity. Since any flame supplied with too little air tends to emit free carbon or soot, it follows that any well-made acetylene burner delivering a gas containing benzene vapour will yield a more or lens smoky flame according to the proportion of benzene in the acetylene. Moreover, at ordinary temperatures benzene is a liquid, for it boils at 81° C., and although, as was explained above in the case of water, it is capable of remaining in the state of vapour far below its boiling-point so long as it is suspended in a sufficiency of some permanent gas like acetylene, if the proportion of vapour in the gas at any given temperature exceeds a certain amount the excess will be precipitated in the liquid form; while as the temperature falls the proportion of vapour which can be retained in a given volume of gas also diminishes to a noteworthy extent. Should any liquid, be it water or benzene, or any other substance, separate from the acetylene under the influence of cold while the gas is passing through pipes, the liquid will run downwards to the lowest points in those pipes; and unless due precautions are taken, by the insertion of draw-off cocks, collecting wells, or the like, to withdraw the deposited water or other liquid, it will accumulate in all bends, angles, and dips till the pipes are partly or completely sealed against the passage of gas, and the lights will either "jump" or be extinguished altogether. In the specific case of an acetylene generator this trouble is very likely to arise, even when the gas is not heated sufficiently during evolution for polymerisation to occur and benzene or other liquid hydrocarbons to be formed, because any excess of water present in the decomposing vessel is liable to be vaporised by the heat of the reaction--as already stated it is desirable that water shall be so vaporised--and will remain safely vaporised as long as the pipes are kept warm inside or near the generator; but directly the pipes pass away from the hot generator the cooling action of the air begins, and some liquid water will be immediately produced. Like the phenomenon of after- generation, this equally inevitable phenomenon of water condensation will be either an inconvenience or source of positive danger, or will be a matter of no consequence whatever, simply as the whole acetylene installation, including the service-pipes, is ignorantly or intelligently built.

As long as nothing but pure polymerisation happens to the acetylene, as long, that is to say, as it is merely converted into other hydrocarbons also having the general formula C_(2n)H_(2n), no harm will be done to the gas as regards illuminating power, for benzene burns with a still more luminous flame than acetylene itself; nor will any injury result to the gas if it is required for combustion in heating or cooking stoves beyond the fact that the burners, luminous or atmospheric, will be delivering a material for the consumption of which they are not properly designed. But if the temperature should rise much above the point at which benzene is the most conspicuous product of polymerisation, other far more complicated changes occur, and harmful effects may be produced in two separate ways. Some of the new hydrocarbons formed may interact to yield a mixture of one or more other hydrocarbons containing a higher proportion of carbon than that which is present in acetylene and benzene, together with a corresponding proportion of free hydrogen; the former will probably be either liquids or solids, while the latter burns with a perfectly non-luminous flame. Thus the quantity of gas evolved from the carbide and passed into the holder is less than it should be, owing to the condensation of its non-gaseous constituents. To quote an instance of this, Haber has found 15 litres of acetylene to be reduced in volume to 10 litres when the gas was heated to 638° C. By other changes, some "saturated hydrocarbons," i.e., bodies having the general formula C_nH_(2n+2), of which methane or marsh-gas, CH_4 is the best known, may be produced; and those all possess lower illuminating powers than acetylene. In two of those experiments already described, where Lewes observed maximum temperatures ranging from 703° to 807° C., samples of the gas which issued when the heat was greatest were submitted to chemical analysis, and their illuminating powers were determined. The figures he gives are as follows:

I. II.
Per Cent. Per Cent.
Acetylene 70.0 69.7
Saturated hydrocarbons 11.3 11.4
Hydrogen 18.7 18.9
_____ _____
100.0 100.0

The average illuminating power of these mixed gases is about 126 candles per 5 cubic feet, whereas that of pure acetylene burnt under good laboratory conditions is 240 candles per 5 cubic feet. The product, it will be seen, had lost almost exactly 50 per cent. of its value as an illuminant, owing to the excessive heating to which it had been, exposed. Some of the liquid hydrocarbons formed at the same time are not limpid fluids like benzene, which is less viscous than water, but are thick oily substances, or even tars. They therefore tend to block the tubes of the apparatus with great persistence, while the tar adheres to the calcium carbide and causes its further attack by water to be very irregular, or even altogether impossible. In some of the very badly designed generators of a few years back this tarry matter was distinctly visible when the apparatus was disconnected for recharging, for the spent carbide was exceptionally yellow, brown, or blackish in colour, [Footnote: As will be pointed out later, the colour of the spent lime cannot always be employed as a means for judging whether overheating has occurred in a generator.] and the odour of tar was as noticeable as that of crude acetylene.

There is another effect of heat upon acetylene, more calculated to be dangerous than any of those just mentioned, which must not be lost sight of. Being an endothermic substance, acetylene is prone to decompose into its elements--

(7) C_2H_2 -> C_2 + H_2

whenever it has the opportunity; and the opportunity arrives if the temperature of the gas risen to 780° C., or if the pressure under which the gas is stored exceeds two atmospheres absolute (roughly 30 lb. per square inch). It decomposes, be it carefully understood, in the complete absence of air, directly the smallest spark of red-hot material or of electricity, or directly a gentle shock, such as that of a fall or blow on the vessel holding it, is applied to any volume of acetylene existing at a temperature exceeding 780° or at a gross pressure of 30 lb. per square inch; and however large that volume may be, unless it is contained in tubes of very small diameter, as will appear hereafter, the decomposition or dissociation into its elements will extend throughout the whole of the gas. Equation (7) states that 2 volumes of acetylene yield 2 volumes of hydrogen and a quantity of carbon which would measure 2 volumes were it obtained in the state of gas, but which, being a solid, occupies a space that may be neglected. Apparently, therefore, the dissociation of acetylene involves no alteration in volume, and should not exhibit explosive effects. This is erroneous, because 2 volumes of acetylene only yield exactly 2 volumes of hydrogen when both gases are measured at the same temperature, and all gases increase in volume as their temperature rises. As acetylene is endothermic and evolves much heat on decomposition, and as that heat must primarily be communicated to the hydrogen, it follows that the latter must be much hotter than the original acetylene; the hydrogen accordingly strives to fill a much larger space than that occupied by the undecomposed gas, and if that gas is contained in a closed vessel, considerable internal pressure will be set up, which may or may not cause the vessel to burst.

What has been said in the preceding paragraph about the temperature at which acetylene decomposes is only true when the gas is free from any notable quantity of air. In presence of air, acetylene inflames at a much lower temperature, viz., 480° C. In a manner precisely similar to that of all other combustible gases, if a stream of acetylene issues into the atmosphere, as from the orifices of a burner, the gas catches fire and burns quietly directly any substance having a temperature of 480° C. or upwards is brought near it; but if acetylene in bulk is mixed with the necessary quantity of air to support combustion, and any object exceeding 480° C. in temperature comes in contact with it, the oxidation of the hydrocarbon proceeds at such a high rate of speed as to be termed an explosion. The proportion of air needed to support combustion varies with every combustible material within known limits (cf. Chapter VI.), and according to Eitner the smallest quantity of air required to make acetylene burn or explode, as the case may be, is 25 per cent. If, by ignorant design or by careless manipulation, the first batches of acetylene evolved from a freshly charged generator should contain more than 25 per cent. of air; or if in the inauguration of a new installation the air should not be swept out of the pipes, and the first makes of gas should become diluted with 25 to 50 per cent. of air, any glowing body whose temperature exceeds 480° C. will fire the gas; and, as in the former instance, the flame will extend all through the mass of acetylene with disastrous violence and at enormous speed unless the gas is stored in narrow pipes of extremely small diameter. Three practical lessons are to be learnt from this circumstance: first, tobacco-smoking must never be permitted in any building where an escape of raw acetylene is possible, because the temperature of a lighted cigar, &c., exceeds 480° C.; secondly, a light must never be applied to a pipe delivering acetylene until a proper acetylene burner has been screwed into the aperture; thirdly, if any appreciable amount of acetylene is present in the air, no operation should be performed upon any portion of an acetylene plant which involves such processes as scraping or chipping with the aid of a steel tool or shovel. If, for example, the iron or stoneware sludge-pipe is choked, or the interior of the dismantled generator is blocked, and attempts are made to remove the obstruction with a hard steel tool, a spark is very likely to be formed which, granting the existence of sufficient acetylene in the air, is perfectly able to fire the gas. For all such purposes wooden implements only are best employed; but the remark does not apply to the hand-charging of a carbide-to-water generator through its proper shoot. Before passing to another subject, it may be remarked that a quantity of air far less than that which causes acetylene to become dangerous is objectionable, as its presence is apt to reduce the illuminating power of the gas unduly.

EFFECT OF HEAT ON CARBIDE.--Chemically speaking, no amount of heat possible of attainment in the worst acetylene generator can affect calcium carbide in the slightest degree, because that substance may be raised to almost any temperature short of those distinguishing the electric furnace, without suffering any change or deterioration. In the absence of water, calcium carbide is as inert a substance as can well be imagined: it cannot be made to catch fire, for it is absolutely incombustible, and it can be heated in any ordinary flame for reasonable periods of time, or thrown into any non-electrical furnace without suffering in the least. But in presence of water, or of any liquid containing water, matters are different. If the temperature of an acetylene generator rises to such an extent that part of the gas is polymerised into tar, that tar naturally tends to coat the residual carbide lumps, and, being greasy in character, more or less completely protects the interior from further attack. Action of this nature not only means that the acetylene is diminished in quantity and quality by partial decomposition, but it also means that the make is smaller owing to imperfect decomposition of the carbide: while over and above this is the liability to nuisance or danger when a mass of solid residue containing some unaltered calcium carbide is removed from the apparatus and thrown away. In fact, whenever the residue of a generator is not so saturated with excess of water as to be of a creamy consistency, it should be put into an uncovered vessel in the open air, and treated with some ten times its volume of water before being run into any drain or closed pipe where an accumulation of acetylene may occur. Even at temperatures far below those needed to determine a production of tar or an oily coating on the carbide, if water attacks an excess of calcium carbide somewhat rapidly, there is a marked tendency for the carbide to be "baked" by the heat produced; the slaked lime adhering to the lumps as a hard skin which greatly retards the penetration of more water to the interior.

COLOUR OF SPENT CARBIDE.--In the early days of the industry, it was frequently taken for granted that any degradation in the colour of the spent lime left in an acetylene generator was proof that overheating had taken place during the decomposition of the carbide. Since both calcium oxide and hydroxide are white substances, it was thought that a brownish, greyish, or blackish residue must necessarily point to incipient polymerisation of the gas. This view would be correct if calcium carbide were prepared in a state of chemical purity, for it also is a white body. Commercial carbide, however, is not pure; it usually contains some foreign matter which tints the residue remaining after gasification. When a manufacturer strives to give his carbide the highest gas-making power possible he frequently increases the proportion of carbon in the charge submitted to electric smelting, until a small excess is reached, which remains in the free state amongst the finished carbide. After decomposition the fine particles of carbon stain the moist lime a bluish grey tint, the depth of shade manifestly depending upon the amount present. If such a sludge is copiously diluted with water, particles of carbon having the appearance and gritty or flaky nature of coke often rise to the surface or fall to the bottom of the liquid; whence they can easily be picked out and identified as pure or impure carbon by simple tests. Similarly the lime or carbon put into the electric furnace may contain small quantities of compounds which are naturally coloured; and which, reappearing in the sludge either in their original or in a different state of combination, confer upon the sludge their characteristic tinge. Spent lime of a yellowish brown colour is frequently to be met with in circumstances that are clearly no reproach to the generator. Doubtless the tint is due to the presence of some coloured metallic oxide or other compound which has escaped reduction in the electric furnace. The colour which the residual lime afterwards assumes may not be noticeable in the dry carbide before decomposition, either because some change in the colour-giving impurity takes place during the chemical reactions in the generator or because the tint is simply masked by the greyish white of the carbide and its free carbon. Hence it follows that a bad colour in the waste lime removed from a generator only points to overheating and polymerisation of the acetylene when corroborative evidence is obtained--such as a distinct tarry smell, the actual discovery of oily or tarry matters elsewhere, or a grave reduction in the illuminating power of the gas.

MAXIMUM ATTAINABLE TEMPERATURES.--In order to discover the maximum temperature which can be reached in or about an acetylene generator when an apparatus belonging to one of the best types is fed at a proper rate with calcium carbide in lumps of the most suitable size, the following calculation may be made. In the first place, it will be assumed that no loss of heat by radiation occurs from the walls of the generator; secondly, the small quantity of heat taken up by the calcium hydroxide produced will be ignored; and, thirdly, the specific heat of acetylene will be assumed to be 0.25, which is about its most probable value. Now, a hand-fed carbide-to-water generator will work with half a gallon of water for every 1 lb. of carbide decomposed, quantities which correspond with 320 grammes of water per 64 grammes (1 molecular weight) of carbide. Of those 320 grammes of water, 18 are chemically destroyed, leaving 302. The decomposition of 64 grammes of commercial carbide evolves 28 large calories of heat. Assuming all the heat to be absorbed by the water, 28 calories would raise 302 grammes through (28 X 1000 / 302) = 93° C., i.e., from 44.6° F. to the boiling-point. Assuming all the heat to be communicated to the acetylene, those 28 calories would raise the 26 grammes of gas liberated through (28 X 1000 / 26 / 0.25) = 4308° C., if that were possible. But if, as would actually be the case, the heat were distributed uniformly amongst the 302 grammes of water and the 20 grammes of acetylene, both gas and water would be raised through the same number of degrees, viz., 90.8° C. [Footnote: Let x = the number of large calories absorbed by the water; then 28 - x = those taken up by the gas. Then--

1000x / 302 = 1000 (28 - x) / (26 X 0.25)

whence x = 27.41; and 28 - x = 0.59.

Therefore, for water, the rise in temperature is--

27.41 X 1000 / 302 = 90.8° C.;

and for acetylene the rise is--

0.59 X 1000 / 26 / 0.25 = 90.8° C.]

If the generator were designed on lines to satisfy the United States Fire Underwriters, it would contain 8.33 lb. of water to every 1 lb. of carbide attacked; identical calculations then showing that the original temperature of the water and gas would be raised through 53.7° C. Provided the carbide is not charged into such an apparatus in lumps of too large a size, nor at too high a rate, there will be no appreciable amount of local overheating developed; and nowhere, therefore, will the rise in temperature exceed 91° in the first instance, or 54° C. in the second. Indeed it will be considerably smaller than this, because a large proportion of the heat evolved will be lost by radiation through the generator walls, while another portion will be converted from sensible into latent heat by causing part of the water to pass off as vapour with the acetylene.

EFFECT OF HIGH TEMPERATURES ON GENERATORS.--As the temperature amongst the carbide in any generator in which water is not present in large excess may easily reach 200° C. or upwards, no material ought to be employed in the construction of such generators which is not competent to withstand a considerable amount of heat in perfect safety. The ordinary varieties of soft solder applied with the bitt in all kinds of light metal-work usually melt, according to their composition, at about 180° C.; and therefore this method of making joints is only suitable for objects that are never raised appreciably in temperature above the boiling-point of water. No joint in an acetylene generator, the partial or complete failure of which would radically affect the behaviour of the apparatus, by permitting the charges of carbide and of water to come into contact at an abnormal rate of speed, by allowing the acetylene to escape directly through the crack into the atmosphere, or by enabling the water to run out of the seal of any vessel containing gas so as to set up a free communication between that vessel and the air, ought ever to be made of soft solder--every joint of this character should be constructed either by riveting, by bolting, or by doubly folding the metal sheets. Apparently, a joint constantly immersed in water on one side cannot rise in temperature above the boiling-point of the liquid, even when its other side is heated strongly; but since, even if a generator is not charged with naturally hard water, its fluid contents soon become "hard" by dissolution of lime, there is always a liability to the deposition of water scale over the joint. Such water scale is a very bad heat conductor, as is seen in steam boilers, so that a seam coated with an exceedingly thin layer of scale, and heated sharply on one side, will rise above the boiling-point of water even if the liquid on its opposite side is ice-cold. For a while the film of scale may be quite water-tight, but after it has been heated by contact with the hot metal several times it becomes brittle and cracks without warning. But there is a more important reason for avoiding the use of plumbers' solder. It might seem that as the natural hard, protective skin of the metal is liable to be injured or removed by the bending or by the drilling or punching which precedes the insertion of the rivets or studs, an application of soft solder to such a joint should be advantageous. This is not true because of the influence of galvanic action. As all soft solders consist largely of lead, if a joint is soldered, a "galvanic couple" of lead and iron, or of lead and zinc (when the apparatus is built of galvanised steel), is exposed to the liquid bathing it; and since in both cases the lead is highly electro-negative to the iron or zinc, it is the iron or zinc which suffers attack, assuming the liquid to possess any corrosive properties whatever. Galvanised iron which has been injured during the joint-making presents a zinc-iron couple to the water, but the zinc protects the iron; if a lead solder is present, the iron will begin to corrode immediately the zinc has disappeared. In the absence of lead it is the less important metal, but in the presence of lead it is the more important (the foundation) metal which is the soluble element of the couple. Where practicable, joints in an acetylene generator may safely be made by welding or by autogenous soldering ("burning"), because no other metal is introduced into the system; any other process, except that of riveting or folding, only hastens destruction of the plant. The ideal method of making joints about an acetylene generator is manifestly that of autogenous soldering, because, as will appear in Chapter IX. of this book, the most convenient and efficient apparatus for performing the operation is the oxy-acetylene blow-pipe, which can be employed so as to convert two separate pieces of similar metal into one homogeneous whole.

In less critical situations in an acetylene plant, such as the partitions of a carbide container, &c., where the collapse of the seam or joint would not be followed by any of the effects previously suggested, there is less cause for prohibiting the use of unfortified solder; but even here, two or three rivets, just sufficient to hold the metal in position if the solder should give way, are advisedly put into all apparatus. In other portions of an acetylene installation where a merely soldered joint is exposed to warm damp gas which is in process of cooling, instead of being bathed in hard water, an equal, though totally dissimilar, danger is courted. The main constituent of such solders that are capable of being applied with the bitt is lead; lead is distinctly soluble in soft or pure water; and the water which separates by condensation out of a warm damp gas is absolutely soft, for it has been distilled. If condensation takes place at or near a soldered joint in such a way that water trickles over the solder, by slow degrees the metallic lead will be dissolved and removed, and eventually a time will come when the joint is no longer tight to gas. In fact, if an acetylene installation is of more than very small dimensions, e.g., when it is intended to supply any building as large as, or larger than, the average country residence, if it is to give satisfaction to both constructor and purchaser by being quite trustworthy and, possessed of a due lease of life, say ten or fifteen years, it must be built of stouter materials than the light sheets which alone are suitable for manipulation with the soldering-iron or for bending in the ordinary type of metal press. Sound cast-iron, heavy sheet-metal, or light boiler-plate is the proper substance of which to construct all the important parts of a generator, and the joints in wrought metal must be riveted and caulked or soldered autogeneously as mentioned above. So built, the installation becomes much more costly to lay down than an apparatus composed of tinplate, zinc, or thin galvanised iron, but it will prove more economical in the long run. It is not too much to say that if ignorant and short-sighted makers in the earliest days of the acetylene industry had not recommended and supplied to their customers lightly built apparatus which has in many instances already begun to give trouble, to need repairs, and to fail by thorough corrosion--apparatus which frequently had nothing but cheapness in its favour--the use of the gas would have spread more rapidly than it has done, and the public would not now be hearing of partial or complete failures of acetylene installations. Each of these failures, whether accompanied by explosions and injury to persons or not, acts more powerfully to restrain a possible new customer from adopting the acetylene light, than several wholly successful plants urge him to take it up; for the average member of the public is not in a position to distinguish properly between the collapse of a certain generator owing to defective design or construction (which reflects no discredit upon the gas itself), and the failure of acetylene to show in practice those advantages that have been ascribed to it. One peculiar and noteworthy feature of acetylene, often overlooked, is that the apparatus is constructed by men who may have been accustomed to gas-making plant all their lives, and who may understand by mere habit how to superintend a chemical operation; but the same apparatus is used by persons who generally have no special acquaintance with such subjects, and who, very possibly, have not even burnt coal-gas at any period of their lives. Hence it happens that when some thoughtless action on the part of the country attendant of an acetylene apparatus is followed by an escape of gas from the generator, and by an accumulation of that gas in the house where the plant is situated, or when, in disregard of rules, he takes a naked light into the house and an explosion follows, the builder dismisses the episode as a piece of stupidity or wilful misbehaviour for which he can in nowise be held morally responsible; whereas the builder himself is to blame for designing an apparatus from which an escape of gas can be accompanied by sensible risks to property or life. However unpalatable this assertion may be, its truth cannot be controverted; because, short of criminal intention or insanity on the part of the attendant, it is in the first place a mere matter of knowledge and skill so to construct an acetylene plant that an escape of gas is extremely unlikely, even when the apparatus is opened for recharging, or when it is manipulated wrongly; and in the second place, it is easy so to arrange the plant that any disturbance of its functions which may occur shall be followed by an immediate removal of the surplus gas into a place of complete safety outside and above the generator-house.

GENERATION AT LOW TEMPERATURES.--In all that has been said hitherto about the reaction between calcium carbide and water being instantaneous, it has been assumed that the two substances are brought together at or about the usual temperature of an occupied room, i.e., 15 degrees C. If, however, the temperature is materially lower than this, the speed of the reaction falls off, until at -5 degrees C., supposing the water still to remain liquid, evolution of acetylene practically ceases. Even at the freezing-point of pure water gas is produced but slowly; and if a lump of carbide is thrown on to a block of ice, decomposition proceeds so gently that the liberated acetylene may be ignited to form a kind of torch, while heat is generated with insufficient rapidity to cause the carbide to sink into the block. This fact has very important bearings upon the manipulation of an acetylene generator in winter time. It is evident that unless precautions are taken those portions of an apparatus which contain water are liable to freeze on a cold night; because, even if the generator has been at work producing gas (and consequently evolving heat) till late in the evening, the surplus heat stored in the plant may escape into the atmosphere long before more acetylene has to be made, and obviously while frost is still reigning in the neighbourhood. If the water freezes in the water store, in the pipes leading therefrom, in the holder seal, or in the actual decomposing chamber, a fresh batch of gas is either totally incapable of production, because the water cannot be brought into contact with the calcium carbide in the apparatus, or it can only be generated with excessive slowness because the carbide introduced falls on to solid ice. Theoretically, too, there is a possibility that some portion of the apparatus--a pipe in particular--may be burst by the freezing, owing to the irresistible force with which water expands when it changes into the solid condition. Probably this last contingency, clearly accompanied as it would be by grave risk, is somewhat remote, all the plant being constructed of elastic material; but in practice even a simple interference with the functions of a generator by freezing, ideally of no special moment, is highly dangerous, because of the great likelihood that hurried and wholly improper attempts to thaw it will be made by the attendant. As it has been well known for many years that the solidifying point of water can be lowered to almost any degree below normal freezing by dissolving in it certain salts in definite proportions, one of the first methods suggested for preventing the formation of ice in an acetylene generator was to employ such a salt, using, in fact, for the decomposition of the carbide some saline solution which remains liquid below the minimum night temperature of the winter season. Such a process, however, has proved unsuitable for the purpose in view; and the explanation of that fact is found in what has just been stated: the "water" of the generator may admittedly be safely maintained in the fluid state, but from so cold a liquid acetylene will not be generated smoothly, if at all. Moreover, were it not so, a process of this character is unnecessarily expensive, although suitable salts are very cheap, for the water of the generator is constantly being consumed, [Footnote: It has already been said that most generators "consume" a much larger volume of water than the amount corresponding with the chemical reaction involved: the excess of water passing into the sludge or by- product. Thus a considerable quantity of any anti-freezing agent must be thrown aside each time the apparatus is cleaned out or its fluid contents are run off.] and as constantly needs renewal; which means that a fresh batch of salt would be required every time the apparatus was recharged, so long as frost existed or might be expected. A somewhat different condition obtains in the holder of an acetylene installation. Here, whenever the holder is a separate item in the plant, not constituting a portion of the generating apparatus, the water which forms the seal of a rising holder, or which fills half the space of a displacement holder, lasts indefinitely; and it behaves equally well, whatever its temperature may be, so long as it retains a fluid state. This matter will be discussed with greater detail at the end of Chapter III. At present the point to be insisted on is that the temperature in any constituent of an acetylene installation which contains water must not be permitted to fall to the freezing-point; while the water actually used for decomposition must be kept well above that temperature.

GENERATION AT HIGH TEMPERATURES.--At temperatures largely exceeding those of the atmosphere, the reaction between calcium carbide and water tends to become irregular; while at a red heat steam acts very slowly upon carbide, evolving a mixture of acetylene and hydrogen in place of pure acetylene. But since at pressures which do not materially exceed that of the atmosphere, water changes into vapour at 100° C., above that temperature there can be no question of a reaction between carbide and liquid water. Moreover, as has been pointed out, steam or water vapour will continue to exist as such at temperatures even as low as the freezing-point so long as the vapour is suspended among the particles of a permanent gas. Between calcium carbide and water vapour a double decomposition occurs chemically identical with that between carbide and liquid water; but the physical effect of the reaction and its practical bearings are considerably modified. The quantity of heat liberated when 30 parts by weight of steam react with 64 parts of calcium carbide should be essentially unaltered from that evolved when the reagent is in the liquid state; but the temperature likely to be attained when the speed of reaction remains the same as before will be considerably higher for two conspicuous reasons. In the first place, the specific heat of steam in is only 0.48, while that of liquid water is 1.0. Hence, the quantity of heat which is sufficient to raise the temperature of a given weight of liquid water through n thermometric degrees, will raise the temperature of the same weight of water vapour through rather more than 2 n degrees. In the second place, that relatively large quantity of heat which in the case of liquid water merely changes the liquid into a vapour, becoming "latent" or otherwise unrecognisable, and which, as already shown, forms roughly five-sixths of the total heat needed to convert cold water into steam, has no analogue if the water has previously been vaporised by other means; and therefore the whole of the heat supplied to water vapour raises its sensible temperature, as indicated by the thermometer. Thus it appears that, except for the sufficient amount of cooling that can be applied to a large vessel containing carbide by surrounding it with a water jacket, there is no way of governing its temperature satisfactorily if water vapour is allowed to act upon a mass of carbide--assuming, of course, that the reaction proceeds at any moderate speed, e.g., at a rate much above that required to supply one or two burners with gas.

The decomposition which with perfect chemical accuracy has been stated to occur quantitatively between 36 parts by weight, of water and 64 parts of calcium carbide scarcely ever takes place in so simple a fashion in an actual generator. Owing to the heat developed when carbide is in excess, about half the water is converted into vapour; and so the reaction proceeds in two stages: half the water added reacting with the carbide as a liquid, the other half, in a state of vapour, afterwards reacting similarly, [Footnote: This secondary reaction is manifestly only another variety of the phenomenon known as "after-generation" (cf. ante). After-generation is possible between calcium carbide and mechanically damp slaked lime, between carbide and damp gas, or between carbide and calcium hydroxide, as opportunity shall serve. In all cases the carbide must be in excess.] or hardly reacting at all, as the case may be. Suppose a vessel, A B, somewhat cylindrical in shape, is charged with carbide, and that water is admitted at the end called A. Suppose now (1) that the exit for gas is at the opposite end, B. As the lumps near A are attacked by half the liquid introduced, while the other half is changed into steam, a current, of acetylene and water vapour travels over the charge lying between the decomposing spot and the end B. During its passage the second half of the water, as vapour, reacts with the excess of carbide, the first make of acetylene being dried, and more gas being produced. Thus a second quantity of heat is developed, equal by theory to that previously evolved; but a second elevation in temperature, far more serious, and far less under control, than the former also occurs; and this is easily sufficient to determine some of those undesirable effects already described. Digressing for a moment, it may be admitted that the desiccation of the acetylene produced in this manner is beneficial, even necessary; but the advantages of drying the gas at this period of its treatment are outweighed by the concomitant disadvantages and by the later inevitable remoistening thereof. Suppose now (2) that both the water inlet and the gas exit of the carbide cylinder are at the same end, A. Again half the added water, as liquid, reacts with the carbide it first encounters, but the hot stream of damp gas is not permitted to travel over the rest of the lumps extending towards B: it is forced to return upon its steps, leaving B practically untouched. The gas accordingly escapes from the cylinder at A still loaded with water vapour, and for a given weight of water introduced much less acetylene is evolved than in the former case. The gas, too, needs drying somewhere else in the plant; but these defects are preferable to the apparent superiority of the first process because overheating is, or can be, more thoroughly guarded against.

PRESSURE IN GENERATORS.--Inasmuch as acetylene is prone to dissociate or decompose into its elements spontaneously whenever its pressure reaches 2 atmospheres or 30 lb. per square inch, as well as when its temperature at atmospheric pressure attains 780° C., no pressure approaching that of 2 atmospheres is permissible in the generator. A due observance of this rule, however, unlike a proper maintenance of a low temperature in an acetylene apparatus, is perfectly easy to arrange for. The only reason for having an appreciable positive pressure in any form of generating plant is that the gas may be compelled to travel through the pipes and to escape from the burner orifices; and since the plant is only installed to serve the burners, that pressure which best suits the burners must be thrown by the generator or its holder. Therefore the highest pressure it is ever requisite to employ in a generator is a pressure sufficient (a) to lift the gasholder bell, or to raise the water in a displacement holder, (b) to drive the gas through the various subsidiary items in the plant, such as washers and purifiers, (c) to overcome the friction in the service-pipes, [Footnote: This friction manifestly causes a loss of pressure, i.e., a fall in pressure, as a gas travels along a pipe; and, as will be shown in Chapter VII., it is the fall in pressure in a pipe rather than the initial pressure at which a gas enters a pipe that governs the volume of gas passing through that pipe. The proper behaviour and economic working of a burner (acetylene or other, luminous or incandescent) naturally depend upon the pressure in the pipe to which the burner is immediately attached being exactly suited to the design of that burner, and have nothing to do with the fall in pressure occurring in the delivery pipes. It is therefore necessary to keep entirely separate the ideas of proper burner pressure and of maximum desirable fall in pressure within the service due to friction.] and (d) to give at the points of combustion a pressure which is required by the particular burners adopted. In all except village or district installations, (c) may be virtually neglected. When the holder has a rising bell, (a) represents only an inch or so of water; but if a displacement holder is employed the pressure needed to work it is entirely indeterminate, being governed by the size and shape of the said holder. It will be argued in Chapter III. that a rising holder is always preferable to one constructed on the displacement principle. The pressure (d) at the burners may be taken at 4 inches of water as a maximum, the precise figure being dependent upon the kind of burners--luminous, incandescent, boiling, &c.--attached to the main. The pressure (b) also varies according to circumstances, but averages 2 or 3 inches. Thus a pressure in the generator exceeding that of the atmosphere by some 12 inches of water--i.e., by about 7 oz., or less than half a pound per square inch--is amply sufficient for every kind of installation, the less meritorious generators with displacement holders only excepted. This pressure, it should be noted, is the net or effective pressure, the pressure with which the gas raises the liquid in a water-gauge glass out of the level while the opposite end of the water column is exposed to the atmosphere. The absolute pressure in a vessel containing gas at an effective pressure of 12 inches of water is 7 oz. plus the normal, insensible pressure of the atmosphere itself--say 15-1/4 lb. per square inch. The liquid in a barometer which measures the pressure of the atmosphere stands at a height of 30 inches only, because that liquid is mercury, 13.6 times as heavy as water. Were it filled with water the barometer would stand at (30 X 13.6) = 408 inches, or 34 feet, approximately. Gas pressures are always measured in inches of water column, because expressed either as pounds per square inch or as inches of mercury, the figures would be so small as to give decimals of unwieldy length.

It would of course be perfectly safe so to arrange an acetylene plant that the pressure in the generating chamber should reach the 100 inches of water first laid down by the Home Office authorities as the maximum allowable. There is, however, no appreciable advantage to be gained by so doing, or by exceeding that pressure which feeds the burners best. Any higher original pressure involves the use of a governor at the exit of the plant, and a governor is a costly and somewhat troublesome piece of apparatus that can be dispensed with in most single installations by a proper employment of a well-balanced rising holder.

[CHAPTER III]

THE GENERAL PRINCIPLES OF ACETYLENE GENERATION--ACETYLENE GENERATING APPARATUS

Inasmuch as acetylene is produced by the mere interaction of calcium carbide and water, that is to say, by simply bringing those two substances in the cold into mutual contact within a suitable closed space, and inasmuch as calcium carbide can always be purchased by the consumer in a condition perfectly fit for immediate decomposition, the preparation of the gas, at least from the theoretical aspect, is characterised by extreme simplicity. A cylinder of glass or metal, closed at one end and open at the other, filled with water, and inverted in a larger vessel containing the same liquid, may be charged almost instantaneously with acetylene by dropping into the basin a lump of carbide, which sinks to the bottom, begins to decompose, and evolves a rapid current of gas, displacing the water originally held in the inverted cylinder or "bell." If a very minute hole is drilled in the top of the floating bell, acetylene at once escapes in a steady stream, being driven out by the pressure of the cylinder, the surplus weight of which causes it to descend into the water of the basin as rapidly as gas issues from the orifice. As a laboratory experiment, and provided the bell has been most carefully freed from atmospheric air in the first instance, this escaping gas may be set light to with a match, and will burn with a more or loss satisfactory flame of high illuminating power. Such is an acetylene generator stripped of all desirable or undesirable adjuncts, and reduced to its most elementary form; but it is needless to say that so simple an apparatus would not in any way fulfil the requirements of everyday practice.

Owing to the inequality of the seasons, and to the irregular nature of the demand for artificial light and heat in all households, the capacity of the plant installed for the service of any institution or district must be amply sufficient to meet the consumption of the longest winter evening--for, as will be shown in the proper place, attempts to make an acetylene generator evolve gas more quickly than it is designed to do are fraught with many objections--while the operation of the plant, must be under such thorough control that not only can a sudden and unexpected demand for gas be met without delay, but also that a sudden and unexpected interruption or cessation of the demand shall not be followed by any disturbance in the working of the apparatus. Since, on the one hand, acetylene is produced in large volumes immediately calcium carbide is wetted with water, so that the gas may be burnt within a minute or two of its first evolution; and, on the other, that acetylene once prepared can be stored without trouble or appreciable waste for reasonable periods of time in a water-sealed gasholder closely resembling, in everything but size, the holders employed on coal-gas works; it follows that there are two ways of bringing the output of the plant into accord with the consumption of the burners. It is possible to make the gas only as and when it is required, or it is possible in the space of an hour or so, during the most convenient part of the day, to prepare sufficient to last an entire evening, storing it in a gasholder till the moment arrives for its combustion. It is clear that an apparatus needing human attention throughout the whole period of activity would be intolerable in the case of small installations, and would only be permissible in the case of larger ones if the district supplied with gas was populous enough to justify the regular employment of two men at least in or about the generating station. But with the conditions obtaining in such a country as Great Britain, and in other lands where coal is equally cheap and accessible, if a neighbourhood was as thickly populated as has been suggested, it would be preferable on various grounds to lay down a coal- gas or electricity works; for, as has been shown in the first chapter, unless a very material fall in the price of calcium carbide should take place--a fall which at present is not to be expected--acetylene can only be considered a suitable and economical illuminant and heating agent for such places as cannot be provided cheaply with coal-gas or electric current. To meet this objection, acetylene generators have been invented in which, broadly speaking, gas is only produced when it is required, control of the chemical reaction devolving upon some mechanical arrangement. There are, therefore, two radically different types of acetylene apparatus to be met with, known respectively as "automatic" and "non-automatic" generators. In a non-automatic generator the whole of the calcium carbide put into the apparatus is more or less rapidly decomposed, and the entire volume of gas evolved from it is collected in a holder, there to await the moment of consumption. In an automatic apparatus, by means of certain devices which will be discussed in their proper place, the act of turning on a burner-tap causes some acetylene to be produced, and the act of turning it off brings the reaction to an end, thus obviating the necessity for storage. That, at any rate, is the logical definition of the two fundamentally different kinds of generator: in automatic apparatus the decomposition of the carbide is periodically interrupted in such fashion as more or less accurately to synchronise with the consumption of gas; in the non-automatic variety decomposition proceeds without a break until the carbide vessels are empty. Unfortunately a somewhat different interpretation of these two words has found frequent acceptance, a generator being denominated non-automatic or automatic according as the holder attached to it is or is not large enough to store the whole of the acetylene which the charge of carbide is capable of producing if it is decomposed all at once. Apart from the fact that a holder, though desirable, is not an absolutely indispensable part of an acetylene plant, the definition just quoted was sufficiently free from objection in the earliest days of the industry; but now efficient commercial generators are to be met with which become either automatic or non-automatic according to the manner of working them, while some would be termed non-automatic which comprise mechanism of a conspicuously self- acting kind.

AUTOMATIC AND NON-AUTOMATIC GENERATORS.--Before proceeding to a detailed description of the various devices which may be adopted to render an acetylene generator automatic in action, the relative advantages of automatic and non-automatic apparatus, irrespective of type, from the consumer's point of view may be discussed. The fundamental idea underlying the employment of a non-automatic generator is that the whole of the calcium carbide put into the apparatus shall be decomposed into acetylene as soon after the charge is inserted as is natural in the circumstances; so that after a very brief interval of time the generating chambers shall contain nothing but spent lime and water, and the holder be as full of gas as is ever desirable. In an automatic apparatus, the fundamental idea is that the generating chamber, or one at least of several generating chambers, shall always contain a considerable quantity of undecomposed carbide, and some receptacle always contain a store of water ready to attack that carbide, so that whenever a demand for gas shall arise everything may be ready to meet it. Inasmuch as acetylene is an inflammable gas, it possesses all the properties characteristic of inflammable gases in general; one of which is that it is always liable to take fire in presence of a spark or naked light, and another of which is that it is always liable to become highly explosive in presence of a naked light or spark if, accidentally or otherwise, it becomes mixed with more than a certain proportion of air. On the contrary, in the complete absence of liquid or vaporised water, calcium carbide is almost as inert a body as it is possible to imagine: for it will not take fire, and cannot in any circumstances be made to explode. Hence it may be urged that a non-automatic generator, with its holder always containing a large volume of the actually inflammable and potentially explosive acetylene, must invariably be more dangerous than an automatic apparatus which has less or practically no ready-made gas in it, and which simply contains water in one chamber and unaltered calcium carbide in another. But when the generating vessels and the holder of a non-automatic apparatus are properly designed and constructed, the gas in the latter is acetylene practically free from air, and therefore while being, as acetylene inevitably is, inflammable, is devoid of explosive properties, always assuming, as must be the case in a water-sealed holder, that the temperature of the gas is below 780° C.; and also assuming, as must always be the case in good plant, that the pressure under which the gas is stored remains less than two atmospheres absolute. It is perfectly true that calcium carbide is non-inflammable and non-explosive, that it is absolutely inert and incapable of change; but so comprehensive an assertion only applies to carbide in its original drum, or in some impervious vessel to which moisture and water have no access. Until it is exhausted, an automatic acetylene generator contains carbide in one place and water in another, dependence being put upon some mechanical arrangement to prevent the two substances coming into contact prematurely. Many of the devices adopted by builders of acetylene apparatus for keeping the carbide and water separate, and for mixing them in the requisite quantities when the proper time arrives, are as trustworthy, perhaps, as it is possible for any automatic gear to be; but some are objectionably complicated, and a few are positively inefficient. There are two difficulties which the designer of automatic mechanism has to contend with, and it is doubtful whether he always makes a sufficient allowance for them. The first is that not only must calcium carbide and liquid water be kept out of premature contact, but that moisture, or vapour of water, must not be allowed to reach the carbide; or alternatively, that if water vapour reaches the carbide too soon, the undesired reaction shall not determine overheating, and the liberated gas be not wasted or permitted to become a source of danger. The second difficulty encountered by the designer of automata is so to construct his apparatus that it shall behave well when attended to by completely unskilled labour, that it shall withstand gross neglect and resist positive ill-treatment or mismanagement. If the automatic principle is adopted in any part of an acetylene apparatus it must be adopted throughout, so that as far as possible--and with due knowledge and skill it is completely possible--nothing shall be left dependent upon the memory and common sense of the gasmaker. For instance, it must not be necessary to shut a certain tap, or to manipulate several cocks before opening the carbide vessel to recharge it; it must not be possible for gas to escape backwards out of the holder; and either the carbide-feed gear or the water-supply mechanism (as the case may be) must be automatically locked by the mere act of taking the cover off the carbide store, or of opening the sludge-cock at the bottom. It would be an advantage, even, if the purifiers and other subsidiary items of the plant were treated similarly, arranging them in such fashion that gas should be automatically prevented from escaping out of the rest of the apparatus when any lid was removed. In fact, the general notion of interlocking, which has proved so successful in railway signal-cabins and in carburetted water gas-plant for the prevention of accidents duo to carelessness or overnight, might be copied in principle throughout an acetylene installation whenever the automatic system is employed.

It is no part of the present argument, to allege that automatic generators are, and must always be, inherently dangerous. Automatic devices of a suitable kind may be found in plenty which are remarkably simple and highly trustworthy; but it would be too bold a statement to say that any such arrangement is incapable of failure, especially when put into the hands of a person untrained in the superintendence of machinery. The more reliable a piece of automatic mechanism proves itself to be, the more likely is it to give trouble and inconvenience and utterly to destroy confidence when it does break down; because the better it has behaved in the past, and the longer it has lasted without requiring adjustment, the less likely is it that the attendant will be at hand when failure occurs. By suitable design and by an intelligent employment of safety-valves and blow-off pipes (which will be discussed in their proper place) it is quite easy to avoid the faintest possibility of danger arising from an increase of pressure or an improper accumulation of gas inside the plant or inside the building containing the plant; but every time such a safety-valve or blow-off pipe comes into action a waste of gas occurs, which means a sacrifice of economy, and shows that the generator is not working as it should.

As glass is a fragile and brittle substance, and as it is not capable of bearing large, rapid, and oft-repeated alterations of temperature in perfect safety, it is not a suitable material for the construction of acetylene apparatus or of portions thereof. Hence it follows that a generator must be built of some non-transparent material which prevents the interior being visible when the apparatus is at work. Although it is comparatively easy, by the aid of a lamp placed outside the generator- shed in such a position as to throw its beams of light through a window upon the plant inside, to charge a generator after dark; and although it is possible, without such assistance, by methodical habits and a systematic arrangement of utensils inside the building to charge a generator even in perfect darkness, such an operation is to be deprecated, for it is apt to lead to mistakes, it prevents any slight derangement in the installation from being instantly noticed, and it offers a temptation to the attendant to break rules and to take a naked light with him. On all those grounds, therefore, it is highly desirable that every manipulation connected with a generator shall be effected during the daytime, and that the apparatus-house shall be locked up before nightfall. But owing to the irregular habits engendered by modern life it is often difficult to know, during any given day, how much gas will be required in the ensuing evening; and it therefore becomes necessary always to have, as ready-made acetylene, or as carbide in a proper position for instant decomposition, a patent or latent store of gas more than sufficient in quantity to meet all possible requirements. Now, as already stated, a non-automatic apparatus has its store of material in the form of gas in a holder; and since this is preferably constructed on the rising or telescopic principle, a mere inspection of the height of the bell--on which, if preferred, a scale indicating its contents in cubic feet or in burner-hours may be marked--suffices to show how near the plant is to the point of exhaustion. In many types of automatic apparatus the amount of carbide remaining undecomposed at any moment is quite unknown, or at best can only be deduced by a tedious and inexact calculation; although in some generators, where the store of carbide is subdivided into small quantities, or placed in several different receptacles, an inspection of certain levers or indicators gives an approximate idea as to the capacity of the apparatus for further gas production. In any case the position of a rising holder is the most obvious sign of the degree of exhaustion of a generator; and therefore, to render absolutely impossible a failure of the light during an evening, a non-automatic generator fitted with a rising holder is best.

Since calcium carbide is a solid body having a specific gravity of 2.2, water being unity, and since 1 cubic foot of water weighs 62.4 lb., in round numbers 137 lb. of compact carbide only occupy 1 cubic foot of space. Again, since acetylene is a gas having a specific gravity of 0.91, air being unity, and since the specific gravity of air, water being unity, is 0.0013, the specific gravity of acetylene, water being unity, is roughly O.00116. Hence 1 cubic foot of acetylene weighs roughly 0.07 lb. Furthermore, since 1 lb. of good carbide evolves 5 cubic feet of gas on decomposition with water, acetylene stored at atmospheric pressure occupies roundly 680 times as much space as the carbide from which it has been evolved. This figure by no means represents the actual state of affairs in a generator, because, as was explained in the previous chapter, a carbide vessel cannot be filled completely with solid; and, indeed, were it so "filled," in ordinary language, much of its space would be still occupied with air. Nevertheless it is incontrovertible that an acetylene plant calculated to supply so many burners for so long a period of time must be very much larger if it is constructed on the non-automatic principle, when the carbide is decomposed all at once, than if the automatic system is adopted, when the solid remains unattacked until a corresponding quantity of gas is required for combustion. Clearly it is the storage part of a non-automatic plant alone which must be so much larger; the actual decomposing chambers may be of the same size or even smaller, according to the system of generation to which the apparatus belongs. In practice this extra size of the non-automatic plant causes it to exhibit two disadvantages in comparison with automatic apparatus, disadvantages which are less serious than they appear, or than they may easily be represented to be. In the first place, the non- automatic generator requires more space for its erection. If acetylene were an illuminating agent suitable for adoption by dwellers in city or suburb, where the back premises and open-air part of the messuage are reduced to minute proportions or are even non-existent, this objection might well be fatal. But acetylene is for the inhabitant of a country village or the occupier of an isolated country house; and he has usually plenty of space behind his residence which he can readily spare. In the second place, the extra size of the non-automatic apparatus makes it more expensive to construct and more costly to instal. It is more cosily to construct and purchase because of its holder, which must be well built on a firm foundation and accurately balanced; it is more costly to instal because a situation must be found for the erection of the holder, and the apparatus-house may have to be made large enough to contain the holder as well as the generator itself. As regards the last point, it may be said at once that there is no necessity to place the holder under cover: it may stand out of doors, as coal-gas holders do in England, for the seal of the tank can easily be rendered frost-proof, and the gas itself is not affected by changes of atmospheric temperature beyond altering somewhat in volume. In respect of the other objections, it must be remembered that the extra expense is one of capital outlay alone, and therefore only increases the cost of the light by an inappreciable amount, representing interest and depreciation charges on the additional capital expenditure. The increased cost of a year's lighting due to these charges will amount to only 10 or 15 per cent, on the additional capital sunk. The extra capital sunk does not in any way increase the maintenance charges; and if, by having a large holder, additional security and trustworthiness are obtained, or if the holder leads to a definite, albeit illusive, sense of extra security and trustworthiness, the additional expenditure may well be permissible or even advantageous.

The argument is sometimes advanced that inasmuch as for the same, or a smaller, capital outlay as is required to instal a non-automatic apparatus large enough to supply at one charging the maximum amount of light and heat that can ever be needed on the longest winter's night, an automatic plant adequate to make gas for two or three evenings can be laid down, the latter must be preferable, because the attendant, in the latter case, will only need to enter the generator-house two or three times a week. Such an argument is defective because it ignores the influence of habit upon the human being. A watch which must be wound every day, or a clock which must be wound every week, on a certain day of the week, is seldom permitted to run down; but a watch requiring to be re-wound every other day, or a fourteen-day clock (used as such), would rarely be kept going. Similarly, an acetylene generator might be charged once a week or once a day without likelihood of being forgotten; but the operation of charging at irregular intervals would certainly prove a nuisance. With a non-automatic apparatus containing all its gas in the holder, the attendant would note the position of the bell each morning, and would introduce sufficient carbide to fill the holder full, or partly full, as the case might be; with an automatic apparatus he would be tempted to trust that the carbide holders still contained sufficient material to last another night.

The automatic system of generating acetylene has undoubtedly one advantage in those climates where frost tends to occur frequently, but only to prevail for a short period. As the apparatus is in operation during the evening hours, the heat evolved will, or can be made to, suffice to protect the apparatus from freezing until the danger has passed; whereas if the gas is generated of a morning in a non-automatic apparatus the temperature of the plant may fall to that of the atmosphere before evening, and some portion may freeze unless special precautions are taken to protect it.

It was shown in Chapter II that overheating is one of the chief troubles to be guarded against in acetylene generators, and that the temperature attained is a function of the speed at which generation proceeds. Seeing that in an automatic apparatus the rate of decomposition depends on the rate at which gas is being burnt, while in a non-automatic generator it is, or may be, under no control, the critic may urge that the reaction must take place more slowly and regularly, and the maximum temperature therefore be lower, when the plant works automatically. This may be true if the non-automatic generator is unskilfully designed or improperly manipulated; but it is quite feasible to arrange an apparatus, especially one of the carbide-to-water or of the flooded-compartment type, in such fashion that overheating to an objectionable extent is rendered wholly impossible. In a non-automatic apparatus the holder is nothing but a holder and may be placed wherever convenient, even at a distance from the generating plant; in an automatic apparatus the holder, or a small similarly constructed holder placed before the main storage vessel, has to act as a water-supply governor, as the releasing gear for certain carbide-food mechanism, or indeed as the motive power of such mechanism; and accordingly it must be close to the water or carbide store, and more or less intimately connected by means of levers, or the like, with the receptacle in which decomposition occurs. Sometimes the holder surrounds, or is otherwise an integral part of, the decomposing chamber, the whole apparatus being made self-contained or a single structure with the object of gaining compactness. But it is evident that such methods of construction render additionally awkward, or even hazardous, any repair or petty operation to the generating portion of the plant; while the more completely the holder is isolated from the decomposing vessels the more easily can they be cleaned, recharged, or mended, without blowing off the stored gas and without interfering with the action of any burners that may be alight at the time. Owing to the ingenuity of inventors, and the experience they have acquired in the construction of automatic acetylene apparatus during the years that the gas has been in actual employment, it is going too far boldly to assert that non-automatic generators are invariably to be preferred before their rivals. Still in view of the nature of the labour which is likely to be bestowed on any domestic plant, of the difficulty in having repairs or adjustments done quickly in outlying country districts, and of the inconvenience, if not risk, attending upon any failure of the apparatus, the greater capital outlay, and the larger space required by non-automatic generators are in most instances less important than the economy in space and prime cost characteristic of automatic machines when the defects of each are weighed fairly in the balance. Indeed, prolonged experience tends to show that a selection between non-automatic and automatic apparatus may frequently be made on the basis of capacity. A small plant is undoubtedly much more convenient if automatic; a very large plant, such as that intended for a public supply, is certainly better if non-automatic, but between these two extremes choice may be exercised according to local conditions.

CONTROL OF THE CHEMICAL REACTION.--Coming now to study the principles underlying the construction of an acetylene generator more closely it will be seen that as acetylene is produced by bringing calcium carbide into contact with water, the chemical reaction may be started either by adding the carbide to the water, or by adding the water to the carbide. Similarly, at least from the theoretical aspect, the reaction, may be caused to stop by ceasing to add carbide to water, or by ceasing to add water to carbide. Apparently if water is added by degrees to carbide, until the carbide is exhausted, the carbide must always be in excess; and manifestly, if carbide is added in small portions to water, the water must always be in excess, which, as was argued in Chapter II., is emphatically the more desirable position of affairs. But it in quite simple to have carbide present in large excess of the water introduced when the whole generator is contemplated, and yet to have the water always in chemical excess in the desired manner; because to realise the advantages of having water in excess, it is only necessary to subdivide the total charge of carbide into a number of separate charges which are each so small that more than sufficient water to decompose and flood one of them is permitted to enter every time the feed mechanism comes into play, or (in a non-automatic apparatus) every time the water-cock is opened; so arranging the charges that each one is protected from the water till its predecessor, or its predecessor, have been wholly decomposed. Thus it is possible to regard either the carbide or the water as the substance which has to be brought into contact with the other in specified quantity. It is perhaps permissible to repeat that in the construction of an automatic generator there is no advantage to be gained from regulating the supply of both carbide and water, because just as the mutual decomposition will begin immediately any quantity of the one meets any quantity of the other, so the reaction will cease (except in one case owing to "after-generation") directly the whole of that material which is not in chemical excess has been consumed-quite independently of the amount of the other material left unattacked. Being a liquid, and possessing as such no definite shape or form of its own irrespective of the vessel in which it is held, water is by far the more convenient of the two substances to move about or to deliver in predetermined volume to the decomposing chamber. A supply of water can be started instantaneously or cut oil as promptly by the movement of a cock or valve of the usual description; or it may be allowed to run down a depending pipe in obedience to the law of gravitation, and stopped from running down such a pipe by opposing to its passage a gas pressure superior to that gravitational force. In any one of several obvious ways the supply of water to a mass of carbide may be controlled with absolute certainty, and therefore it should apparently follow that the make of acetylene should be under perfect control by controlling the water current. On the other hand, unless made up into balls or cartridges of some symmetrical form, calcium carbide exists in angular masses of highly irregular shape and size. Its lumps alter in shape and size directly liquid water or moisture reaches them; a loose more or loss gritty powder, or a damp cohesive mud, being produced which is well calculated to choke any narrow aperture or to jam any moving valve. It is more difficult, therefore, by mechanical agency to add a supply of carbide to a mass of water than to introduce a supply of water to a stationary mass of carbide; and far more difficult still to bring the supply of carbide under perfect control with the certainty that the movement shall begin and stop immediately the proper time arrives.

But assuming the mechanical difficulties to be satisfactorily overcome, the plan of adding carbide to a stationary mass of water has several chemical advantages, first, because, however the generator be constructed, water will be in excess throughout the whole time of gas production; and secondly, because the evolution of acetylene will actually cease completely at the moment when the supply of carbide is interrupted. There is, however, one particular type of generator in which as a matter of fact the carbide is the moving constituent, viz., the "dipping" apparatus (cf. infra), to which these remarks do not apply; but this machine, as will be seen directly, is, illogically perhaps, but for certain good reasons, classed among the water-to-carbide apparatus. All the mechanical advantages are in favour, as just indicated, of making water the moving substance; and accordingly, when classified in the present manner, a great majority of the generators now on the markets are termed water-to-carbide apparatus. Their disadvantages are twofold, though these may be avoided or circumvented: in all types save one the carbide is in excess at the immediate place and time of decomposition; and in all types without exception the carbide in the whole of the generator is in excess, so that the phenomenon of "after- generation" occurs with more or less severity. As explained in the last chapter, after-generation is the secondary production of acetylene which takes place more or less slowly after the primary reaction is finished, proceeding either between calcium hydroxide, merely damp lime, or damp gas and calcium carbide, with an evolution of more acetylene. As it is possible, and indeed usual, to fit a holder of some capacity even to an automatic generator, the simple fact that more acetylene is liberated after the main reaction is over does not matter, for the gas can be safely stored without waste and entirely without trouble or danger. The real objection to after-generation is the difficulty of controlling the temperature and of dissipating the heat with which the reaction is accompanied. It will be evident that the balance of advantage, weighing mechanical simplicity against chemical superiority, is somewhat even between carbide-to-water and water-to-carbide generators of the proper type; but the balance inclines towards the former distinctly in the ease of non-automatic apparatus, and points rather to the latter when automatism is desired. In the early days of the industry it would have been impossible to speak so favourably of automatic carbide-to-water generators, for they were at first constructed with absurdly complicated and unreliable mechanism; but now various carbide-feed gears have been devised which seem to be trustworthy even when carbide not in cartridge form is employed.

NON-AUTOMATIC CARBIDE-TO-WATER GENERATORS.--There is little to be said in the present place about the principles underlying the construction of non-automatic generators. Such apparatus may either be of the carbide-to- water or the water-to-carbide type. In the former, lumps of carbide are dropped by hand down a vertical or sloping pipe or shoot, which opens at its lower end below the water-level of the generating chamber, and which is fitted below its mouth with a deflector to prevent the carbide from lodging immediately underneath that mouth. The carbide falls through the water which stands in the shoot itself almost instantaneously, but during its momentary descent a small quantity of gas is evolved, which produces an unpleasant odour unless a ventilating hood is fixed above the upper end of the tube. As the ratio of cubical contents to superficial area of a lump is greater as the lump itself is larger, and as only the outer surface of the lump can be attacked by the water in the shoot during its descent, carbide for a hand-fed carbide-to-water generator should be in fairly large masses--granulated material being wholly unsuitable--and this quite apart from the fact that large carbide is superior to small in gas-making capacity, inasmuch as it has not suffered the inevitable slight deterioration while being crushed and graded to size. If carbide is dropped too rapidly into such a generator which is not provided with a false bottom or grid for the lumps to rest upon, the solid is apt to descend among a mass of thick lime sludge produced at a former operation, which lies at the bottom of the decomposing chamber; and here it may be protected from the cooling action of fresh water to such an extent that its surface is baked or coated with a hard layer of lime, while overheating to a degree far exceeding the boiling-point of water may occur locally. When, however, it falls upon a grid placed some distance above the bottom of the water vessel, the various convection currents set up as parts of the liquid become warm, and the mechanical agitations produced by the upward current of gas rinse the spent lime from the carbide, and entirely prevent overheating, unless the lumps are excessively large in size. If the carbide charged into a hand-fed generator is in very large lumps there is always a possibility that overheating may occur in the centre of the masses, due to the baking of the exterior, even if the generator is fitted with a reaction grid. Manifestly, when carbide in lumps of reasonable size is dropped into excess of water which is not merely a thick viscid cream of lime, the temperature cannot possibly exceed the boiling-point--i.e., 100° C.--provided always the natural convection currents of the water are properly made use of.

The defect which is, or rather which may be, characteristic of a hand-fed carbide-to-water generator is a deficiency of gas yield due to solubility. At atmospheric temperatures and pressure 10 volumes of water dissolve 11 volumes of acetylene, and were the whole of the water in a large generator run to waste often, a sensible loss of gas would ensue. If the carbide falls nearly to the bottom of the water column, the rising gas is forced to bubble through practically the whole of the liquid, so that every opportunity is given it to dissolve in the manner indicated till the liquid is completely saturated. The loss, however, is not nearly so serious as is sometimes alleged, because (1) the water becomes heated and so loses much of its solvent power; and (2) the generator is worked intermittently, with sufficiently long intervals to allow the spent lime to settle into a thick cream, and only that thick cream is run off, which represents but a small proportion of the total water present. Moreover, a hand-fed carbide-to-water generator will work satisfactorily with only half a gallon [Footnote: The United States National Board of Fire Underwriters stipulates for the presence of 1 (American) gallon of water for every 1 lb. of carbide before such an apparatus is "permitted." This quantity of liquid might retain nearly 4 per cent. of the total acetylene evolved. Even this is an exaggeration; for neither her, nor in the corresponding figure given in the text, is any allowance made for the diminution in solvent power of the water as it becomes heated by the reaction.] of liquid present for every 1 lb. of carbide decomposed, and were all this water run off and a fresh quantity admitted before each fresh introduction of carbide, the loss of acetylene by dissolution could not exceed 2 per cent. of the total make, assuming the carbide to be capable of yielding 5 cubic feet of gas per lb. Admitting, however, that some loss of gas does occur in this manner, the defect is partly, if not wholly, neutralised by the concomitant advantages of the system: (1) granted that the generator is efficiently constructed, decomposition of the carbide is absolutely complete, so that no loss of gas occurs in this fashion; (2) the gas is evolved at a low temperature, so that it is unaccompanied, by products of polymerisation, which may block the leading pipes and must reduce the illuminating power; (3) the acetylene is not mixed with air (as always happens at the first charging of a water-to- carbide apparatus), which also lowers the illuminating power; and (4) the gas is freed from two of its three chief impurities, viz., ammonia and sulphuretted hydrogen, in the generating chamber itself. To prevent the loss of acetylene by dissolution, carbide-to-water generators are occasionally fitted with a reaction grid placed only just below the water-level, so that the acetylene has no more than 1 inch or so of liquid to bubble through. The principle is wrong, because hot water being lighter than cold, the upper layers may be raised to the boiling-point, and even converted into steam, while the bulk of the liquid still remains cold; and if the water actually surrounding the carbide is changed into vapour, nearly all control over the temperature attending the reaction is lost.

The hand-fed carbide-to-water generator is very simple and, as already indicated, has proved itself perhaps the best type of all for the construction of very large installations; but the very simplicity of the generator has caused it more than once to be built in a manner that has not given entire satisfaction. As shown at L in Fig. 6, p. 84, the generator essentially consists of a closed cylindrical vessel communicating at its top with a separate rising holder. At one side as drawn, or disposed concentrically if so preferred, is an open-mouthed pipe or shoot (American "shute") having its lower open extremity below the water-level. Into this shoot are dropped by hand or shovel lumps of carbide, which fall into the water and there suffer decomposition. As the bottom of the shoot is covered with water, which, owing to the small effective gas pressure in the generator given by the holder, stands a few inches higher in the shoot than in the generator, gas cannot escape from the shoot; because before it could do so the water in the generator would have to fall below the level of the point a, being either driven out through the shoot or otherwise. Since the point b of the shoot extends further into the generator than a, the carbide drops centrally, and as the bubbles of gas rise vertically, they have no opportunity of ascending into the shoot. In practice, the generator is fitted with a conical bottom for the collection of the lime sludge and with a cock or other aperture at the apex of the cone for the removal of the waste product. As it is not desirable that the carbide should be allowed to fall directly from the shoot into the thicker portion of the sludge within the conical part of the generator, one or more grids is usually placed in the apparatus as shown by the dotted lines in the sketch. It does not seem that there is any particular reason for the employment of more than one grid, provided the size of the carbide decomposed is suited to the generator, and provided the mesh of the grid is suited to the size of the carbide. A great improvement, however, is made if the grid is carried on a horizontal spindle in such a way that it can be rocked periodically in order to assist in freeing the lumps of carbide from the adhering particles of lime. As an alternative to the movable grid, or even as an adjunct thereto, an agitator scraping the conical sides of the generator may be fitted which also assists in ensuring a reasonably complete absence of undecomposed carbide from the sludge drawn off at intervals. A further point deserves attention. If constructed in the ideal manner shown in Fig. 6 removal of some of the sludge in the generator would cause the level of the liquid to descend and, by carelessness, the level might fall below the point a at the base of the shoot. In these circumstances, if gas were unable to return from the holder, a pressure below that of the atmosphere would be established in the gas space of the generator and air would be drawn in through the shoot. This air might well prove a source of danger when generation was started again. Any one of three plans may be adopted to prevent the introduction of air. A free path may be left on the gas-main passing from the generator to the holder so that gas may be free to return and so to maintain the usual positive pressure in the decomposing vessel; the sludge may be withdrawn into some vessel so small in capacity that the shoot cannot accidentally become unsealed; or the waterspace of the generator may be connected with a water-tank containing a ball-valve attached to a constant service of water be that liquid runs in as quickly as sludge is removed, and the level remains always at the same height. The first plan is only a palliative and has two defects. In the first place, the omission of any non-return valve between, the generator and the next item in the train of apparatus is objectionable of itself; in the second place, should a very careless attendant withdraw too much liquid, the shoot might become unsealed and the whole contents of the holder be passed into the air of the building containing the apparatus through the open mouth of the shoot. The second plan is perfectly sound, but has the practical defect of increasing the labour of cleaning the generator. The third plan is obviously the best. It can indeed be adopted where no real constant service of water is at hand by connecting the generator to a water reservoir of relatively large size and by making the latter of comparatively large transverse area, in proportion to its depth; so that the escape of even a largo volume of water from the reservoir may not involve a large reduction in the level at which it stands there.

The dust that always clings to lumps of carbide naturally decomposes with extreme rapidity when the material is thrown into the shoot of a carbide- to-water generator, and the sudden evolution of gas so produced has on more than one occasion seriously alarmed the attendant on the plant. Moreover, to a trifling extent the actual superficial layers of the carbide suffer attack before the lumps reach the true interior of the generator, and a small loss of gas thereby occurs through the open mouth of the shoot. To remove these objections to the hand-fed generator it has become a common practice in large installations to cause the lower end of the shoot to dip under the level of some oil contained in an appropriate receptacle, the carbide falling into a basket carried upon a horizontal spindle. The basket and its support are so arranged that when a suitable charge of carbide has been dropped into it, a partial rotation of an external hand-wheel lifts the basket and carbide out of the oil into an air-tight portion of the generator where the surplus oil can drain away from the lumps. A further rotation of the hand-wheel then tips the basket over a partition inside the apparatus, allowing the carbide to fall into the actual decomposing chamber. This method of using oil has the advantage of making the evolution of acetylene on a large scale appear to proceed more quietly than usual, and also of removing the dust from the carbide before it reaches the water of the generator. The oil itself obviously does not enter the decomposing chamber to any appreciable extent and therefore does not contaminate the final sludge. The whole process accordingly lies to be favourably distinguished from those other methods of employing oil in generators or in the treatment of carbide which are referred to elsewhere in this book.

NON-AUTOMATIC WATER-TO-CARBIDE GENERATORS.--The only principle underlying the satisfactory design of a non-automatic water-to-carbide generator is to ensure the presence of water in excess at the spot where decomposition is taking place. This may be effected by employing what is known as the "flooded-compartment" system of construction, i.e., by subdividing the total carbide charge into numerous compartments arranged either vertically or horizontally, and admitting the water in interrupted quantities, each more than sufficient thoroughly to decompose and saturate the contents of one compartment, rather than in a slow, steady stream. It would be quite easy to manage this without adopting any mechanism of a moving kind, for the water might be stored in a tank kept full by means of a ball-valve, and admitted to an intermediate reservoir in a slow, continuous current, the reservoir being fitted with an inverted syphon, on the "Tantalus-cup" principle, so that it should first fill itself up, and then suddenly empty into the pipe leading to the carbide container. Without this refinement, however, a water-to-carbide generator, with subdivided charge, behaves satisfactorily as long as each separate charge of carbide is so small that the heat evolved on its decomposition can be conducted away from the solid through the water- jacketed walls of the vessel, or as the latent heat of steam, with sufficient rapidity. Still it must be remembered that a water-to-carbide generator, with subdivided charge, does not belong to the flooded- compartment type if the water runs in slowly and continuously: it is then simply a "contact" apparatus, and may or may not exhibit overheating, as well as the inevitable after-generation. All generators of the water-to- carbide type, too, must yield a gas containing some air in the earlier portions of their make, because the carbide containers can only be filled one-third or one-half full of solid. Although the proportion of air so passed into the holder may be, and usually is, far too small in amount to render the gas explosive or dangerous in the least degree, it may well be sufficient to reduce the illuminating power appreciably until it is swept out of the service by the purer gas subsequently generated. Moreover, all water-to-carbide generators are liable, as just mentioned, to produce sufficient overheating to lower the illuminating power of the gas whenever they are wilfully driven too fast, or when they are reputed by their makers to be of a higher productive capacity than they actually should be; and all water-to-carbide generators, excepting those where the carbide is thoroughly soaked in water at some period of their operation, are liable to waste gas by imperfect decomposition.

DEVICES TO SECURE AUTOMATIC ACTION,--The devices which are commonly employed to render a generator automatic in action, that is to say, to control the supply of one of the two substances required in the intermittent evolution of gas, may be divided into two broad classes: (A) those dependent upon the position of a rising-holder bell, and (B) those dependent upon the gas pressure inside the apparatus. As the bell of a rising holder descends in proportion as its gaseous contents are exhausted, it may (A^1) be fitted with some laterally projecting pin which, arrived at a certain position, actuates a series of rods or levers, and either opens a cock on the water-supply pipe or releases a mechanical carbide-feed gear, the said cock being closed again or the feed-gear thrown out of action when the pin, rising with the bell, once more passes a certain position, this time in its upward path. Secondly (A^2), the bell may be made to carry a perforated receptacle containing carbide, which is dipped into the water of the holder tank each time the bell falls, and is lifted out of the water when it rises again. Thirdly (A^3), by fitting inside the upper part of the bell a false interior, conical in shape, the descent of the bell may cause the level of the water in the holder tank to rise until it is above some lateral aperture through which the liquid may escape into a carbide container placed elsewhere. These three methods are represented in the annexed diagram (Fig. 1). In Al the water-levels in the tank and bell remain always at l, being higher in the tank than in the bell by a distance corresponding with the pressure produced by the bell itself. As the bell falls a pin X moves the lever attached to the cock on the water- pipe, and starts, or shuts off, a current passing from a store-tank or reservoir to a decomposing vessel full of carbide. It is also possible to make X work some releasing gear which permits carbide to fall into water--details of this arrangement are given later on. In A^1 the water in the tank serves as a holder seal only, a separate quantity being employed for the purposes of the chemical reaction. This arrangement has the advantage that the holder water lasts indefinitely, except for evaporation in hot weather, and therefore it may be prevented from freezing by dissolving in it some suitable saline body, or by mixing with it some suitable liquid which lowers its point of solidification. It will be observed, too, that in A^1 the pin X, which derives its motive power from the surplus weight of the falling bell, has always precisely the same amount of work to do, viz., to overcome the friction of the plug of the water-cock in its barrel. Hence at all times the pressure obtaining in the service-pipe is uniform, except for a slight jerk momentarily given each time the cock is opened or closed. When X actuates a carbide-feed arrangement, the work it does may or may not vary on different occasions, as will appear hereafter. In A^2 the bell itself carries a perforated basket of carbide, which is submerged in the water when the bell falls, and lifted out again when it rises. As the carbide is thus wetted from below, the lower portion of the mass soon becomes a layer of damp slaked lime, for although the basket is raised completely above the water-level, much liquid adheres to the spent carbide by capillary attraction. Hence, even when the basket is out of the water, acetylene is being produced, and it is produced in circumstances which prevent any control over the temperature attained. The water clinging to the lower part of the basket is vaporised by the hot, half-spent carbide, and the steam attacks the upper part, so that polymerisation of the gas and baking of the carbide are inevitable. In the second place, the pressure in the service-pipe attached to A^2 depends as before upon the net weight of the holder bell; but here that net weight is made up of the weight of the bell itself, that of the basket, and that of the carbide it contains. Since the carbide is being gradually converted into damp slaked lime, it increases in weight to an indeterminate extent as the generator in exhausted; but since, on the other hand, some lime may be washed out of the basket each time it is submerged, and some of the smaller fragments of carbide may fall through the perforations, the basket tends to decrease in weight as the generator is exhausted. Thus it happens in A^2 that the combined weight of bell plus basket plus contents is wholly indefinite, and the pressure in the service becomes so irregular that a separate governor must be added to the installation before the burners can be expected to behave properly. In the third place, the water in the tank serves both for generation and for decomposition, and this involves the employment of some arrangement to keep its level fairly constant lest the bell should become unsealed, while protection from frost by saline or liquid additions is impossible. A^2 is known popularly as a "dipping" generator, and it will be seen to be defective mechanically and bad chemically. In both A^1 and A^2 the bell is constructed of thin sheet- metal, and it is cylindrical in shape; the mass of metal in it is therefore negligible in comparison with the mass of water in the tank, and so the level of the liquid is sensibly the same whether the bell be high or low. In A^3 the interior of the bell is fitted with a circular plate which cuts off its upper corners and leaves a circumferential space S triangular in vertical section. This space is always full of air, or air and water, and has to be deducted from the available storage capacity of the bell. Supposing the bell transparent, and viewing it from above, its effective clear or internal diameter will be observed to be smaller towards the top than near the bottom; or since the space S is closed both against the water and against the gas, the walls of the bell may be said to be thicker near its top. Thus it happens that as the bell descends into the water past the lower angle of S, it begins to require more space for itself in the tank, and so it displaces the water until the levels rise. When high, as shown in the sketch marked A^3(a), the water-level is at l, below the mouth of a pipe P; but when low, as in A^3(b), the water is raised to the point l', which is above P. Water therefore flows into P, whence it reaches the carbide in an attached decomposing chamber. Here also the water in the tank is used for decomposition as well as for sealing purposes, and its normal level must be maintained exactly at l, lest the mouth of P should not be covered whenever the bell falls.

The devices employed to render a generator automatic which depend upon pressure (B) are of three main varieties: (B^1) the water-level in the decomposing chamber may be depressed by the pressure therein until its surface falls below a stationary mass of carbide; (B^2) the level in a water-store tank may be depressed until it falls below the mouth of a pipe leading to the carbide vessel; (B^3) the current of water passing down a pipe to the decomposing chamber may be interrupted by the action of a pressure superior to the force of gravitation. These arrangements are indicated roughly in Fig. 2. In B^1, D is a hollow cylinder closed at all points except at the cock G and the hole E, which are always below the level of the water in the annulus F, the latter being open to the air at its top. D is rigidly fastened to the outer vessel F so that it cannot move vertically, and the carbide cage is rigidly fastened to D. Normally the water-levels are at l, and the liquid has access to the carbide through perforations in the basket. Acetylene is thus produced; but if G is shut, the gas is unable to escape, and so it presses downwards upon the water until the liquid falls in D to the dotted line l", rising in F to the dotted line l'. The carbide is then out of water, and except for after-generation, evolution of gas ceases. On opening G more or less fully, the water more or less quickly reaches its original position at l, and acetylene is again produced. Manifestly this arrangement is identical with that of A^2 as regards the periodical immersion of the carbide holder in the liquid; but it is even worse than the former mechanically because there is no rising holder in B^1, and the pressure in the service is never constant. B^2 represents the water store of an unshown generator which works by pressure. It consists of a vessel divided vertically by means of a partition having a submerged hole N. One-half, H, is cloned against the atmosphere, but communicates with the gas space of the generator through L; the other half, K, is open to the air. M is a pipe leading water to the carbide. When gas is being burnt as fast as, or faster than, it is being evolved, the pressure in the generator is small, the level of the water stands at l, and the mouth of M is below it. When the pressure rises by cessation of consumption, that pressure acts through L upon the water in H, driving it down in H and up in K till it takes the positions l", and l', the mouth of M being then above the surface. It should be observed that in the diagrams B^1 and B^3, the amount of pressure, and the consequent alteration in level, is grossly exaggerated to gain clearness; one inch or less in both cases may be sufficient to start or retard evolution of acetylene. Fig. B^3 is somewhat ideal, but indicates the principle of opposing gas pressure to a supply of water depending upon gravitation; a method often adopted in the construction of portable acetylene apparatus. The arrangement consists of an upper tank containing water open to the air, and a lower vessel holding carbide closed everywhere except at the pipe P, which leads to the burners, and at the pipe S, which introduces water from the store-tank. If the cock at T is closed, pressure begins to rise in the carbide holder until it is sufficient to counterbalance the weight of the column of water in the pipe S, when a further supply is prevented until the pressure sinks again. This idea is simply an application of the displacement-holder principle, and as such is defective (except for vehicular lamps) by reason of lack of uniformity in pressure.

DISPLACEMENT GASHOLDERS.--An excursion may here be made for the purpose of studying the action of a displacement holder, which in its most elementary form is shown at C. It consists of an upright vessel open at the top, and divided horizontally into two equal portions by a partition, through which a pipe descends to the bottom of the lower half. At the top of the closed lower compartment a tube is fixed, by means of which gas can be introduced below the partition. While the cock is open to the air, water is poured in at the open top till the lower compartment is completely full, and the level of the liquid is at l. If now, gas is driven in through the side tube, the water is forced downwards in the lower half, up through the depending pipe till it begins to fill the upper half of the holder, and finally the upper half is full of water and the lower half of gas an shown by the levels l' and l". But the force necessary to introduce gas into such an apparatus, which conversely is equal to the force with which the apparatus strives to expel its gaseous contents, measured in inches of water, is the distance at any moment between the levels l' and l"; and as these are always varying, the effective pressure needed to fill the apparatus, or the effective pressure given by the apparatus, may range from zero to a few inches less than the total height of the whole holder. A displacement holder, accordingly, may be used either to store a varying quantity of gas, or to give a steady pressure just above or just below a certain desired figure; but it will not serve both purposes. If it is employed as a holder, it in useless as a governor or pressure regulator; if it is used as a pressure regulator, it can only hold a certain fixed volume of gas. The rising holder, which is shown at A^1 in Fig. 1 (neglecting the pin X, &c.) serves both purposes simultaneously; whether nearly full or nearly empty, it gives a constant pressure--a pressure solely dependent upon its effective weight, which may be increased by loading its crown or decreased by supporting it on counterpoises to any extent that may be required. As the bell of a rising holder moves, it must be provided with suitable guides to keep its path vertical; these guides being arranged symmetrically around its circumference and carried by the tank walls. A fixed control rod attached to the tank over which a tube fastened to the bell slides telescope-fashion is sometimes adopted; but such an arrangement is in many respects less admirable than the former.

Two other devices intended to give automatic working, which are scarcely capable of classification among their peers, may be diagrammatically shown in Fig. 3. The first of these (D) depends upon the movements of a flexible diaphragm. A vessel (a) of any convenient size and shape is divided into two portions by a thin sheet of metal, leather, caoutchouc, or the like. At its centre the diaphragm is attached by some air-tight joint to the rod c, which, held in position by suitable guides, is free to move longitudinally in sympathy with the diaphragm, and is connected at its lower extremity with a water-supply cock or a carbide-feed gear. The tube e opens at its base into the gas space of the generator, so that the pressure below the diaphragm in a is the same as that elsewhere in the apparatus, while the pressure in a above the diaphragm is that of the atmosphere. Being flexible and but slightly stretched, the diaphragm is normally depressed by the weight of c until it occupies the position b; but if the pressure in the generator (i.e., in e) rises, it lifts the diaphragm to somewhat about the position b'--the extent of movement being, as usual, exaggerated in the sketch. The movement of the diaphragm is accompanied by a movement of the rod c, which can be employed in any desirable way. In E the bell of a rising holder of the ordinary typo is provided with a horizontal striker which, when the bell descends, presses against the top of a bag g made of any flexible material, such as india-rubber, and previously filled with water. Liquid is thus ejected, and may be caused to act upon calcium carbide in some adjacent vessel. The sketch is given because such a method of obtaining an intermittent water-supply has at one time been seriously proposed; but it is clearly one which cannot be recommended.

ACTION OF WATER-TO-CARBIDE GENERATORS.--Having by one or other of the means described obtained a supply of water intermittent in character, it remains to be considered how that supply may be made to approach the carbide in the generator. Actual acetylene apparatus are so various in kind, and merge from one type to another by such small differences, that it is somewhat difficult to classify them in a simple and intelligible fashion. However, it may be said that water-to-carbide generators, i.e., such as employ water as the moving material, may be divided into four categories: (F^1) water is allowed to fall as single drops or as a fine stream upon a mass of carbide--this being the "drip" generator; (F^2) a mass of water is made to rise round and then recede from a stationary vessel containing carbide--this being essentially identical in all respects save the mechanical one with the "dip" or "dipping" generator shown in A^2, Fig. 1; (F^3) a supply of water is permitted to rise round, or to flow upon, a stationary mass of carbide without ever receding from the position it has once assumed--this being the "contact" generator; and (F^4) a supply of water is admitted to a subdivided charge of carbide in such proportion that each quantity admitted is in chemical excess of the carbide it attacks. With the exception of F^2, which has already been illustrated as A^2 Fig. 1, or as B^1 in Fig. 2, these methods of decomposing carbide are represented in Figs. 4 and 5. It will be observed that whereas in both F^1 and F^3 the liberated acetylene passes off at the top of the apparatus, or rather from the top of the non-subdivided charge of carbide, in F^1 the water enters at the top, and in F^3 it enters at the bottom. Thus it happens that the mixture of acetylene and steam, which is produced at the spot where the primary chemical reaction is taking place, has to travel through the entire mass of carbide present in a generator belonging to type F^3, while in F^1 the damp gas flows directly to the exit pipe without having to penetrate the lumps of solid. Both F^1 and F^3 exhibit after-generation caused by a reaction between the liquid water mechanically clinging to the mass of spent lime and the excess of carbide to an approximately equal extent; but for the reason just mentioned, after-generation due to a reaction between the vaporised water accompanying the acetylene first evolved and the excess of carbide is more noticeable in F^3 than in F^1; and it is precisely this latter description of after-generation which leads to overheating of the most ungovernable kind. Naturally both F^1 and F^3 can be fitted with water jackets, as is indicated by the dotted lines in the second sketch; but unless the generating chamber in quite small and the evolution of gas quite slow, the cooling action of the jacket will not prove sufficient. As the water in F^1 and F^3 is not capable of backward motion, the decomposing chambers cannot be employed as displacement holders, as is the case in the dipping generator pictured at B^1, Fig. 2. They must be coupled, accordingly, to a separate holder of the displacement or, preferably, of the rising type; and, in order that the gas evolved by after-generation may not be wasted, the automatic mechanism must cut off the supply of water to the generator by the time that holder is two-thirds or three-quarters full.

The diagrams G, H, and K in Figs. 4 and 5 represent three different methods of constructing a generator which belongs either to the contact type (F^3) if the supply of water is essentially continuous, i.e., if less is admitted at each movement of the feeding mechanism than is sufficient to submerge the carbide in each receptacle; or to the flooded- compartment type (F') if the water enters in large quantities at a time. In H the main carbide vessel is arranged horizontally, or nearly so, and each partition dividing it into compartments is taller than its predecessor, so that the whole of the solid in (1) must be decomposed, and the compartment entirely filled with liquid before it can overflow into (2), and so on. Since the carbide in all the later receptacles is exposed to the water vapour produced in that one in which decomposition is proceeding at any given moment, at least at its upper surface, some after-generation between vapour and carbide occurs in H; but a partial control over the temperature may be obtained by water-jacketing the container. In G the water enters at the base and gas escapes at the top, the carbide vessels being disposed vertically; hero, perhaps, more after- generation of the same description occurs, as the moist gas streams round and over the higher baskets. In K, the water enters at the top and must completely fill basket (1) before it can run down the depending pipe into (2); but since the gas also leaves the generator at the top, the later carbide receptacles do not come in contact with water vapour, but are left practically unattacked until their time arrives for decomposition by means of liquid water. K, therefore, is the best arrangement of parts to avoid after-generation, overheating, and polymerisation of the acetylene whether the generator be worked as a contact or as a flooded-compartment apparatus; but it may be freely admitted that the extent of the overheating due to reaction between water vapour and carbide may be kept almost negligible in either K, H, or G, provided the partitions in the carbide container be sufficient in number--provided, that is to say, that each compartment holds a sufficiently small quantity of carbide; and provided that the quantity of water ultimately required to fill each compartment is relatively so large that the temperature of the liquid never approaches the boiling-point where vaporisation is rapid. The type of generator indicated by K has not become very popular, but G is fairly common, whilst H undoubtedly represents the apparatus which is most generally adopted for use in domestic and other private installations in the United Kingdom and the Continent of Europe. The actual generators made according to the design shown by H usually have a carbide receptacle designed in the form of a semi-cylindrical or rectangular vessel of steel sliding fairly closely into an outside container, the latter being either built within the main water space of the entire apparatus or placed within a separate water-jacketed casing. Owing to its shape and the sliding motion with which the carbide receptacle is put into the container these generators are usually termed "drawer" generators. In comparison with type G, the drawer generator H certainly exhibits a lower rise in temperature when gas is evolved in it at a given speed and when the carbide receptacles are constructed of similar dimensions. It is very desirable that the whole receptacle should be subdivided into a sufficient number of compartments and that it should be effectively water-cooled from outside. It would also be advantageous if the water- supply were so arranged that the generator should be a true flooded- compartment apparatus, but experience has nevertheless shown that generators of type H do work very well when the water admitted to the carbide receptacle, each time the feed comes into action, is not enough to flood the carbide in one of the compartments. Above a certain size drawer generators are usually constructed with two or even more complete decomposing vessels, arrangements being such that one drawer can be taken out for cleaning, whilst the other is in operation. When this is the case a third carbide receptacle should always be employed so that it may be dry, lit to receive a charge of carbide, and ready to insert in the apparatus when one of the others is withdrawn. The water-feed should always be so disposed that the attendant can see at a glance which of the two (or more) carbide receptacles is in action at any moment, and it should be also so designed that the supply is automatically diverted to the second receptacle when the first is wholly exhausted and back again to the first (unless there are more than two) when the carbide in the second is entirely gasified. In the sketches G, H, and K, the total space occupied by the various carbide receptacles is represented as being considerably smaller than the capacity of the decomposing chamber. Were this method of construction copied in actual acetylene apparatus, the first makes of gas would be seriously (perhaps dangerously) contaminated with air. In practice the receptacles should fit so tightly into the outer vessel and into one another that when loaded to the utmost extent permissible--space being left for the swelling of the charge and for the passage of water and gas--but little room should be left for the retention of air in the chamber.

ACTION OF CARBIDE-TO-WATER GENERATORS.--The methods which may be adopted to render a generator automatic when carbide is employed as the moving material are shown at M, N, and P, in Fig. 6; but the precise devices used in many actual apparatus are so various that it is difficult to portray them generically. Moreover it is desirable to subdivide automatic carbide-to-water generators, according to the size of the carbide they are constructed to take, into two or three classes, which are termed respectively "large carbide-feed," "small carbide-feed," and "granulated carbide-feed" apparatus. (The generator represented at L does not really belong to the present class, being non-automatic and fed by hand; but the sketch is given for completeness.) M is an automatic carbide-feed generator having its store of carbide in a hopper carried by the rising- holder bell. The hopper is narrowed at its mouth, where it is closed by a conical or mushroom valve d supported on a rod held in suitable guides. When the bell falls by consumption of gas, it carries the valve and rod with it; but eventually the button at the base of c strikes the bottom of the generator, or some fixed distributing plate, and the rod can descend no further. Then, when the bell falls lower, the mushroom d rises from its seat, and carbide drops from the hopper into the water. This type of apparatus has the defect characteristic of A^2, Fig. 1; for the pressure in the service steadily diminishes as the effective weight of bell plus hopper decreases by consumption of carbide. But it has also two other defects--(1) that ordinary carbide is too irregular in shape to fall smoothly through the narrow annular space between the valve and its seat; (2) that water vapour penetrates into the hopper, and liberates some gas there, while it attacks the lumps of carbide at the orifice, producing dust or causing them to stick together, and thus rendering the action of the feed worse than ever. Most of these defects can be avoided by using granulated carbide, which is more uniform in size and shape, or by employing a granulated and "treated" carbide which has been dipped in some non-aqueous liquid to make it less susceptible to the action of moisture. Both these plans, however, are expensive to adopt; first, because of the actual cost of granulating or "treating" the carbide; secondly, because the carbide deteriorates in gas-making capacity by its inevitable exposure to air during the granulating or "treating" process. The defects of irregularity of pressure and possible waste of gas by evolution in the hopper may be overcome by disposing the parts somewhat differently; making the holder an annulus round the hopper, or making it cylindrical with the hopper inside. In this case the hopper is supported by the main portion of the apparatus, and does not move with the bell: the rod and valve being given their motion in some fashion similar to that figured. Apparatus designed in accordance with the sketch M, or with the modification just described, are usually referred to under the name of "hopper" generators. On several occasions trouble has arisen during their employment owing to the jamming of the valve, a fragment of carbide rather larger than the rest of the material lodging between the lips of the hopper and the edges of the mushroom valve. This has been followed by a sudden descent of all the carbide in the store into the water beneath, and the evolution of gas has sometimes been too rapid to pass away at the necessary speed into the holder. The trouble is rendered even more serious should the whole charge of carbide fall at a time when, by neglect or otherwise, the body of the generator contains much lime sludge, the decomposition then proceeding under exceptionally bad circumstances, which lead to the production of an excessively high temperature. Hopper generators are undoubtedly very convenient for certain purposes, chiefly, perhaps, for the construction of table-lamps and other small installations. Experience tends to show that they may be employed, first, provided they are designed to take granulated carbide--which in comparison with larger grades is much more uniform and cylindrical in shape--and secondly, provided the quantity of carbide in the hopper does not exceed a few pounds. The phenomenon of the sudden unexpected descent of the carbide, popularly known as "dumping," can hardly be avoided with carbide larger in size than the granulated variety; and since the results of such an accident must increase in severity with the size of the apparatus, a limit in their capacity is desirable.

When it is required to construct a carbide-feed generator of large size or one belonging to the large carbide-feed pattern, it is preferable to arrange the store in a different manner. In N the carbide is held in a considerable number of small receptacles, two only of which are shown in the drawing, provided with detachable lids and hinged bottoms kept shut by suitable catches. At proper intervals of time those catches in succession are knocked on one side by a pin, and the contents of the vessel fall into the water. There are several methods available for operating the pins. The rising-holder bell may be made to actuate a train of wheels which terminate in a disc revolving horizontally on a vertical axis somewhere just below the catches; and this wheel may bear an eccentric pin which hits each catch as it rotates. Alternatively the carbide boxes may be made to revolve horizontally on a vertical axis by the movements of the bell communicated through a clutch; and thus each box in succession may arrive at a certain position where the catch is knocked aside by a fixed pin. The boxes, again, may revolve vertically on a horizontal axis somewhat like a water-wheel, each box having its bottom opened, or, by a different system of construction, being bodily upset, when it arrives at the bottom of its circular path. In no case, however, are the carbide receptacles carried by the bell, which is a totally distinct part of the apparatus; and therefore in comparison with M, the pressure given by the bell is much more uniform. Nevertheless, if the system of carbide boxes moves at all, it becomes easier to move by decrease in weight and consequent diminution in friction as the total charge is exhausted; and accordingly the bell has less work to do during the later stages of its operation. For this reason the plan actually shown at N is preferable, since the work done by the moving pin, i.e., by the descending bell, is always the same. P represents a carbide-feed effected by a spiral screw or conveyor, which, revolved periodically by a moving bell, draws carbide out of a hopper of any desired size and finally drops it into a shoot communicating with a generating chamber such as that shown in L. Here the work done by the bell is large, as the friction against the blades of the screw and the walls of the horizontal tube is heavy; but that amount of work must always be essentially identical. The carbide-feed may similarly be effected by means of some other type of conveyor instead of the spiral screw, such as an endless band, and the friction in these cases may be somewhat less than with the screw, but the work to be done by the bell will always remain large, whatever type of conveyor may be adopted. A further plan for securing a carbide-feed consists in employing some extraneous driving power to propel a charge of carbide out of a reservoir into the generator. Sometimes the propulsive effort is obtained from a train of clockwork, sometimes from a separate supply of water under high pressure. The clockwork or the water power is used either to drive a piston travelling through the vessel containing the carbide so that the proper quantity of material is dropped over the open mouth of a shoot, or to upset one after another a series of carbide receptacles, or to perform some analogous operation. In these cases the pin or other device fitted to the acetylene apparatus itself has nothing to do beyond releasing the mechanism in question, and therefore the work required from the bell is but small. The propriety of employing a generator belonging to these latter types must depend upon local conditions, e.g., whether the owner of the installation has hydraulic power on a small scale (a constant supply of water under sufficient pressure) at disposal, or whether he does not object to the extra labour involved in the periodical winding up of a train of clockwork.

It must be clear that all these carbide-feed arrangements have the defect in a more or less serious degree of leaving the carbide in the main storage vessel exposed to the attack of water vapour rising from the decomposing chamber, for none of the valves or operating mechanism can be made quite air-tight. Evolution of gas produced in this way does not matter in the least, because it is easy to return the gas so liberated into the generator or into the holder; while the extent of the action, and the consequent production of overheating, will tend to be less than in generators such as those shown in G and H of Figs. 4 and 5, inasmuch as the large excess of water in the carbide-feed apparatus prevents the liquid arriving at a temperature at which it volatilises rapidly. The main objection to the evolution of gas in the carbide vessel of a carbide-to-water generator depends on the danger that the smooth working of the feed-gear may be interfered with by the formation of dust or by the aggregation of the carbide lumps.

USE OF OIL IN GENERATORS.--Calcium carbide is a material which is only capable of attack for the purpose of evolving acetylene by a liquid that is essentially water, or by one that contains some water mixed with it. Oils and the like, or even such non-aqueous liquids as absolute alcohol, have no effect upon carbide, except that the former naturally make it greasy and somewhat more difficult to moisten. This last property has been found of service in acetylene generation, especially on the small scale; for if carbide is soaked in, or given a coating of, some oil, fat, or solid hydrocarbon like petroleum, cocoanut oil, or paraffin wax, the substance becomes comparatively indifferent towards water vapour or the moisture present in the air, while it still remains capable of complete, albeit slow, decomposition by liquid water when completely immersed therein. The fact that ordinary calcium carbide is attacked so quickly by water is really a defect of the substance; for it is to this extreme rapidity of reaction that the troubles of overheating are due. Now, if the basket in the generator B^1 of Fig. 2, or, indeed, the carbide store in any of the carbide-to-water apparatus, is filled with a carbide which has been treated with oil or wax, as long as the water-level stands at l' and l" or the carbide still remains in the hopper, it is essentially unattacked by the vapour arising from the liquid; but directly the basket is submerged, or the lumps fall into the water, acetylene is produced, and produced more slowly and regularly than otherwise. Again, oils do not mix with water, but usually float thereon, and a mass of water covered by a thick film or layer of oil does not evaporate appreciably. If, now, a certain quantity of oil, say lamp paraffin or mineral lubricating oil, is poured on to the water in B^1, Fig. 2, it moves upwards and downwards with the water. When the water takes the position l, the oil is driven upwards away from the basket of carbide, and acetylene is generated in the ordinary manner; but when the water falls to l" the oil descends also, rinses off much of the adhering water from the carbide lumps, covers them with a greasy film, and almost entirely stops generation till it is in turn washed off by the next ascent of the water. Similarly, if the carbide in generators F, G, and H (also K) has been treated with a solid or semi-solid grease, it is practically unattacked by the stream of warm damp gas, and is only decomposed when the liquid itself arrives in the basket. For the same reason treated carbide can be kept for fairly long periods of time, even in a drum with badly fitting lid, without suffering much deterioration by the action of atmospheric moisture. The problem of acetylene generation is accordingly simplified to a considerable degree by the use of such treated carbide, and the advantage becomes more marked as the plant decreases in size till a portable apparatus is reached, because the smaller the installation the more relatively expensive or inconvenient is a large holder for surplus gas. The one defect of the method is the extra cost of such treated carbide; and in English conditions ordinary calcium carbide is too expensive to permit of any additional outlay upon the acetylene if it is to compete with petroleum or the product of a tiny coal-gas works. The extra cost of using treated carbide falls upon the revenue account, and is much more noticeable than that of a large holder, which is capital expenditure. When fluid oil is employed in a generator of type B^1, evolution of gas becomes so regular that any holder beyond the displacement one which the apparatus itself constitutes is actually unnecessary, though still desirable; but B^1, with or without oil, still remains a displacement apparatus, and as such gives no constant pressure. It must be admitted that the presence of oil so far governs the evolution of gas that the movement of the water, and the consequent variation of pressure, is rendered very small; still a governor or a rising holder would be required to give the best result at the burners. One point in connexion with the use of liquid oil must not be overlooked, viz., the extra trouble it may give in the disposal of the residues. This matter will be dealt with more fully in Chapter V.; here it is sufficient to say that as the oil does not mix with the water but floats on the surface, care has to be taken that it is not permitted to enter any open stream. The foregoing remarks about the use of oil manifestly only apply to those cases where it is used in quantity and where it ultimately becomes mixed with the sludge or floats on the water in the decomposing chamber. The employment of a limpid oil, such as paraffin, as an intermediate liquid into which carbide is introduced on its way to the water in the decomposing vessel of a hand-fed generator in the manner described on page 70 is something quite different, because, except for trifling losses, one charge of oil should last indefinitely.

RISING GASHOLDERS.--Whichever description of holder is employed in an acetylene apparatus, the gas is always stored over, or in contact with, a liquid that is essentially water. This introduces three subjects for consideration: the heavy weight of a large body of liquid, the loss of gas by dissolution in that liquid, and the protection of that liquid from frost in the winter. The tanks of rising holders are constructed in two different ways. In one the tank is a plain cylindrical vessel somewhat larger in diameter than the bell which floats in it; and since there must be nearly enough water in the tank to fill the interior of the bell when the latter assumes its lowest position, the quantity of water is considerable, its capacity for dissolving acetylene is large, and the amount of any substance that may have to be added to it to lower its freezing-point becomes so great as to be scarcely economical. All these defects, including that of the necessity for very substantial foundations under the holder to support its enormous weight, may be overcome by adopting the second method of construction. It is clear that the water in the centre of the tank is of no use,--all that is needed being a narrow trough for the bell to work in. Large rising holders are therefore advantageously built with a tank formed in the shape of an annulus, the effective breadth of which is not more than 2 or 3 inches, the centre portion being roofed over so as to prevent escape of gas. The same principle may be retained with modified details by fitting inside a plain cylindrical tank a "dummy" or smaller cylinder, closed by a flat or curved top and fastened water- and air-tight to the bottom of the main vessel. The construction of annular tanks or the insertion of a "dummy" may be attended with difficulty if the tank is wholly or partly sunk below the ground level, owing to the lifting force of water in the surrounding soil. Where a steel tank is sunk, or a masonry tank is constructed, regard must be paid, both in the design of the tank and in the manner of construction, to the level of the underground water in the neighbourhood, as in certain cases special precautions will be needed to avoid trouble from the pressure of the water on the outside of the tank until it is balanced by the pressure of the water with which the tank is filled. So far as mere dissolution of gas is concerned, the loss may be reduced by having a circular disc of wood, &c., a little smaller in diameter than the boll, floating on the water of a plain tank.

EFFECT OF STORAGE IN GASHOLDER ON ACETYLENE.--It is perfectly true, as has been stated elsewhere, that the gas coming from an acetylene generator loses some of its illuminating power if it is stored over water for any great length of time; such loss being given by Nichols as 94 per cent, in five months, and having been found by one of the authors as 0.63 per cent. per day--figures which stand in fair agreement with one another. This wastage is not due to any decomposition of the acetylene in contact with water, but depends on the various solubilities of the different gases which compose the product obtained from commercial calcium carbide. Inasmuch as an acetylene evolved in the best generator contains some foreign ingredients, and inasmuch as an inferior product contains more (cf. Chapter V.), the contents of a holder are never pure; but as those contents are principally made up of acetylene itself, that gas stands at a higher partial pressure in the holder than the impurities. Since acetylene is more soluble in water than any of its diluents or impurities, sulphuretted hydrogen and ammonia excepted, and since the solubility of all gases increases as the pressure at which they are stored rises, the true acetylene in an acetylene holder dissolves in the water more rapidly and comparatively more copiously than the impurities; and thus the acetylene tends to disappear and the impurities to become concentrated within the bell. Simultaneously at the outer part of the seal, air is dissolved in the water; and by processes of diffusion the air so dissolved passes through the liquid from the outside to the inside, where it escapes into the bell, while the dissolved acetylene similarly passes from the inside to the outside of the seal, and there mingles with the atmosphere. Thus, the longer a certain volume of acetylene is stored over water, the more does it become contaminated with the constituents of the atmosphere and with the impurities originally present in it; while as the acetylene is much more soluble than its impurities, more gas escapes from, than enters, the holder by diffusion, and so the bulk of stored gas gradually diminishes. However, the figures previously given show that this action is too slow to be noticeable in practice, for the gas is never stored for more than a few days at a time. The action cannot be accepted as a valid argument against the employment of a holder in acetylene plant. Such deterioration and wastage of gas may be reduced to some extent by the use of a film of some cheap and indifferent oil floating on the water inside an acetylene holder; the economy being caused by the lower solubility of acetylene in oils than in aqueous liquids not saturated with some saline material. Probably almost any oil would answer equally well, provided it was not volatile at the temperature of the holder, and that it did not dry or gum on standing, e.g., olive oil or its substitutes; but mineral lubricating oil is not so satisfactory. It is, however, not necessary to adopt this method in practice, because the solvent power of the liquid in the seal can be reduced by adding to it a saline body which simultaneously lowers its freezing-point and makes the apparatus more trustworthy in winter.

FREEZING OF GASHOLDER SEAL.--The danger attendant upon the congelation of the seal in an acetylene holder is very real, not so much because of the fear that the apparatus may be burst, which is hardly to be expected, as because the bell will be firmly fixed in a certain position by the ice, and the whole establishment lighted by the gas will be left in darkness. In these circumstances, hurried and perhaps injudicious attempts may be made to thaw the seal by putting red-hot bars into it or by lighting fires under it, or the generator-house may be thoughtlessly entered with a naked light at a time when the apparatus is possibly in disorder through the loss of storage-room for the gas it is evolving. Should a seal ever freeze, it must be thawed only by the application of boiling water; and the plant-house must be entered, if daylight has passed, in perfect darkness or with the assistance of an outside lamp whining through a closed window. [Footnote: By "closed window" is to be understood one incapable of being opened, fitted with one or two thicknesses of stout glass well puttied in, and placed in a wall of the house as far as possible from the door.] There are two ways of preventing the seal from freezing. In all large installations the generator-house will be fitted with a warm-water heating apparatus to protect the portion of the plant where the carbide is decomposed, and if the holder is also inside the same building it will naturally be safe. If it is outside, one of the flow-pipes from the warming apparatus should be led into and round the lowest part of the seal, care being taken to watch for, or to provide automatic arrangements for making good, loss of water by evaporation. If the holder is at a distance from the generator-house, or if for any other reason it cannot easily be brought into the warming circuit, the seal can be protected in another way; for unlike the water in the generator, the water in the holder-seal will perform its functions equally well however much it be reduced in temperature, always providing it is maintained in the liquid condition. There are numerous substances which dissolve in, or mix with, water, and yield solutions or liquids that do not solidify until their temperature falls far below that of the natural freezing- point. Assuming that those substances in solution do not attack the acetylene, nor the metal of which the holder is built, and are not too expensive, choice may be made between them at will. Strictly speaking the cost of using them is small, because unless the tank is leaky they last indefinitely, not evaporating with the water as it is vaporised into the gas or into the air. The water-seal of a holder standing within the generator-house may eventually become so offensive to the nostrils that the liquid has to be renewed; but when this happens it is due to the accumulation in the water of the water-soluble impurities of the crude acetylene. If, as should be done, the gas is passed through a washer or condenser containing much water before it enters the holder the sulphuretted hydrogen and ammonia will be extracted, and the seal will not acquire an obnoxious odour for a very long time.

Four principal substances have been proposed for lowering the freezing- point of the water in an acetylene-holder seal; common salt (sodium chloride), calcium chloride (not chloride of lime), alcohol (methylated spirit), and glycerin. A 10 per cent. solution of common salt has a specific gravity of 1.0734, and does not solidify above -6° C. or 21.2° F.; a 15 per cent. solution has a density of 1.111, and freezes at -10° C. or 14° F. Common salt, however, is not to be recommended, as its solutions always corrode iron and steel vessels more or less quickly. Alcohol, in its English denatured form of methylated spirit, is still somewhat expensive to use, but it has the advantage of not increasing the viscosity of the water; so that a frost-proof mixture of alcohol and water will flow as readily through minute tubes choked with needle- valves, or through felt and the like, or along wicks, as will plain water. For this reason, and for the practically identical one that it is quite free from dirt or insoluble matter, diluted spirit is specially suitable for the protection of the water in cyclists' acetylene lamps, [Footnote: As will appear in Chapter XIII., there is usually no holder in a vehicular acetylene lamp, all the water being employed eventually for the purpose of decomposing the carbide. This does not affect the present question. Dilute alcohol does not attack calcium carbide so energetically as pure water, because it stands midway between pure water and pure alcohol, which is inert. The attack, however, of the carbide is as complete as that of pure water, and the slower speed thereof is a manifest advantage in any holderless apparatus.] where strict economy is less important than smooth working. For domestic and larger installations it is not indicated. As between calcium chloride and glycerin there is little to choose; the former will be somewhat cheaper, but the latter will not be prohibitively expensive if the high-grade pure glycerins of the pharmacist are avoided. The following tables show the amount of each substance which must be dissolved in water to obtain a liquid of definite solidifying point. The data relating to alcohol were obtained by Pictet, and those for calcium chloride by Pickering. The latter are materially different from figures given by other investigators, and perhaps it would be safer to make due allowance for this difference. In Germany the Acetylene Association advocates a 17 per cent. solution of calcium chloride, to which Frank ascribes a specific gravity of 1.134, and a freezing-point of -8° C. or 17.6° F.

Freezing-Points of Dilute Alcohol. _________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| Alcohol. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 4.8 | 0.9916 | -2.0 | +28.4 |
| 11.3 | 0.9824 | 5.0 | 23.0 |
| 16.4 | 0.9761 | 7.5 | 18.5 |
| 18.8 | 0.9732 | 9.4 | 15.1 |
| 20.3 | 0.9712 | 10.6 | 12.9 |
| 22.1 | 0.9689 | 12.2 | 10.0 |
| 24.2 | 0.9662 | 14.0 | 6.8 |
| 26.7 | 0.9627 | 16.0 | 3.2 |
| 29.9 | 0.9578 | 18.9 | -2.0 |
|_______________|___________________|__________|__________|

Freezing-Points of Dilute Glycerin. _________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| Glycerin. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 10 | 1.024 | -1.0 | +30.2 |
| 20 | 1.051 | 2.5 | 27.5 |
| 30 | 1.075 | 6.0 | 21.2 |
| 40 | 1.105 | 17.5 | 0.5 |
| 50 | 1.127 | 31.3 | -24.3 |
|_______________|___________________|__________|__________|

Freezing-Points of Calcium Chloride Solutions. _________________________________________________________
| | | |
| Percentage of | Specific Gravity. | Freezing-point. |
| CaCl_2. | | |
|_______________|___________________|_____________________|
| | | | |
| | | Degs. C. | Degs. F. |
| 6 | 1.05 | -3.0 | +26.6 |
| 8 | 1.067 | 4.3 | 24.3 |
| 10 | 1.985 | 5.9 | 21.4 |
| 12 | 1.103 | 7.7 | 18.1 |
| 14 | 1.121 | 9.8 | 14.4 |
| 16 | 1.140 | 12.2 | 10.0 |
| 18 | 1.159 | 15.2 | 4.6 |
| 20 | 1.170 | 18.6 | -1.5 |
|_______________|___________________|__________|__________|

Calcium chloride will probably be procured in the solid state, but it can be purchased as a concentrated solution, being sold under the name of "calcidum" [Footnote: This proprietary German article is a liquid which begins to solidify at -42° C. (-43.6° F.), and is completely solid at -56° C. (-69)° F.). Diluted with one-third its volume of water, it freezes between -20° and -28° C. (-4° and-l8.4° F.). The makers recommend that it should be mixed with an equal volume of water. Another material known as "Gefrierschutzflüssigkeit" and made by the Flörsheim chemical works, freezes at -35° C. (-3° F.). Diluted with one-quarter its volume of water, it solidifies at -18° C. (-0.4° F.); with equal parts of water it freezes at -12° C. (10.4° F.). A third product, called "calcidum oxychlorid," has been found by Caro and Saulmann to be an impure 35 per cent. solution of calcium chloride. Not one of these is suitable for addition to the water used in the generating chamber of an acetylene apparatus, the reasons for this having already been mentioned.] for the protection of gasholder seals. Glycerin itself resembles a strong solution of calcium chloride in being a viscid, oily-looking liquid; and both are so much heavier than water that they will not mix with further quantities unless they are thoroughly agitated therewith. Either may be poured through water, or have water floated upon it, without any appreciable admixture taking place; and therefore in first adding them to the seal great care must be taken that they are uniformly distributed throughout the liquid. If the whole contents of the seal cannot conveniently be run into an open vessel in which the mixing can be performed, the sealing water must be drawn off a little at a time and a corresponding quantity of the protective reagent added to it. Care must be taken also that motives of economy do not lead to excessive dilution of the reagent; the seal must be competent to remain liquid under the prolonged influence of the most severe frost ever known to occur in the neighbourhood where the plant is situated. If the holder is placed out of doors in an exposed spot where heavy rains may fall on the top of the bell, or where snow may collect there and melt, the water is apt to run down into the seal, diluting the upper layers until they lose the frost- resisting power they originally had. This danger may be prevented by erecting a sloping roof over the bell crown, or by stirring up the seal and adding more preservative whenever it has been diluted with rain water. Quite small holders would probably always be placed inside the generator-house, where their seals may be protected by the same means as are applied to the generator itself. It need hardly be said that all remarks about the dangers incidental to the freezing of holder seals and the methods for obviating them refer equally to every item in the acetylene plant which contains water or is fitted with a water-sealed cover; only the water which is actually used for decomposing the calcium carbide cannot be protected from frost by the addition of calcium chloride or glycerin--that water must be kept from falling to its natural freezing-point. From Mauricheau-Beaupré's experiments, referred to on page 106, it would appear that a further reason for avoiding an addition of calcium chloride to the water used for decomposing carbide should lie in the danger of causing a troublesome production of froth within the generator.

It will be convenient to digress here for the purpose of considering how the generators of an acetylene apparatus themselves should be protected from frost; but it may be said at the outset that it is impossible to lay down any fixed rules applicable to all cases, since local conditions, such as climate, available resources, dimensions, and exposed or protected position of the plant-house vary so largely in different situations. In all important installations every item of the plant, except the holder, will be collected in one or two rooms of a single building constructed of brick or other incombustible material. Assuming that long-continued frost reigns at times in the neighbourhood, the whole of such a building, with the exception of one apartment used as a carbide store only, is judiciously fitted with a heating arrangement like those employed in conservatories or hothouses; a system of pipes in which warm water is kept circulating being run round the walls of each chamber near the floor. The boiler, heated with coke, paraffin, or even acetylene, must naturally be placed in a separate room of the apparatus-house having no direct (indoor) communication with the rooms containing the generators, purifiers, &c. Instead of coils of pipe, "radiators" of the usual commercial patterns may be adopted; but the immediate source of heat should be steam, or preferably hot water, and not hot air or combustion products from the stove. In exposed situations, where the holder is out of doors, one branch of the flow-pipe should enter and travel round the seal as previously suggested. Most large country residences are already provided with suitable heating apparatus for warming the greenhouses, and part of the heat may be capable of diversion into the acetylene generator-shed if the latter is erected in a convenient spot. In fact, if any existing hot-water warming appliances are already at hand, and if they are powerful enough to do a little more work, it may be well to put the generator-building in such a position that it can be efficiently supplied with artificial warmth from those boilers; for any extra length of main necessary to lead the gas into the residence from a distant generator will cost less on the revenue account than the fuel required to feed a special heating arrangement. In smaller installations, especially such as are to be found in mild climates, it may be possible to render the apparatus-house sufficiently frost-proof without artificial heat by building it partly underground, fitting it with a double skylight in place of a window for the entrance of daylight, and banking up its walls all round with thick layers of earth. The house must have a door, however, which must open outwards and easily, so that no obstacle may prevent a hurried exit in emergencies. Such a door can hardly be made very thick or double without rendering it heavy and difficult to open; and the single door will be scarcely capable of protecting the interior if the frost is severe and prolonged. Ventilators, too, must be provided to allow of the escape of any gas that may accidentally issue from the plant during recharging, &c.; and some aperture in the roof will be required for the passage of the vent pipe or pipes, which, in certain types of apparatus, move upwards and downwards with the bell of the holder. These openings manifestly afford facilities for the entry of cold air, so that although this method of protecting generator-houses has proved efficient in many places, it can only be considered inferior to the plan of installing a proper heating arrangement. Occasionally, where local regulations do not forbid, the entire generator-house may be built as a "lean-to" against some brick wall which happens to be kept constantly warm, say by having a furnace or a large kitchen stove on its other side.

In less complicated installations, where there are only two distinct items in the plant to be protected from frost--generator and holder--or where generator and holder are combined into one piece of apparatus, other methods of warming become possible. As the reaction between calcium carbide and water evolves much heat, the most obvious way of preventing the plant from freezing is to economise that heat, i.e., to retain as much of it as is necessary within the apparatus. Such a process, clearly, is only available if the plant is suitable in external form, is practically self-contained, and comprises no isolated vessels containing an aqueous liquid. It is indicated, therefore, rather for carbide-to- water generators, or for water-to-carbide apparatus in which the carbide chambers are situated inside the main water reservoir--any apparatus, in fact, where much water is present and where it is all together in one receptacle. Moreover, the method of heat economy is suited for application to automatic generators rather than to those belonging to the opposite system, because automatic apparatus will be generating gas, and consequently evolving heat, every evening till late at night--just at the time when frost begins to be severe. A non-automatic generator will usually be at work only in the mornings, and its store of heat will accordingly be much more difficult to retain till nightfall. With the object of storing up the heat evolved in the generator, it must be covered with some material possessed of the lowest heat-conducting power possible; and the proper positions for that material in order of decreasing importance are the top, sides, and bottom of the plant. The generator may either be covered with a thick layer of straw, carpet, flannel, or the like, as is done in the protection of exposed water- pipes; or it may be provided with a jacket filled with some liquid. In view of the advisability of not having any organic or combustible material near the generator, the solid substances just mentioned may preferably be replaced by one of those partially inorganic compositions sold for "lagging" steam-pipes and engine-cylinders, such as "Fossil meal." Indeed, the exact nature of the lagging matters comparatively little, because the active substance in retaining the heat in the acetylene generator or the steam-pipe is the air entangled in the pores of the lagging; and therefore the value of any particular material depends mainly on its exhibiting a high degree of porosity. The idea of fitting a water jacket round an acetylene generator is not altogether good, but it may be greatly improved upon by putting into the jacket a strong solution of some cheap saline body which has the property of separating from its aqueous solution in the form of crystals containing water of crystallisation, and of evolving much heat in so separating. This method of storing much heat in a small space where a fire cannot be lighted is in common use on some railways, where passengers' foot-warmers are filled with a strong solution of sodium acetate. When sodium acetate is dissolved in water it manifestly exists in the liquid state, and it is presumably present in its anhydrous condition (i.e., not combined with water of crystallisation). The common crystals are solid, and contain 3 molecules of water of crystallisation--also clearly in the solid state. Now, the reaction

NaC_2H_3O_2 + 3H_2O = NaC_2H_3O_2.3H_2O

(anhydrous acetate) (crystals)

evolves 4.37 calories (Berthelot), or 1.46 calorie for each molecule of water; and whereas 1 kilo. of water only evolves 1 large calorie of heat as its temperature falls 1° C., 18 grammes of water (1 gramme-molecule) evolve l.46 large calorie when they enter into combination with anhydrous sodium acetate to assist in forming crystals--and this 1.46 calorie may either be permitted to warm the mass of crystals, or made to do useful work by raising the temperature of some adjacent substance. Sodium acetate crystals dissolve in 3.9 parts by weight of water at 6° C. (43° F.) or in 2.4 parts at 37° C. (99° F.). If, then, a jacket round an acetylene apparatus is filled with a warm solution of sodium acetate crystals in (say) 3 parts by weight of water, the liquid will crystallise when it reaches some temperature between 99° and 43° F.; but when the generator comes into action, the heat liberated will change the mass of crystals into a liquid without raising its sensible temperature to anything like the extent that would happen were the jacket full of simple water. Not being particularly warm to the touch, the liquefied product in the jacket will not lose much heat by radiation, &c., into the surrounding air; but when the water in the generator falls again (after evolution of acetylene ceases) the contents of the jacket will also cool, and finally will begin to crystallise once more, passing a large amount of low-temperature heat into the water of the generator, and safely maintaining it for long periods of time at a temperature suitable for the further evolution of gas. Like the liquid in the seal of an isolated gasholder, the liquid in such a jacket will last indefinitely; and therefore the cost of the sodium acetate in negligible.

Another method of keeping warm the water in any part of an acetylene installation consists in piling round the apparatus a heap of fresh stable manure, which, as is well known, emits much heat as it rots. Where horses are kept, such a process may be said to cost nothing. It has the advantage over methods of lagging or jacketing that the manure can be thrown over any pipe, water-seal, washing apparatus, &c., even if the plant is constructed in several separate items. Unfortunately the ammonia and the volatile organic compounds which are produced during the natural decomposition of stable manure tend seriously to corrode iron and steel, and therefore this method of protecting an apparatus from frost should only be employed temporarily in times of emergency.

CORROSION IN APPARATUS.--All natural water is a solution of oxygen and may be regarded also as a weak solution of the hypothetical carbonic acid. It therefore causes iron to rust more or less quickly; and since no paint is absolutely waterproof, especially if it has been applied to a surface already coated locally with spots of rust, iron and steel cannot be perfectly protected by its aid. More particularly at a few inches above and below the normal level of the water in a holder, therefore, the metal soon begins to exhibit symptoms of corrosion which may eventually proceed until the iron is eaten away or becomes porous. One method of prolonging the life of such apparatus is to give it fresh coats of paint periodically; but unless the old layers are removed where they have cracked or blistered, and the rust underneath is entirely scraped off (which is practically impossible), the new paint films will not last very long. Another more elegant process for preserving any metal like iron which is constantly exposed to the attack of a corrosive liquid, and which is readily applicable to acetylene holders and their tanks, depends on the principle of galvanic action. When two metals in good electrical contact are immersed in some liquid that is capable of attacking both, only that metal will be attacked which is the more electro-positive, or which (the same thing in other words) is the more readily attacked by the liquid, evolving the more heat during its dissolution. As long as this action is proceeding, as long, that is, as some of the more electro- positive material is present, the less electro-positive material will not suffer. All that has to be done, therefore, to protect the walls of an acetylene-holder tank and the sides of its bell is to hang in the seal, supported by a copper wire fastened to the tank walls by a trustworthy electrical joint (soldering or riveting it), a plate or rod of some more electro-positive metal, renewing that plate or rod before it is entirely eaten away. [Footnote: Contact between the bell and the rod may be established by means of a flexible metallic wire; or a separate rod might be used for the bell itself.] If the iron is bare or coated with lead (paint may be overlooked), the plate may be zinc; if the iron is galvanised, i.e., coated with zinc, the plate may be aluminium or an alloy of aluminium and zinc. The joint between the copper wire and the zinc or aluminium plate should naturally be above the water-level. The foregoing remarks should be read in conjunction with what was said in Chapter II., about the undesirability of employing a soft solder containing lead in the construction of an acetylene generator. Here it is proposed intentionally to set up a galvanic couple to prevent corrosion; there, with the same object in view, the avoidances of galvanic action is counselled. The reason for this difference is self-evident; here a foreign metal is brought into electrical contact with the apparatus in order that the latter may be made electro-negative; but when a joint is soldered with lead, the metal of the generator is unintentionally made electro-positive. Here the plant is protected by the preferential corrosion of a cheap and renewable rod; in the former case the plant is encouraged to rust by the unnecessary presence of an improperly selected metal.

OTHER ITEMS IN GENERATING PLANT.--It has been explained in Chapter II. that the reaction between calcium carbide and water is very tumultuous in character, and that it occurs with great rapidity. Clearly, therefore, the gas comes away from the generator in rushes, passing into the next item of the plant at great speed for a time, and then ceasing altogether. The methods necessarily adopted for purifying the crude gas are treated of in Chapter V.; but it is manifest now that no purifying material can prove efficient unless the acetylene passes through it at a uniform rate, and at one which is as slow as other conditions permit. For this reason the proper position of the holder in an acetylene installation is before the purifier, and immediately after the condenser or washer which adjoins the generator. By this method of design the holder is filled up irregularly, the gas passing into it sometimes at full speed, sometimes at an imperceptible rate; but if the holder is well balanced and guided this is a matter of no consequence. Out of the holder, on the other hand, the gas issues at a rate which is dependent upon the number and capacity of the burners in operation at any moment; and in ordinary conditions this rate is so much more uniform during the whole of an evening than the rate at which the gas is evolved from the carbide, that a purifier placed after the holder is given a far better opportunity of extracting the impurities from the acetylene than it would have were it situated before the holder, as is invariably the case on coal-gas works.

For many reasons, such as capacity for isolation when being recharged or repaired, it is highly desirable that each item in an acetylene plant shall be separated, or capable of separation, from its neighbours; and this observation applies with great force to the holder and the decomposing vessel of the generator. In all large plants each vessel should be fitted with a stopcock at its inlet and, if necessary, one at its outlet, being provided also with a by-pass so that it can be thrown out of action without interfering with the rest of the installation. In the best practice the more important vessels, such as the purifiers, will be in duplicate, so that unpurified gas need not be passed into the service while a solitary purifier is being charged afresh. In smaller plants, where less skilled labour will probably be bestowed on the apparatus, and where hand-worked cocks are likely to be neglected or misused, some more, automatic arrangement for isolating each item is desirable. There are two automatic devices which may be employed for the purposes in view, the non-return valve and the water-seal. The non-return valve is simply a mushroom or ball valve without handle, lifted off its seat by gas passing from underneath whenever the pressure of the gas exceeds the weight of the valve, but falling back on to its seat and closing the pipe when the pressure decreases or when pressure above is greater than that below. The apparatus works perfectly with a clean gas or liquid which is not corrosive; but having regard to the possible presence of tarry products, lime dust, or sludge, condensed water loaded with soluble impurities, &c., in the acetylene, a non-return valve is not the best device to adopt, for both it and the hand-worked cock or screw- down valve are liable to stick and give trouble. The best arrangement in all respects, especially between the generator and the holder, is a water-seal. A water-seal in made by leading the mouth of a pipe delivering gas under the level of water in a suitable receptacle, so that the issuing gas has to bubble through the liquid. Gas cannot pass backwards through the pipe until it has first driven so much liquid before it that the level in the seal has fallen below the pipe's mouth; and if the end of the pipe is vertical more pressure than can possibly be produced in the apparatus is necessary to effect this. Omitting the side tube b, one variety of water-seal is shown at D in Fig. 7 on page 103. The water being at the level l, gas enters at a and bubbles through it, escaping from the apparatus at c. It cannot return from c to a without driving the water out of the vessel till its level falls from f to g; and since the area of the vessel is much greater than that of the pipe, so great a fall in the vessel would involve a far greater rise in a. It is clear that such a device, besides acting as a non-return valve, also fulfils two other useful functions: it serves to collect and retain all the liquid matter that may be condensed in the pipe a from the spot at which it was originally level or was given a fall to the seal, as well as that condensing in c as far as the spot where c dips again; and it equally acts as a washer to the gas, especially if the orifice g of the gas-inlet pipe is not left with a plain mouth as represented in the figure, but terminates in a large number of small holes, the pipe being then preferably prolonged horizontally, with minute holes in it so as to distribute the gas throughout the entire vessel. Such an apparatus requires very little attention. It may with advantage be provided with the automatic arrangement for setting the water-level shown at d and e. d is a tunnel tube extending almost to the bottom of the vessel, and e is a curved run-off pipe of the form shown. The lower part of the upper curve in e is above the level f, being higher than f by a distance equal to that of the gas pressure in the pipes; and therefore when water is poured into the funnel it fills the vessel till the internal level reaches f, when the surplus overflows of itself. The operation thus not only adjusts the quantity of water present to the desired level so that a cannot become unsealed, but it also renews the liquid when it has become foul and nearly saturated with dissolved and condensed impurities from the acetylene. It would be a desirable refinement to give the bottom of the vessel a slope to the mouth of e, or to some other spot where a large-bore draw-off cock could be fitted for the purpose of extracting any sludge of lime, &c., that may collect. By having such a water-seal, or one simpler in construction, between the generator and the holder, the former may be safely opened at any time for repairs, inspection, or the insertion of a fresh charge of carbide while the holder is full of gas, and the delivery of acetylene to the burners at a specified pressure will not be interrupted. If a cock worked by hand were employed for the separation of the holder from the generator, and the attendant were to forget to close it, part or all of the acetylene in the holder would escape from the generator when it was opened or disconnected.

Especially when a combined washer and non-return valve follows immediately after a generator belonging to the shoot type, and the mouth of the shoot is open to the air in the plant-house, it is highly desirable that the washer shall be fitted with some arrangement of an automatic kind for preventing the water level rising much above its proper position. The liquid in a closed washer tends to rise as the apparatus remains in use, water vapour being condensed within it and liquid water, or froth of lime, being mechanically carried forward by the stream of acetylene coming from the decomposing chamber. In course of time, therefore, the vertical depth to which the gas-inlet pipe in the washer is sealed by the liquid increases; and it may well be that eventually the depth in question, plus the pressure thrown by the holder bell, may become greater than the pressure which can be set up inside the generator without danger of gas slipping under the lower edge of the shoot. Should this state of things arise, the acetylene can no longer force its way through the washer into the holder bell, but will escape from the mouth of the shoot; filling the apparatus-house with gas, and offering every opportunity for an explosion if the attendant disobeys orders and takes a naked light with him to inspect the plant.

It is indispensable that every acetylene apparatus shall be fitted with a safety-valve, or more correctly speaking a vent-pipe. The generator must have a vent-pipe in case the gas-main leading to the holder should become blocked at any time, and the acetylene which continues to be evolved in all water-to-carbide apparatus, even after the supply of water has been cut off be unable to pass away. Theoretically a non-automatic apparatus does not require a vent-pipe in its generator because all the gas enters the holder immediately, and is, or should be, unable to return through the intermediate water seal; practically such a safeguard is absolutely necessary for the reason given. The holder must have a safety-valve in case the cutting-off mechanism of the generator fails to act, and more gas passes into it than it can store. Manifestly the pressure of the gas in a water-sealed holder or in any generator fitted with a water-sealed lid cannot rise above that corresponding with the depth of water in the seal; for immediately the pressure, measured in inches of water, equals the depth of the sealing liquid, the seal will be blown out, and the gas will escape. Such an occurrence, however, as the blowing of a seal must never be possible in any item of an acetylene plant, more especially in those items that are under cover, for the danger that the issuing gas might be fired or might produce suffocation would be extremely great. Typical simple forms of vent-pipe suitable for acetylene apparatus are shown in Fig. 7. In each case the pipe marked "vent" is the so-called safety-valve; it is open at its base for the entry of gas, and open at its top for the escape of the acetylene into the atmosphere, such top being in all instances carried through the roof of the generator-house into the open air, and to a spot distant from any windows of that house or of the residence, where it can prove neither dangerous nor a nuisance by reason of its odour. At A is represented the vent-pipe of a displacement vessel, which may either be part of a displacement holder or of a generator working on the displacement principle. The vent-pipe is rigidly fixed to the apparatus. If gas is generated within the closed portion of the holder or passes through it, and if the pressure so set up remains less than that which is needed to move the water from the level l to the levels l' and l", the mouth of the pipe is under water, and acetylene cannot enter it; but immediately such an amount of gas is collected, or such pressure is produced that the interior level sinks below l", which is that of the mouth of the pipe, it becomes unsealed, and the surplus gas freely escapes. There are two minor points in connexion with this form of vent-pipe often overlooked. At the moment when the water arrives at l" in the closed half of the apparatus, its level in the interior of the vent-pipe stands at l', identical with that in the open hall of the apparatus (for the mouth of the vent-pipe and the water in the open hall of the apparatus are alike exposed to the pressure of the atmosphere only). When the water, then, descends just below l" there is an amount of water inside the pipe equal in height to the distance between l' and l"; and before the acetylene can escape, it must either force this water as a compact mass out of the upper mouth of the vent-pipe (which it is clearly not in a position to do), drive it out of the upper mouth a little at a time, or bubble through it till the water is gradually able to run downwards out of the pipe as its lower opening is more fully unsealed. In practice the acetylene partly bubbles through this water and partly drives it out of the mouth of the pipe; on some occasions temporarily yielding irregular pressures at the burners which cause them to jump, and always producing a gurgling noise in the vent- pipe which in calculated to alarm the attendant. If the pipe is too small in diameter, and especially if its lower orifice is cut off perfectly horizontal and constricted slightly, the water may refuse to escape from the bottom altogether, and the pipe will fail to perform its allotted task. It is better therefore to employ a wide tube, and to cut off its mouth obliquely, or to give its lower extremity the shape of an inverted funnel. At the half of the central divided drawing marked B (Fig. 7) is shown a precisely similar vent-pipe affixed to the bell of a rising holder, which behaves in an identical fashion when by the rising of the bell its lower end is lifted out of the water in the tank. The features described above as attendant, upon the act of unsealing of the displacement-holder vent-pipe occur here also, but to a less degree; for the water remaining in the pipe at the moment of unsealing is only that which corresponds with the vertical distance between l' and l", and in a rising holder this is only a height always equal to the pressure given by the bell. Nevertheless this form of vent-pipe produces a gurgling noise, and would be better for a trumpet-shaped mouth. A special feature of the pipe in B is that unless it is placed symmetrically about the centre of the bell its weight tends to throw the bell out of the vertical, and it may have to be supported at its upper part; conversely, if the pipe is arranged concentrically in the bell, it may be employed as part of the guiding arrangement of the bell itself. Manifestly, as the pipe must be long enough to extend through the roof of the generator-house, its weight materially increases the weight of the bell, and consequently the gas pressure in the service; this fact is not objectionable provided due allowance is made for it. So tall a vent-pipe, however, seriously raises the centre of gravity of the bell and may make it top-heavy. To work well the centre of gravity of a holder bell should be as low as possible, any necessary weighting being provided symmetrically about its circumference and close to its bottom edge. The whole length of an ascending vent-pipe need not be carried by the rising bell, because the lower portion, which must be supported by the bell, can be arranged to slide inside a wider length of pipe which is fixed to the roof of the generator-house at the point where it passes into the open air.

A refinement upon this vent-pipe is represented at C, where it is rigidly fastened to the tank of the holder, and has its internal aperture always above the level of the water in the apparatus. Rigidly fixed to the crown of the bell is a tube of wider diameter, h, which is closed at its upper end. h is always full of gas, and its mouth is normally beneath the level of the water in the seal; but when the bell rises to its highest permissible position, the mouth of h comes above the water, and communication is opened between the holder and the outer atmosphere. No water enters the vent-pipe from the holder, and therefore no gurgling or irregular pressure is produced. Another excellent arrangement of a vent-pipe, suggested by Klinger of Gumpoldskirchen, is shown at D, a drawing which has already been partly considered as a washer and water-seal. For the present purpose the main vessel and its various pipes are so dimensioned that the vertical height g to f is always appreciably greater than the gas pressure in the service or in the generator or gasholder to which it is connected. In these circumstances the gas entering at a depresses the water in the pipe below the level f to an extent equal to the pressure at which it enters that pipe--an extent normally less than the distance f to g; and therefore gas never passes into the body of the vessel, but travels away by the side tube b (which in former references to this drawing was supposed to be absent). If, however, the pressure at a exceeds that of the vertical height f to g, gas escapes at g through the water, and is then free to reach the atmosphere by means of the vent c. As before, d serves to charge the apparatus with water, and e to ensure a proper amount being added. Clearly no liquid can enter the vent-pipe in this device. Safety-valves such as are added to steam-boilers and the like, which consist of a weighted lever holding a conical valve down against its seat, are not required in acetylene apparatus, for the simpler hydraulic seals discussed above can always be fitted wherever they may be needed. It should be noticed that these vent-pipes only come into operation in emergencies, when they are required to act promptly. No economy is to be effected by making them small in diameter. For obvious reasons the vent-pipe of a holder should have a diameter equal to that of the gas-inlet tube, and the vent-pipe of a generator be equal in size to the gas-leading tube.

FROTHING IN GENERATORS.--A very annoying trouble which crops up every now and then during the evolution of acetylene consists in the production of large masses of froth within the generator. In the ordinary way, decomposition of carbide is accompanied by a species of effervescence, but the bubbles should break smartly and leave the surface of the liquid reasonably free from foam. Sometimes, however, the bubbles do not break, but a persistent "head" of considerable height is formed. Further production of gas only increases the thickness of the froth until it rises so high that it is carried forward through the gas-main into the next item of the plant. The froth disappears gradually in the pipes, but leaves in them a deposit of lime which sooner or later causes obstructions by accumulating at the angles and dips; while during its presence in the main the steady passage of gas to the holder is interrupted and the burners may even be made to jump. Manifestly the defect is chiefly, if not always, to be noticed in the working of carbide-to-water generators. The phenomenon has been examined by Mauricheau-Beaupré, who finds that frothing is not characteristic of pure carbide and that it cannot be attributed to any of the impurities normally present in commercial carbide. If, however, the carbide contains calcium chloride, frothing is liable to occur. A 0.1 per cent. solution of calcium chloride appears to yield some foam when carbide is decomposed in it, and a 1 per cent. solution to foam in a pronounced manner. In the absence of calcium chloride, the main cause of frothing seems to be the presence in the generator of new paint or tar. If a generator is taken into use before the paint in any part of it which becomes moistened by warm lime-water has had opportunity of drying thoroughly hard, frothing is certain to occur; and even if the carbide has been stored for only a short time in a tin or drum which has been freshly painted, a production of froth will follow when it is decomposed in water. The products of the polymerisation of acetylene also tend to produce frothing, but not to such an extent as the turpentine in paint and the lighter constituents of coal-tar. Carbide stored even temporarily in a newly painted tin froths on decomposition because it has absorbed among its pores some of the volatile matter given off by the paint during the process of desiccation.

THE "DRY" PROCESS OF GENERATION.--A process for generating acetylene, totally different in principle from those hitherto considered, has been introduced in this country. According to the original patents of G. J. Atkins, the process consisted in bringing small or powdered carbide into mechanical contact with some solid material containing water, the water being either mixed with the solid reagent or attached to it as water of crystallisation. Such reagents indeed were claimed as crude starch and the like, the idea being to recover a by-product of pecuniary value. Now the process seems to be known only in that particular form in which granulated carbide is treated with crystallised sodium carbonate, i.e., common washing soda. Assuming the carbide employed to be chemically pure and the reaction between it and the water of crystallisation contained in ordinary soda crystals to proceed quantitatively, the production of acetylene by the dry process should be represented by the following chemical equation:

5CaC_2 + Na_2CO_3.10H_2O = 5C_2H_2 + 5Ca(OH)_2 + Na_2CO_3.

On calculating out the molecular weights, it will be seen that 286 parts of washing soda should suffice for the decomposition of 320 parts of pure calcium carbide, or in round numbers 9 parts of soda should decompose 10 parts of carbide. In practice, however, it seems to be found that from 1 to 1.5 parts of soda are needed for every part of carbide.

The apparatus employed is a metal drum supported on a hollow horizontal spindle, one end of which is closed and carries a winch handle, and the other end of which serves to withdraw the gas generated in the plant. The drum is divided into three compartments by means of two vertical partitions so designed that when rotation proceeds in one particular direction portions of the two reagents stored in one end compartment pass into the centre compartment; whereas when rotation proceeds in the opposite direction, the material in the centre compartment is merely mixed together, partly by the revolution of the drum, partly with the assistance of a stationary agitator slung loosely from the central spindle. The other end compartment contains coke or sawdust or other dry material through which the gas passes for the removal of lime or other dust carried in suspension as it issues from the generating compartment. The gas then passes through perforations into the central spindle, one end of which is connected by a packed joint with a fixed pipe, which leads to a seal or washer containing petroleum. Approached from a theoretical standpoint, it will be seen that this method of generation entirely sacrifices the advantages otherwise accruing from the use of liquid water as a means for dissipating the heat of the chemical reaction, but on the other hand, inasmuch as the substances are both solid, the reaction presumably occurs more slowly than it would in the presence of liquid water; and moreover the fact that the water employed to act upon the carbide is in the solid state and also more or less combined with the rest of the sodium carbonate molecule, means that, per unit of weight, the water decomposed must render latent a larger amount of heat than it would were it liquid. Experiments made by one of the authors of this book tend to show that the gas evolved from carbide by the dry process contains rather less phosphorus than it might in other conditions of generation, and as a fact gas made by the dry process is ordinarily consumed without previous passage through any chemical purifying agent. It is obvious, however, that the use of the churn described above greatly increases the labour attached to the production of the gas; while it is not clear that the yield per unit weight of carbide decomposed should be as high as that obtained in wet generation. The inventor has claimed that his by-product should be valuable and saleable, apparently partly on the ground that it should contain caustic soda. Evidence, however, that a reaction between the calcium oxide or hydroxide and the sodium carbonate takes place in the prevailing conditions is not yet forthcoming, and the probabilities are that such decomposition would not occur unless the residue were largely diluted with water. [Footnote: The oldest process employed for manufacturing caustic soda consisted in mixing a solution of sodium carbonate with quick or slaked lime, and it has been well established that the causticisation of the soda will not proceed when the concentration of the liquid is greater than that corresponding with a specific gravity of about 1-10, i.e., when the liquid contains more than some 8 to 10 per cent, of sodium hydroxide.] Conversely there are some grounds for believing that the dry residue is less useful than an ordinary wet residue for horticultural purposes, and also for the production of whitewash. From a financial standpoint, the dry process suffers owing to the expense involved in the purchase of a second raw material, for which but little compensation can be discovered unless it is proved that the residue is intrinsically more valuable than common acetylene-lime and can be sold or used advantageously by the ordinary owner of an installation. The discarding of the chemical purifier at the present day is a move of which the advantage may well be overrated.

ARTIFICIAL LIGHTING OF GENERATOR SHEDS.--It has already been argued that all normal or abnormal operations in connexion with an acetylene generating plant should be carried out, if possible, by daylight; and it has been shown that on no account must a naked light ever be taken inside the house containing such a plant. It will occasionally happen, however, that the installation must be recharged or inspected after nightfall. In order to do this in safety, a double window, incapable of being opened, should be fitted in one wall of the house, as far as possible from the door, and in such a position that the light may fall on to all the necessary places. Outside this window may be suspended an ordinary hand- lantern burning oil or paraffin; or, preferably, round this window may be built a closed lantern into which some source of artificial light may be brought. If the acetylene plant has an isolated holder of considerable size, there is no reason at all why a connexion should not be made with the service-pipes, and an acetylene flame be used inside this lantern; but with generators of the automatic variety, an acetylene light is not so suitable, because of the fear that gas may not be available precisely at the moment when it is necessary to have light in the shed. It would, however, be a simple matter to erect an acetylene burner inside the lantern in such a way that when needed an oil-lamp or candle could be used instead. Artificial internal light of any kind is best avoided; the only kind permissible being an electric glow-lamp. If this is employed, it should be surrounded by a second bulb or gas-tight glass jacket, and preferably by a wire cage as well; the wires leading to it must be carefully insulated, and all switches or cut-outs (which may produce a spark) must be out of doors. The well-known Davy safety or miner's lamp is not a trustworthy instrument for use with acetylene because of (a) the low igniting-point of acetylene; (b) the high temperature of its flame; and (c) the enormous speed at which the explosive wave travels through a mixture of acetylene and air. For these reasons the metallic gauze of the Davy lamp is not so efficient a protector of the flame as it is in cases of coal-gas, methane, &c. Moreover, in practice, the Davy lamp gives a poor light, and unless in constant use is liable to be found out of order when required. It should, however, be added that modern forms of the safety lamp, in which the light is surrounded by a stout glass chimney and only sufficient gauze is used for the admission of fresh air and for the escape of the combustion products, appear quite satisfactory when employed in an atmosphere containing some free acetylene.

[CHAPTER IV]

THE SELECTION OF AN ACETYLENE GENERATOR

In Chapter II. an attempt has been made to explain the physical and chemical phenomena which accompany the interaction of calcium carbide and water, and to show what features in the reaction are useful and what inconvenient in the evolution of acetylene on a domestic or larger scale. Similarly in Chapter III. have been described the various typical devices which may be employed in the construction of different portions of acetylene plant, so that the gas may be generated and stored under the best conditions, whether it is evolved by the automatic or by the non- automatic system. This having been done, it seemed of doubtful utility to include in the first edition of this work a long series of illustrations of such generators as had been placed on the markets by British, French, German, and American makers. It would have been difficult within reasonable limits to have reproduced diagrams of all the generators that had been offered for sale, and absolutely impossible within the limits of a single hand-book to picture those which had been suggested or patented. Moreover, some generating apparatus appeared on the market ephemerally; some was constantly being modified in detail so as to alter parts which experience or greater knowledge had shown the makers to be in need of alteration, while other new apparatus was constantly being brought out. On these and other grounds it did not appear that much good purpose would have been served by describing the particular apparatus which at that time would have been offered to prospective purchasers. It seemed best that the latter should estimate the value and trustworthiness of apparatus by studying a section of it in the light of the general principles of construction of a satisfactory generator as enunciated in the book. While the position thus taken by the authors in 1903 would still not be incorrect, it has been represented to them that it would scarcely be inconsistent with it to give brief descriptions of some of the generators which are now being sold in Great Britain and a few other countries. Six more years' experience in the design and manufacture of acetylene plant has enabled the older firms of manufacturers to fix upon certain standard patterns for their apparatus, and it may confidently be anticipated that many of these will survive a longer period. Faulty devices and designs have been weeded out, and there are lessons of the past as well as theoretical considerations to guide the inventor of a new type of generator. On those grounds, therefore, an attempt has now been made to give brief descriptions, with sectional views, of a number of the generators now on the market in Great Britain. Moreover, as the first edition of this book found many readers in other countries, in several of which there is greater scope for the use of acetylene, it has been decided to describe also a few typical or widely used foreign generators. All the generators described must stand or fall on their merits, which cannot be affected by any opinion expressed by the authors. In the descriptions, which in the first instance have generally been furnished by the manufacturers of the apparatus, no attempt has therefore been made to appraise the particular generators, and comparisons and eulogistic comments have been excluded. The descriptions, however, would nevertheless have been somewhat out of place in the body of this book; they have therefore been relegated to a special Appendix. It has, of course, been impossible to include the generators of all even of the English manufacturers, and doubtless many trustworthy ones have remained unnoticed. Many firms also make other types of generators in addition to those described. It must not be assumed that because a particular make of generator is not mentioned it is necessarily faulty. The apparatus described may be regarded as typical or well known, and workable, but it is not by reason of its inclusion vouched for in any other respect by the authors. The Appendix is intended, not to bias or modify the judgment of the would-be purchaser of a generator, but merely to assist him in ascertaining what generators there are now on the market.

The observations on the selection of a generator which follow, as well as any references in other chapters to the same matter, have been made without regard to particular apparatus of which a description may (or may not) appear in the Appendix. With this premise, it may be stated that the intending purchaser should regard the mechanism of a generator as shown in a sectional view or on inspection of the apparatus itself. If the generator is simple in construction, he should be able to understand its method of working at a glance, and by referring it to the type (vide Chapter III.) to which it belongs, be able to appraise its utility from a chemical and physical aspect from what has already been said. If the generator is too complicated for ready understanding of its mode of working, it is not unlikely to prove too complicated to behave well in practice. Not less important than the mechanism of a generator is good construction from the mechanical point of view, i.e., whether stout metal has been employed, whether the seams and joints are well finished, and whether the whole apparatus has been built in the workman- like fashion which alone can give satisfaction in any kind of plant. Bearing these points in mind, the intending purchaser may find assistance in estimating the mechanical value of an apparatus by perusing the remainder of this chapter, which will be devoted to elaborating at length the so-called scientific principles underlying the construction of a satisfactory generator, and to giving information on the mechanical and practical points involved.

It is perhaps desirable to remark that there is scarcely any feature in the generation of acetylene from calcium carbide and water--certainly no important feature--which introduces into practice principles not already known to chemists and engineers. Once the gas is set free it ranks simply as an inflammable, moisture-laden, somewhat impure, illuminating and heat-giving gas, which has to be dried, purified, stored, and led to the place of combustion; it is in this respect precisely analogous to coal- gas. Even the actual generation is only an exothermic, or heat-producing, reaction between a solid and a liquid, in which rise of temperature and pressure must be prevented as far as possible. Accordingly there is no fundamental or indispensable portion of an acetylene apparatus which lends itself to the protection of the patent laws; and even the details (it may be said truthfully, if somewhat cynically) stand in patentability in inverse ratio to their simplicity and utility.

During the early part of 1901 a Committee appointed by the British Home Office, "to advise as to the conditions of safety to which acetylene generators should conform, and to carry out tests of generators in the market in order to ascertain how far those conform with such conditions," issued a circular to the trade suggesting that apparatus should be sent them for examination. In response, forty-six British generators were submitted for trial, and were examined in a fashion which somewhat exceeded the instructions given to the Committee, who finally reported to the Explosives Department of the Home Office in a Blue Book, No. Cd. 952, which can be purchased through any bookseller. This report comprises an appendix in which most of the apparatus are illustrated, and it includes the result of the particular test which the Committee decided to apply. Qualitatively the test was useful, as it was identical in all instances, and only lacks full utility inasmuch as the trustworthiness of the automatic mechanism applied to such generators as were intended to work on the automatic system was not estimated. Naturally, a complete valuation of the efficiency of automatic mechanism cannot be obtained from one or even several tests, it demands long-continued watching; but a general notion of reliability might have been obtained. Quantitatively, however, the test applied by the Committee is not so free from reproach, for, from the information given, it would appear to have been less fair to some makers of apparatus than to others. Nevertheless the report is valuable, and indicates the general character of the most important apparatus which were being offered for sale in the United Kingdom in 1900-1901.

It is not possible to give a direct answer to the question as to which is the best type of acetylene generator. There are no generators made by responsible firms at the present time which are not safe. Some may be easier to charge and clean than others; some require more frequent attention than others; some have moving parts less likely to fail, when handled carelessly, than others; some have no moving mechanism to fail. For the illumination of a large institution or district where one man can be fully occupied in attending to the plant, cleaning, lighting, and extinguishing the lamps, or where other work can be found for him so as to leave him an hour or so every day to look after the apparatus, the hand-fed carbide-to-water generator L (Fig. 6) has many advantages, and is probably the best of all. In smaller installations choice must be made first between the automatic and the non-automatic principle--the advantages most frequently lying with the latter. If a non-automatic generator is decided upon, the hand carbide-feed or the flooded- compartment apparatus is almost equally good; and if automatism is desired, either a flooded-compartment machine or one of the most trustworthy types of carbide-feed apparatus may be taken. There are contact apparatus on the markets which appear never to have given trouble, and those are worthy of attention. Some builders advocate their own apparatus because the residue is solid and not a cream. If there is any advantage in this arising from greater ease in cleaning and recharging the generator and in disposing of the waste, that advantage is usually neutralised by the fear that the carbide may not have been wholly decomposed within the apparatus; and whereas any danger arising from imperfectly spent carbide being thrown into a closed drain may be prevented by flooding the residue with plenty of water in an open vessel, imperfect decomposition in the generator means a deficiency in the amount of gas evolved from a unit weight of solid taken or purchased. In fact, setting on one side apparatus which belong to a notoriously defective system and such as are constructed in large sizes on a system that is only free from overheating, &c., in small sizes; setting aside all generators which are provided with only one decomposing chamber when they are of a capacity to require two or more smaller ones that can more efficiently be cooled with water jackets; and setting aside any form of plant which on examination is likely to exhibit any of the more serious objections indicated in this and the previous chapters, there is comparatively little to choose, from the chemical and physical points of view, between the different types of generators now on the markets. A selection may rather be made on mechanical grounds. The generator must be well able to produce gas as rapidly as it will ever be required during the longest or coldest evening; it must be so large that several more brackets or burners can be added to the service after the installation is complete. It must be so strong that it will bear careless handling and the frequent rough manipulation of its parts. It must be built of stout enough material not to rust out in a few years. Each and all of its parts must be accessible and its exterior visible. Its pipes, both for gas and sludge, must be of large bore (say 1 inch), and fitted at every dip with an arrangement for withdrawing into some closed vessel the moisture, &c., that may condense. The number of cocks, valves, and moving parts must be reduced to a minimum; cocks which require to be shut by hand before recharging must give way to water-seals. It must be simple in all its parts, and its action intelligible at a glance. It must be easy to charge--preferably even by the sense of touch in darkness. It must be easy to clean. The waste lime must be easily removed. It must be so fitted with vent-pipes that the pressure can never rise above that at which it is supposed to work. Nevertheless, a generator in which these vent-pipes are often brought into use is badly constructed and wasteful, and must be avoided. The water of the holder seal should be distinct from that used for decomposing the carbide; and those apparatus where the holder is entirely separated from the generator are preferable to such as are built all in one, even if water-seals are fitted to prevent return of gas. Apparatus which is supposed to be automatic should be made perfectly automatic, the water or the carbide-feed being locked automatically before the carbide store, the decomposing chamber, or the sludge-cock can be opened. The generating chamber must always be in communication with the atmosphere through a water-sealed vent-pipe, the seal of which, if necessary, the gas can blow at any time. All apparatus should be fitted with rising holders, the larger the better. Duplicate copies of printed instructions should be demanded of the maker, one copy being kept in the generator-house, and the other elsewhere for reference in emergencies. These instructions must give simple and precise information as to what should be done in the event of a breakdown as well as in the normal manipulation of the plant. Technical expressions and descriptions of parts understood only by the maker must be absent from these rules.

ADDENDUM.

BRITISH AND FOREIGN REGULATIONS FOR THE CONSTRUCTION AND INSTALLATION OF ACETYLENE GENERATING PLANT

Dealing with the "conditions which a generator should fulfil before it can be considered as being safe," the HOME OFFICE COMMITTEE of 1901 before mentioned write as follows:

1. The temperature in any part of the generator, when run at the maximum rate for which it is designed, for a prolonged period, should not exceed 130° C. This may be ascertained by placing short lengths of wire, drawn from fusible metal, in those parts of the apparatus in which heat is liable to be generated.

2. The generator should have an efficiency of not less than 90 per cent., which, with carbide yielding 5 cubic feet per pound, would imply a yield of 4.5 cubic feet for each pound of carbide used.

3. The size of the pipes carrying the gas should be proportioned to the maximum rate of generation, so that undue back pressure from throttling may not occur.

4. The carbide should be completely decomposed in the apparatus, so that lime sludge discharged from the generator shall not be capable of generating more gas.

5. The pressure in any part of the apparatus, on the generator side of the holder, should not exceed that of 20 inches of water, and on the service side of same, or where no gasholder is provided, should not exceed that of 5 inches of water.

6. The apparatus should give no tarry or other heavy condensation products from the decomposition of the carbide.

7. In the use of a generator regard should be had to the danger of stoppage of passage of the gas and resulting increase of pressure which may arise from the freezing of the water. Where freezing may be anticipated, steps should be taken to prevent it.

8. The apparatus should be so constructed that no lime sludge can gain access to any pipes intended for the passage of gas or circulation of water.

9. The use of glass gauges should be avoided as far as possible, and, where absolutely necessary, they should be effectively protected against breakage.

10. The air space in a generator before charging should be as small as possible.

11. The use of copper should be avoided in such parts of the apparatus as are liable to come in contact with acetylene.

The BRITISH ACETYLENE ASSOCIATION has drawn up the following list of regulations which, it suggests, shall govern the construction of generators and the installation of piping and fittings:

1. Generators shall be so constructed that, when used in accordance with printed instructions, it shall not be possible for any undecomposed carbide to remain in the sludge removed therefrom.

2. The limit of pressure in any part of the generator shall not exceed that of 20 inches of water, subject to the exception that if it be shown to the satisfaction of the Executive of the Acetylene Association that higher pressures up to 50 inches of water are necessary in certain generators, and are without danger, the Executive may, with the approval of the Home Office, grant exemption for such generators, with or without conditions.

3. The limit of pressure in service-pipes, within the house, shall not exceed 10 inches of water.

4. Except when used for special industrial purposes, such as oxy- acetylene welding, factories, lighthouses, portable apparatus containing not more than four pounds of carbide, and other special conditions as approved by the Association, the acetylene plant, such as generators, storage-holders, purifiers, scrubbers, and for washers, shall be in a suitable and well-ventilated outhouse, in the open, or in a lean-to, having no direct communication with a dwelling-house. A blow-off pipe or safety outlet shall be arranged in such a manner as to carry off into the open air any overmake of gas and to open automatically if pressure be increased beyond 20 inches water column in the generating chamber or beyond 10 inches in the gasholder, or beyond the depth of any fluid seal on the apparatus.

5. Generators shall have sufficient storage capacity to make a serious blow-off impossible.

6. Generators and apparatus shall be made of sufficiently strong material and be of good workmanship, and shall not in any part be constructed of unalloyed copper.

7. It shall not be possible under any conditions, even by wrong manipulation of cocks, to seal the generating chamber hermetically.

8. It shall not be possible for the lime sludge to choke any of the gas- pipes in the apparatus, nor water-pipes if such be alternately used as safety-valves.

9. In the use of a generator, regard shall be had to the danger of stoppage of passage of the gas, and resulting increase of pressure, which may arise from the freezing of the water. Where freezing may be anticipated, steps shall be taken to prevent it.

10. The use of glass gauges shall be avoided as far as possible, and where absolutely necessary they shall be effectively protected against breakage.

11. The air space in the generator before charging shall be as small as possible, i.e., the gas in the generating chamber shall not contain more than 8 per cent. of air half a minute after commencement of generation. A sample of the contents, drawn from the holder any time after generation has commenced, shall not contain an explosive mixture, i.e., more than 18 per cent, of air. This shall not apply to the initial charges of the gasholder, when reasonable precautions are taken.

12. The apparatus shall produce no tarry or other heavy condensation products from the decomposition of the carbide.

13. The temperature of the gas, immediately on leaving the charge, shall not exceed 212° F. (100° C.)

14. No generator shall be sold without a card of instructions suitable for hanging up in some convenient place. Such instructions shall be of the most detailed nature, and shall not presuppose any expert knowledge whatever on the part of the operator.

15. Notice to be fixed on Generator House Door, "NO LIGHTS OR SMOKING ALLOWED."

Acetylene, the Principles of Its Generation and Use / A Practical Handbook on the Production, Purification, and Subsequent Treatment of Acetylene for the Development of Light, Heat, and Power

ACETYLENE

THE PRINCIPLES OF ITS GENERATION AND USE

A PRACTICAL HANDBOOK ON THE PRODUCTION, PURIFICATION, AND SUBSEQUENT TREATMENT OF ACETYLENE FOR THE DEVELOPMENT OF LIGHT, HEAT, AND POWER

BY

F. H. LEEDS, F.I.C.

FOR SOME YEARS TECHNICAL EDITOR OF THE JOURNAL "ACETYLENE"

AND

W. J. ATKINSON BUTTERFIELD, M.A.

AUTHOR OF "THE CHEMISTRY OF GAS MANUFACTURE"

Second Edition

REVISED AND ENLARGED

PREFATORY NOTE TO THE FIRST EDITION

NOTE TO THE SECOND EDITION

ACETYLENE

[CHAPTER I]

INTRODUCTORY--THE COST AND ADVANTAGES OF ACETYLENE LIGHTING

[CHAPTER II]

THE PHYSICS AND CHEMISTRY OF THE REACTION BETWEEN CARBIDE AND WATER

[CHAPTER III]

THE GENERAL PRINCIPLES OF ACETYLENE GENERATION--ACETYLENE GENERATING APPARATUS

[CHAPTER IV]

THE SELECTION OF AN ACETYLENE GENERATOR