Please see the [Transcriber’s Notes] at the end of this text.

The cover image has been created for this text and is placed in the public domain.

ROCK BLASTING.

ROCK BLASTING.

A
PRACTICAL TREATISE
ON THE
MEANS EMPLOYED IN BLASTING ROCKS
FOR INDUSTRIAL PURPOSES.

BY
GEO. G. ANDRÉ, F.G.S., Assoc. Inst. C.E.,
MINING CIVIL ENGINEER; MEMBER OF THE SOCIETY OF ENGINEERS.

LONDON:
E. & F. N. SPON, 46, CHARING CROSS.
NEW YORK:
446, BROOME STREET.
1878.

PREFACE.

During the past decade, numerous and great changes have taken place in the system followed and the methods adopted for blasting rocks in industrial operations. The introduction of the machine drill led naturally to these important changes. The system which was suitable to the operations carried on by hand was inefficient under the requirements of machine labour, and the methods which had been adopted as the most appropriate in the former case were found to be more or less unsuitable in the latter. Moreover, the conditions involved in machine boring are such as render necessary stronger explosive agents than the common gunpowder hitherto in use, and a more expeditious and effective means of firing them than that afforded by the ordinary fuse. These stronger agents have been found in the nitro-cotton and the nitro-glycerine compounds, and in the ordinary black powder improved in constitution and fired by detonation; and this more expeditious and effective means of firing has been discovered in the convenient application of electricity. Hence it is that the changes mentioned have been brought about, and hence, also, has arisen a need for a work like the present, in which the subjects are treated of in detail under the new aspects due to the altered conditions.

GEO. G. ANDRÉ.

London, 17, King William Street, Strand,
January 1st, 1878.

CONTENTS.

ROCK BLASTING.

CHAPTER I.
THE TOOLS, MACHINES, AND OTHER APPLIANCES USED IN BLASTING ROCKS.

Section I.—Hand Boring.

Drills.

—The operations of blasting consist in boring suitable holes in the rock to be dislodged, in inserting a charge of some explosive compound into the lower portion of these holes, in filling up, sometimes, the remaining portion of the holes with suitable material, and in exploding the charge. The subjects which naturally first present themselves for consideration are: the nature, form, and construction of the tools, machines, and other appliances used. Of these tools, the “drill” or “borer” constitutes the chief. To understand clearly the action of the rock drill, we must consider the nature of the substance which has to be perforated. He who has examined the mineral constitution of rocks will have recognised the impossibility of cutting them, using that term in its ordinary acceptation, inasmuch as the rock constituents are frequently harder than the material of the tools employed to penetrate them. As a rock cannot be cut, the only way of removing portions of it is to fracture or to disintegrate it by a blow delivered through the medium of a suitable instrument. Each blow so delivered may be made to chip off a small fragment, and by this means the rock may be gradually worn away. To effect this chipping, however, the instrument used must present only a small surface to the rock, in order to concentrate the force, and that surface must be bounded by inclined planes or wedge surfaces, to cause a lateral pressure upon the particles of rock in contact with them. In other words, the instrument must be provided with an edge similar to that possessed by an ordinary cutting tool.

The conditions under which the instrument is worked are obviously such that this edge will be rapidly worn down by attrition from the hard rock material, and by fracture. To withstand these destructive actions, two qualities are requisite in the material of which the instrument is composed, namely, hardness and toughness. Thus there are three important conditions concurring to determine the nature and the form of a cutting tool to be used in rock boring—1, a necessity for a cutting edge; 2, a necessity for a frequent renewal of that edge; and 3, a necessity for the qualities of hardness and toughness in the material of the tool.

In very hard rock, a few minutes of work suffice to destroy the cutting edge, and then the tool has to be returned to the smithy to be re-sharpened. Hence it is manifest that the form of the edge should not be one that is difficult to produce, since, were it so, much time would be consumed in the labour of re-sharpening. Experience has shown that the foregoing conditions are most fully satisfied in the steel rod terminating in a simple chisel edge, now universally adopted.

This form of drill is exhibited in [Fig. 1], which represents a common “jumper” borer. It consists of a rod terminating at each end in a chisel edge, and having a swell, technically described as the “bead,” between the extremities to give it weight. The bead divides the jumper into two unequal portions, each of which constitutes a chisel bit, with its shank or “stock.” The shorter stock is used while the hole is shallow, and the longer one to continue it to a greater depth.

Fig. 1.

Fig. 2.

Fig. 3.

With the jumper, the blow is obtained from the direct impact of the falling tool. The mode of using the instrument is to lift it with both hands to a height of about a foot, and then to let it drop. In lifting the jumper, care is taken to turn it partially round, that the edge may not fall twice in the same place. By this means, the edge is made to act most favourably in chipping away the rock, and the hole is kept fairly circular. So long as the holes are required to be bored vertically downwards, the jumper is a convenient and very efficient tool, and hence in open quarrying operations, it is very commonly employed. But in mining, the shot-holes are more often required to be bored in some other direction, or, as it is termed, “at an angle;” that is, at an angle with the vertical. Or it may be that a shot-hole is required to be bored vertically upward. It is obvious that in any one of these directions the jumper is useless. To meet the requirements of such cases, recourse is had to the hammer wherewith to deliver the blow, and the drill is constructed to be used with the hammer. We have a suitable form of tool for application in this wise when we cut out the bead of the jumper and leave the ends flat for a striking face, as shown in [Figs. 2] and [3]. The form of the two chisels thus obtained is that adopted for the ordinary rock drill.

It will be understood from these descriptions that a rock drill consists of the chisel edge or bit, the stock, and the striking face. Formerly drills were made of wrought iron, and steeled at each end to form the bit and the striking face. Now they are commonly made of cast steel, which is supplied for that purpose in octagonal bars of the requisite diameter. The advantages offered by steel stocks are numerous. The superior solidity of texture of that material renders it capable of transmitting the force of a blow more effectively than iron. Being stronger than the latter material, a smaller diameter of stock, and, consequently, a less weight, are sufficient. This circumstance also tends to increase the effect of the blow by diminishing the mass through which it is transmitted. On the other hand, a steel stock is more easily broken than one of iron.

The cutting edge of a drill demands careful consideration. To enable the tool to free itself readily in the bore-hole, and also to avoid introducing unnecessary weight into the stock, the bit is made wider than the latter; the difference in width may be as much as 1 inch. It is evident that in hard rock, the liability of the edge to fracture increases as the difference of width. The edge of the drill may be straight or slightly curved. The straight edge cuts its way somewhat more freely than the curved, but it is weaker at the corners than the latter, a circumstance that renders it less suitable for very hard rock. It is also slightly more difficult to forge. The width of the bit varies, according to the size of the hole required, from 1 inch to 2¹⁄₂ inches. [Figs. 4], [5], and [6] show the straight and the curved bits, and the angles of the cutting edges for use in rock.

Fig. 4.

Fig. 5.

Fig. 6.

The stock is octagonal in section; it is made in lengths varying from 20 inches to 42 inches. The shorter the stock the more effectively does it transmit the force of the blow, and therefore it is made as short as possible. For this reason, several lengths are employed in boring a shot-hole, the shortest being used at the commencement of the hole, a longer one to continue the depth, and a still longer one, sometimes, to complete it. To ensure the longer drills working freely in the hole, the width of the bit should be very slightly reduced in each length. It has already been remarked that the diameter of the stock is less than the width of the bit; this difference may be greater in coal drills than in rock or “stone” drills; a common difference in the latter is ³⁄₈ of an inch for the longer. The following proportions may be taken as the average adopted:—

The striking face of the drill should be flat. The diameter of the face is less than that of the stock in all but the smallest sizes, the difference being made by drawing in the striking end. The amount of reduction is greater for the largest diameters; that of the striking face being rarely more than one-eighth of an inch.

The making and re-sharpening of rock drills constitute an extremely important part of the labour of the mine smith. The frequent use of the drill, and its rapid wear, necessitate a daily amount of work of no trifling proportions, and the judgment and skill required in proper tempering render some degree of intelligence in the workman indispensable; indeed, so much depends upon the smith whose duty it is to repair the miners’ tools, that no pains should be spared to obtain a man capable of fulfilling that duty in the most efficient manner possible.

When the borer-steel bars are supplied to the smith, he cuts them up, as required, into the desired lengths. To form the bit, the end of the bar is heated and flattened out by hammering to a width a little greater than the diameter of the hole to be bored. The cutting edge is then hammered up with a light hammer to the requisite angle, and the corners beaten in to give the exact diameter of the bore-hole intended. As the drills are made in sets, the longer stocks will have a bit slightly narrower than the shorter ones, for reasons already given. The edge is subsequently touched up with a file. In performing these operations, heavy hammering should be avoided, as well as high heats, and care should be taken in making the heat that the steel should be well covered with coal, and far enough removed from the tuyere to be protected from the “raw” air. Overheated or “burned” steel is liable to fly, and drills so injured are useless until the burned portion has been cut away.

Fig. 7.

Fig. 8.

Fig. 9.

Both in making and in re-sharpening drills, great care is required to form the cutting edge evenly, and of the full form and dimensions. If the corners get hammered in, as shown in [Fig. 7], they are said to be “nipped,” and the tool will not free itself in cutting. When a depression of the straight, or the curved, line forming the edge occurs, as shown in [Fig. 8], the bit is said to be “backward,” and when one of the corners is too far back, as in [Fig. 9], it is spoken of as “odd-cornered.” When either of these defects exist—and they are unfortunately common—not only does the bit work less effectively on the rock, but the force of the blow is thrown upon a portion only of the edge, which, being thereby overstrained, is liable to fracture.

The hardening and tempering of steel is a matter requiring careful study and observation. It is a well-known fact that a sudden and great reduction of temperature causes a notable increase of hardness in the metal. The reason of this phenomenon is not understood, but it is certain that it is in some way dependent upon the presence of carbon. The degree of hardness imparted to steel by this means depends upon the amount of the reduction of the temperature, and the proportion of carbon present in the metal, highly carburetted steel being capable of hardening to a higher degree, under the same conditions, than steel containing less carbon. Thus, for steel of the same quality, the wider the range of temperature the higher is the degree of hardness. But here we encounter another condition, which limits the degree of hardness practically attainable.

The change which takes place among the molecules of the metal in consequence of the change of temperature causes internal strains, and thereby puts portions in a state of unequal tension. This state renders the strained parts liable to yield when an additional strain is thrown upon them while the tool is in use; in other words, the brittleness of the steel increases with its hardness. Here again the proportion of carbon present comes into play, and it must be borne in mind that for equal degrees of hardness the steel which contains the least carbon will be the most brittle. In hardening borer-steel, which has to combine as far as possible the qualities of hardness and toughness, this matter is one deserving careful attention. It is a remarkable fact, and one of considerable practical value, that when oil is employed as the cooling medium instead of water, the toughness of steel is enormously increased.

The tempering of steel, which is a phenomenon of a similar character to that of hardening, also claims careful consideration. When a bright surface of steel is subjected to heat, a series of colours is produced, which follow each other in a regular order as the temperature increases. This order is as follows: pale yellow, straw yellow, golden yellow, brown, brown and purple mingled, purple, light blue, full clear blue, and dark blue. Experience has shown that some one of these colours is more suitable than the rest for certain kinds of tools and certain conditions of working.

The selection of the proper colour constitutes a subject for the exercise of judgment and skill on the part of the smith. For rock drills, straw colour is generally the most suitable when the work is in very hard rock, and light blue when the rock is only of moderate hardness.

The processes of hardening and tempering drills are as follows: When the edge of the bit has been formed in the manner already described, from 3 to 4 inches of the end is heated to cherry redness, and dipped in cold water to a depth of about an inch to harden it. While in the water, the bit should be moved slightly up and down, for, were this neglected, the hardness would terminate abruptly, and the bit would be very liable to fracture along the line corresponding with the surface of the water. In cold weather, the water should be slightly warmed, by immersing a piece of hot iron in it, before dipping the steel. When a sufficient degree of hardness has been attained, the remainder of the hot portion is immersed until the heat is reduced sufficiently for tempering. At this stage it is withdrawn, and the colours carefully watched for. The heat which is left in the stock will pass down to the edge of the bit, and as the temperature increases in that part the colours will appear in regular succession upon the filed surface of the edge. When the proper hue appears, the whole drill is plunged into the water and left there till cold, when the tempering is complete. When the edge is curved or “bowed,” the colours will reach the corners sooner than the middle of the bit. This tendency must be checked by dipping the corners in the water, for otherwise the edge will not be of equal hardness throughout. As the colour can be best observed in the dark, it is a good plan to darken that portion of the smithy in which tempering is being carried on.

The degree of temper required depends upon the quality of the steel and the nature of the work to be performed. The larger the proportion of carbon present in the metal, the lower must be the temper. Also the state of the blunted edges, whether battered or fractured, will show what degree of hardness it is desirable to produce. From inattention to these matters, good steel is not unfrequently condemned as unsuitable.

To form the striking face, the end of the stock is heated to a dull red, and drawn out by a hammer to form a conical head. The extremity is then flattened to form a face from ¹⁄₂ inch to 1 inch in diameter. This head is then annealed to a degree that will combine considerable toughness with hardness. The constant blows to which the head is subjected tend to wear it down very rapidly. There is great difference in the lasting qualities of steel in this respect; some drills will wear away more quickly at the striking than at the bit end.

A smith will, with the assistance of a striker, sharpen and temper about thirty single-hand drills of medium size in an hour, or twenty double-hand drills of medium size in the same time. Of course, much will depend on the degree of bluntness in the cutting edge; but assuming the drills to be sent up only moderately blunted, this may be taken as a fair average of the work of two men.

It will be evident from the foregoing remarks, that to enable a drill to stand properly it must be made of good material, be skilfully tempered in the smithy, and provided with a cutting edge having an angle and a shape suited to the character of the rock in which it is used. To these conditions, may be added another, namely, proper handling; for if the drill be carelessly turned in the hole so as to bring all the work upon a portion only of the cutting edge, or unskilfully struck by the sledge, fracture or blunting will speedily result. Improper handling often destroys the edge in the first five minutes of using.

Drills, as before remarked, are used in sets of different lengths. The sets may be intended for use by one man or by two. In the former case, the sets are described as “single-hand” sets, and they contain a hammer for striking the drills; in the latter case, the sets are spoken of as “double-handed,” and they contain a sledge instead of a hammer for striking. It may appear at first sight that there is a waste of power in employing two men, or, as it is termed, the double set, for that two men cannot bore twice as fast as one. This rate of speed can, however, be obtained, and is due less to the greater effectiveness of the stroke than to the fact that two men can, by repeatedly changing places with each other, keep up almost without intermission a succession of blows for an indefinite length of time; whereas, with the single set, the man is continually obliged to cease for rest.

Hammers.

—To deliver the blow upon a rock drill, hammers and sledges are used. The distinction between a hammer and a sledge is founded on dimensions only: the hammer being intended for use in one hand, is made comparatively light and is furnished with a short handle, while the sledge, being intended for use in both hands, is furnished with a much longer handle and is made heavier. The striking face of the blasting sledge should be flat, to enable the striker to deliver a direct blow with certainty upon the head of the drill; and to facilitate the directing of the blow, as well as to increase its effect, the mass of metal composing the head should be concentrated within a short length. To cause the sledge to fly off from the head of the drill in the case of a false blow being struck, and thereby to prevent it from striking the hand of the man who holds the drill, the edges of the striking face should be chamfered or bevelled down till the diameter is reduced by nearly one-half. This requirement is, however, but seldom provided for.

Fig. 10.

Fig. 11.

Fig. 12.

Fig. 13.

The head of a sledge is of iron; it consists of a pierced central portion called the “eye,” and two shanks or “stumps,” the steeled ends of which form the striking faces or “panes.” The form of the head varies in different localities, but whatever the variations may be, the form may be classed under one of four types or “patterns.” A very common form is that shown in [Fig. 10] and known as the “bully” pattern. By varying the width, as shown in [Fig. 11], we obtain the “broad bully,” the former being called for the sake of distinction the “narrow” bully. Another common form is the “pointing” pattern, represented in [Fig. 12]. The form shown in [Fig. 13] is designated as the “bloat” pattern; and that given in [Fig. 14] the “plug” pattern. Each of these forms possesses peculiar merits which renders it more suitable for certain uses than the others. The same forms are used for hammers. The eye is generally made oval in shape, but sometimes, especially with the bloat pattern, it is made circular, as shown in [Fig. 13]. The weight of a sledge head may vary from 5 lb. to 10 lb., but a common and convenient weight is 7 lb. The length of the helve varies from 20 inches to 30 inches; a common length for blasting sledges is 24 inches. The average weight of hammer heads is about 3 lb., and the average length of the helve 10 inches.

Fig. 14.

Fig. 15.

[Fig. 15] represents a blasting sledge used in South Wales. The stumps are octagonal in section, and spring from a square block in the centre. The panes or striking faces, however, are circular and flat. The length of the head is 8³⁄₄ inches, and that of the helve 27 inches, and the weight of the tool complete 7 lb.

Fig. 16.

[Fig. 16] represents a blasting sledge used in North Wales. The central block is an irregular octagon in section, formed by slightly chamfering the angles of a square section, and the stumps are chamfered down to form a regular octagon at the panes, which are flat. The length of the head is 7³⁄₄ inches, and that of the helve 22 inches, and the weight of the tool complete 6 lb. 7 oz.

Fig. 17.

The sledges used in the north of England have shorter heads, and are lighter than the foregoing. [Fig. 17] represents one of these blasting sledges. The head is nearly square in section at the centre, and the panes are flat. The length of the head is 5 inches, and that of the helve 24¹⁄₂ inches, and the weight of the sledge complete 4 lb. 14 oz.

Auxiliary Tools.

—Besides the drill and the hammer, other tools are needed in preparing the hole for the blasting charge. If the bore-hole is inclined downwards, the débris or “bore-meal” made by the drill remains on the bottom of the hole, where it is converted into mud or “sludge” by the water there present. This sludge has to be removed as the work progresses, to keep the rock exposed to the action of the drill. The removal of the sludge is effected by a simple tool called a “scraper.” It consists of a rod of iron from ¹⁄₄ inch to ¹⁄₂ inch in diameter, and of sufficient length to reach the bottom of the bore-hole. One end of the rod is flattened out on the anvil and made circular in form, and then turned up at right angles to the stem. The disc thus formed must be less in diameter than the bore-hole, to allow it to pass readily down. When inserted in the hole, the scraper is turned round while it is being pressed to the bottom; on withdrawing the instrument, the sludge is brought up upon the disc. The operation, two or three times repeated, is sufficient to clear the bore-hole. The other end of the scraper is sometimes made to terminate in a ring for convenience in handling, as shown in [Fig. 18]. Instead of the ring, however, at one end, a disc may be made at each end, as shown in [Fig. 19], the discs in this case being of different diameter, to render the scraper suitable for different size bore-holes. Sometimes the scraper is made to terminate in a spiral hook or “drag-twist,” as represented in [Fig. 20]. The use of the drag is to thoroughly cleanse the hole before inserting the charge. A wisp of hay is pushed down the hole, and the drag end of the scraper introduced after it, and turned round till it has become firmly entangled. The withdrawal of the hay by the drag wipes the bore-hole clean. Instead of the twist drag, the “loop” drag is frequently employed. This consists of a loop or eye, through which a piece of rag or tow is passed. The rag or tow is used for the same purpose as the hay, namely, to thoroughly cleanse and dry the bore-hole previous to the introduction of the charge. Very frequently the “swab-stick” is used instead of the scraper to clear out the bore-hole. This is simply a deal rod bruised at one end by blows with a hammer until the fibres separate to form a kind of stumpy brush or “swab.” When this is pushed down the hole, the sludge passes up around and between the fibres, which are then spread out by being pressed against the bottom of the hole. On withdrawing the swab, the sludge is brought out with it.

Fig. 18.

Fig. 19.

Fig. 20.

When the charge has been placed in the bore-hole, and the fuse laid to it, the hole needs to be tamped, that is, the portion above the charge has to be filled up with some suitable substance. For this purpose, a “rammer,” “stemmer,” or “tamping iron,” as the instrument is variously called, is required. This instrument is illustrated in [Fig. 21]. It consists of a metal bar, the tamping end of which is grooved to receive the fuse lying against the side of the bore-hole. The other end is flat, to afford a pressing surface for the hand, or a striking face for the hammer when the latter is needed. To prevent the danger of accidental ignition from sparks caused by the friction of the metal against silicious substances, the employment of iron stemmers has been prohibited by law. They are usually made of copper or phosphor-bronze, the latter substance being more resisting than the former.

Fig. 21.

Fig. 22.

Fig. 23.

Sometimes in wet ground it becomes necessary to shut back the water from the bore-hole before introducing the charge of gunpowder. This happens very frequently in shaft sinking. The method employed in such cases is to force clay into the interstices through which the water enters. The instrument used for this purpose is the “claying-iron” or “bull,” represented in [Fig. 22]. It consists of a round bar of iron, called the stock or shaft, a little smaller in diameter than the bore-hole, and a thicker portion, called the head or poll, terminating in a striking face. The lower end of the shaft is pointed, to enable it to penetrate the clay, and the head is pierced by a hole about an inch in diameter to receive a lever. Clay in a plastic state having been put into the bore-hole, the bull is inserted and driven down by blows with the sledge. As the shaft forces its way down, the clay is driven into the joints and crevices of the rock on all sides. To withdraw the bull, a bar of iron is placed in the eye and used as a lever to turn it round to loosen it; the rod is then taken by both hands and the bull lifted out. To allow the bull to be withdrawn more readily, the shaft should be made with a slight taper and kept perfectly smooth. As the bull is subjected to a good deal of heavy hammering on the head, the latter part should be made stout. This tool, which should be considered as an extra instrument rather than as an essential part of a blasting set, is a very serviceable one, and should always be at hand in wet ground when loose gunpowder is employed.

Another instrument of this auxiliary character is the beche, [Fig. 23], used for extracting a broken drill. It consists of an iron rod of nearly the diameter of the bore-hole, and hollow at the lower end. The form of the aperture is slightly conical, so that the lower end may easily pass over the broken stock of the drill, and, on being pressed down with some force, may grasp the stock in the higher portion of the aperture with sufficient firmness to allow of the two being raised together. When only a portion of the bit remains in the hole, it may often be extracted by means of the drag-twist end of the scraper, or the swab-stick may be driven down upon the broken portion, and latter withdrawn with the swab.

Sets of Blasting Gear.

—On [Plates I.], [II.], and [III.], will be found three sets of blasting gear; a set of coal-blasting gear; a set of single-hand stone-blasting gear; and a set of double-hand stone-blasting gear. In the first set, the drill, shown in [Fig. 1], is 22 inches in length; the cutting edge is straight and 1¹⁄₂ inch wide, and the weight is 2¹⁄₂ lb. The other drill, [Fig. 2], is 42 inches in length; it has a straight cutting edge 1⁷⁄₁₆ inch wide, and weighs 4 lb. 10 oz. The hammer used in this set and shown in [Fig. 3] weighs 2 lb. 14 oz.; the length of the head is 4¹⁄₂ inches, and that of the handle 7³⁄₄ inches. In the second or single-hand stone set, the shorter drill, [Fig. 6, Plate II.], is 22 inches in length; the cutting edge is strongly curved, and is 1¹⁄₂ inch in width, and the weight is 3 lb. 10 oz. The longer drill, [Fig. 7], is 36 inches in length; the width of the cutting edge, which is curved as in the shorter drill, is 1⁷⁄₁₆ inch, and the weight is 6 lb. 5 oz. The hammer used with this set, and represented in [Fig. 8], weighs 3 lb. 6 oz.; the length of the head is 5 inches, and that of the handle 10 inches. In the third or double-hand stone set, [Plate III.], the first or shortest drill, [Fig. 12], is 18 inches in length, 1³⁄₄ inch wide on the cutting edge, and weighs 4¹⁄₄ lb. The second drill, [Fig. 13], is 27 inches in length, 1¹¹⁄₁₆ wide on the cutting edge, and weighs 6 lb. The third or longest drill, [Fig. 14], is 40 inches in length, 1⁵⁄₈ inch wide on the cutting edge, and weighs 9¹⁄₄ lb. The cutting edges of all these drills are strongly curved as in the preceding set. The sledge used with this set, and represented in [Fig. 15], weighs about 5 lb.

Section II.—Machine Boring.

Machine Rock-Drills.

—The most remarkable advance, which in recent, or perhaps in any, times has been made in the practice of mining consists in the substitution of machine for hand labour in rock boring. The importance of this change is obvious, and very great. Not only is the miner relieved thereby of the labour of boring, but the speed with which the shot-holes may be bored is increased a hundredfold. This gain of speed offers many practical advantages. The ability to sink a shaft or to drive a heading rapidly may ensure the success of an undertaking, and save indirectly the expenditure of large sums of money; and, in all cases, it allows the time spent in preparatory work to be materially shortened. Indeed, it would be difficult to over-estimate the magnitude of the advantage accruing from the increased rate of progress due to the substitution of machine power for hand labour, and in the future we may expect to see its application greatly extended. In making this substitution, numerous difficulties have had to be overcome, and in encountering these many failures have had to be recorded. But it must now be conceded by the most prejudiced that rock-boring machines have successfully passed through what may be described as the tentative stage of their existence, and have taken a foremost place among the mechanical appliances which experience has shown to be capable of effectually performing the work required of them. In the author’s work on ‘Mining Engineering,’ the requirements of a rock drill will be found fully discussed, and the principles and the construction of the most important machines now in use carefully explained and described. In the present work, only one example can be given.

Machine drills penetrate rock in the same way as the ordinary hand drills already described, namely, by means of a percussive action. The cutting tool is in most cases attached directly to the piston rod, with which it consequently reciprocates. Thus the piston with its rod is made to constitute a portion of the cutting tool, and the blow is then given by the direct action of the steam, or the compressed air, upon the tool. As no work is done upon the rock by the back stroke of the piston, the area of the forward side is reduced to the dimensions necessary only to lift the piston, and to overcome the resistance due to the friction of the tool in the bore-hole. The piston is made to admit steam or air into the cylinder, and to cut off the supply, and to open the exhaust, as required, by means of tappet valves, or other suitable devices; and provision is made to allow, within certain limits, a variation in the length of the stroke. During a portion of the stroke, means are brought into action to cause the piston to rotate to some extent, for the purposes that have been already explained. To keep the cutting edge of the tool up to its work, the whole machine is moved forward as the rock is cut away. This forward or “feed” motion is usually given by hand, but in some cases it is communicated automatically. The machine is supported upon a stand or framing which varies in form according to the situation in which it is to be used. This support is in all cases constructed to allow of the feed motion taking place, and also of the cutting tool being directed at any angle. The support for a rock drill constitutes an indispensable and a very important adjunct to the machine, for upon the suitability of its form, material, and construction, the efficiency of the machine will largely depend.

The foregoing is a general description of the construction and mode of action of percussive rock-drills. The numerous varieties now in use differ from each other rather in the details of their construction than in the principles of their action, and the importance of the difference is, of course, dependent upon that of the details. It is but just to remark here that the first really practical solution of the rock-drilling problem is due to M. Sommeiller, whose machine was employed in excavating the Mont Cenis tunnel.

The Darlington Drill.

—The machine which, in England, has stood the test of experience most satisfactorily, and which, consequently, is surely working itself into general favour in this country, and also in some of the important mining districts of the Continent, is the invention of John Darlington, and is known as the “Darlington drill.” This drill is remarkable as the attainment of the highest degree of simplicity of parts possible in a machine. The valve gear of a machine drill is especially liable to derangement. It must necessarily consist of several parts, and these parts must as necessarily be of a somewhat fragile character. Besides this, when actuated by the piston through the intervention of tappets, the violence of the blow delivered at each stroke is such as to rapidly destroy the parts. In some machines, the force of these blows and their destructive tendency have been reduced to a minimum; but when every means of remedying the evil has been employed, there remains a large amount of inevitable wear and tear, and a liability to failure from fracture or displacement exists in a greater or less degree. Moreover, as these effects are greatly intensified by increasing the velocity of the piston, it becomes at least undesirable to use a high piston speed. To remedy these defects, which are inherent in the system, Darlington proposed to remove altogether the necessity for a valve gear by radically changing the mode of admitting the motor fluid to the cylinder. This proposal he has realized in the machine which is illustrated on [Plate IV.]

The Darlington rock-drill consists essentially of only two parts: the cylinder A, [Figs. 20 and 21], with its cover; and the piston B, with its rod. The cover, when bolted on, forms a part of the cylinder; the piston rod is cast solid with the piston, and is made sufficiently large at its outer end to receive the tool. These two parts constitute an engine, and with less than one fixed and one moving part it is obviously impossible to develop power in a machine by the action of an elastic fluid. The piston itself is made to do the work of a valve in the following manner: The annular space affording the area for pressure on the fore part of the piston gives a much smaller extent of surface than that afforded by the diameter of the cylinder, as shown in the drawing; and it is obvious that by increasing or diminishing the diameter of the piston rod, the area for pressure on the one side of the piston may be made to bear any desired proportion to that on the other side. The inlet aperture, or port C, being in constant communication with the interior of the cylinder, the pressure of the fluid is always acting upon the front of the piston, consequently when there is no pressure upon the other side, the piston will be forced backward in the cylinder. During this backward motion, the piston first covers the exhaust port D, and then uncovers the equilibrium port E, by means of which communication is established between the front and back ends of the cylinder, and, consequently, the fluid is made to act upon both sides of the piston. The area of the back face of the piston being greater than that of the front face by the extent occupied by the piston rod, the pressure upon the former first acts to arrest the backward motion of the piston, which, by its considerable weight and high velocity, has acquired a large momentum, and then to produce a forward motion, the propelling force being dependent for its amount upon the difference of area on the two sides of the piston. As the piston passes down, it cuts off the steam from the back part of the cylinder and opens the exhaust. The length or thickness of the piston is such that the exhaust port D is never open to its front side, but, in the forward stroke, it is opened almost immediately after the equilibrium port is closed, and nearly at the time of striking the blow. It will be observed that the quantity of fluid expended is only that which passes over to the back face of the piston, since that which is used to effect the return stroke is not discharged.

The means employed to give a rotary motion to the tool are deserving of special attention, as being simple in design, effective in action, and well situate within the cylinder. These means consist of a spiral or rifled bar H, having three grooves, and being fitted at its head with a ratchet wheel G, recessed into the cover of the cylinder. Two detents J, J, [Fig. 22], also recessed into the cover, are made to fall into the teeth of the ratchet wheel by spiral springs. These springs may, in case of breakage, be immediately renewed without removing the cover. It will be observed that this arrangement of the wheel and the detents allow the spiral bar H to turn freely in one direction, while it prevents it from turning in the contrary direction. The spiral bar drops into a long recess in the piston, which is fitted with a steel nut made to accurately fit the grooves of the spiral. Hence the piston, during its instroke, is forced to turn upon the bar; but, during its outstroke, it turns the bar, the latter being free to move in the direction in which the straight outstroke of the piston tends to rotate it. Thus the piston, and with it the tool, assumes a new position after each stroke.

The mode of fixing the cutting tool to the piston rod is a matter deserving some attention. As the tool has to be changed more than once during the progress of a bore-hole, it is important that the change should be accomplished in as short a time as possible; and as the vibration of the machine and the strain upon the tool are necessarily great, it is equally important that the tool be firmly held. It is also desirable that the mode of fixing the tool shall not require a shoulder upon the latter, a slot in it, or any peculiarity of form difficult to be made in the smithy. The Darlington machine fulfils the requirements of expedition in fixing, firmness of retention, and simplicity of form most satisfactorily. The means and the method are the following: The outer end of the rod or holder is first flattened to afford a seat for the nut, as shown in [Figs. 21 and 25]. The slot is then cut and fitted tightly with a piece of steel K forged of the required shape for the clamp, and the holder is afterwards bored to receive the tool while the clamp is in place. This clamp K is then taken out, its fittings eased a little, and its end screwed and fitted with a nut. When returned to its place in the holder, the clamp, in consequence of the easing, can be easily drawn tight against the tool, by which means it is firmly held in position. The shank of the tool is turned to fit the hole easily, and the end of it is made hemispherical to fit the bottom of the hole, upon which the force of the reaction of the blow is received.

It would seem impossible to attain a higher degree of simplicity of form, or to construct a machine with fewer parts. The absence of a valve or striking gear of any kind ensures the utmost attainable degree of durability, and allows a high piston speed to be adopted without risk or injury. As the piston controls its own motion, there is no liability to strike against the cylinder cover. The stroke may be varied in length from half an inch to four inches, and as the machine will work effectively with a pressure of 10 lb. to the inch, holes may be started with the greatest ease. With a pressure of 40 lb., the machine makes 1000 blows a minute, a speed that may be attained without causing undue strains or vibration. This alone constitutes a very great advantage. It must indeed be conceded that an unprejudiced consideration of the merits of this drill shows it to be admirably adapted to the work required of it.

Borer-Bits.

—The form and the dimensions of the cutting tools, variously described as “drills,” “borers,” and “bits,” used with machine rock-perforators are matters of great practical importance. The dimensions are determined mainly by two conditions, namely, the necessity for sufficient strength in the shank of the tool, and the necessity for sufficient space between the shank and the sides of the hole to allow the débris to escape. Experience has shown that the latter condition is best fulfilled when the distance between the sides of the hole and the shank of the tool is from ³⁄₁₆ inch to ¹⁄₄ inch, regard being had to the former condition.

The form of the cutting edge is determined by several conditions, some of which have been already discussed in relation to hand drills. The form first adopted was naturally that possessed by the hand drill, namely, the chisel edge. To increase the useful effect of the blow, the cutting edge was subsequently doubled, the bit being formed of two chisel edges crossing each other at right angles. This bit, which from its form was called the “cross” bit, was found to penetrate the rock more rapidly than the straight or chisel bit. The gain in speed was very marked at the commencement of the hole; but it diminished gradually as the hole progressed in depth, owing to the difficulty with which the débris escaped. To remedy this defect, the cutting edges were next made to cross each other obliquely, so as to form the letter X. In this way, the two chisel edges were retained, while the breadth of the bit was considerably reduced. This form, described as the X bit, cleared the hole much more effectively than the cross, but not in a manner that was altogether satisfactory. Another modification of the form was, therefore, made, and this time that of the Z was adopted, the upper and the lower portions of which were arcs of circles struck from the centre of the bit in the direction contrary to that of the rotation.

This form of tool, which is known as the Z bit, readily cleared itself of the débris. But besides this advantage, it was found to possess others of an important character. With the chisel-edge forms, the corners of the bit were rapidly worn off by friction against the sides of the hole. With the Z form, this wearing no longer occurred, by reason of the large surface exposed to friction. Another advantage of the Z form of bit lies in its tendency to bore the hole truly circular. Generally then, it may be stated that this form satisfies most fully the determining conditions. The form of bit, however, that is most suitable in a given case will, in some degree, be determined by particular circumstances. Of these, the nature and the character of the rock will operate most strongly to influence the choice. Thus the cross bit will generally be found the most suitable in fissured rock, while the single chisel edge may be used with advantage in rock of a very solid and hard character. Indeed, on the judicious selection of the most suitable form of cutting edge, the success of machine boring largely depends. The chisel bit, the cross bit, the X bit, and the Z bit, are shown in [Figs. 24] to [27].

Fig. 24.

Fig. 25.

Fig. 26.

Fig. 27.

The sharpening of bits of a form other than that of the chisel is done by means of “swages.” The tempering is effected in the way already [described] in reference to hand drills. As in the latter case, the degree of temper must be suited to the hardness of the rocks to be penetrated. Generally the straw colour will be found to be the best degree. It is a remarkable fact that the wear of the cutting edge of a machine drill is, for a given length of boring, five or six times less than that of a hand drill. Steel of the best quality should always be used.

As in the case of hand boring, each successive length of drill must diminish slightly in the width of its cutting edge; a diminution of about ¹⁄₃₂ inch may be considered sufficient. Care should, however, be taken to ensure the proper dimensions being given to the edge, and it will be found advantageous to have at hand an accurate gauge through which the tool may be passed previously to its being fixed to the machine. It is important that the tool be truly “centred,” that is, the centres of the edge of the bit, of the shank, and of the piston rod, should be perfectly coincident.

Rock-Drill Supports.

—A machine rock-drill may satisfy every requirement, and yet, by reason of the defective character of the support to which it is attached, it may be unsuitable to the work required of it. Hence it becomes desirable to carefully study the design and construction of a drill support, and to consider the requirements which it is needful to fulfil. Assuming the necessity for a high degree of strength and rigidity in the support, a primary condition is that it shall allow the machine to be readily adjusted to any angle, so that the holes may be bored in the direction and with the inclination required. When this requirement is not fulfilled, the machine is placed, in this respect, at a great disadvantage with hand labour. If a machine drill were not capable of boring in any position and in any direction, hand labour would have to be employed in conjunction with it, and such incompleteness in the work of a machine would constitute a serious objection to its adoption.

Besides allowing of the desired adjustment of the machine, the support must be itself adjustable to uneven ground. The bottom of a shaft which is being sunk, or the sides, roof, and floor of a heading which is being driven, present great irregularities of surface, and, as the support must of necessity in most cases be fixed to these, it is obvious that its design and construction must be such as will allow of its ready adjustment to these irregularities. The means by which the adjustment is effected should be few and simple, for simplicity of parts is important in the support as well as in the machine, and for the same reasons. A large proportion of the time during which a machine drill is in use is occupied in shifting it from one position or one situation to another; this time reduces, in a proportionate degree, the superiority of machine over hand labour, in respect of rapidity of execution, and it is evidently desirable that it should be shortened as far as possible. Hence the necessity for the employment of means of adjustment which shall be few in number, rapid in action, and of easy management.

For reasons similar to the foregoing, the drill support must be of small dimensions, and sufficiently light to allow of its being easily portable. The limited space in which rock drills are used renders this condition, as in the case of the machine itself, a very important one. It must be borne in mind that, after every blast, the dislodged rock has to be removed, and rapidity of execution requires that the operations of removal should be carried on without hindrance. A drill support that occupies a large proportion of the free space in a shaft or a heading is thus a cause of inconvenience and a source of serious delay. Moreover, as it has to be continually removed from one situation to another, it should be of sufficiently light weight to allow of its being lifted or run along without difficulty. In underground workings, manual power is generally the only power available, and therefore it is desirable that both the machine and its support should be of such weight that each may be lifted by one man. Of course, when any endeavour is made to reduce the weight of the support, the necessity for great strength and rigidity must be kept in view.

In spacious headings, such as are driven in railway tunnel work, supports of a special kind may be used. In these situations, the conditions of work are different from those which exist in mines. The space is less limited, the heading is commenced at surface, and the floor is laid with a tramway and sidings. In such a case, the support may consist of a more massive structure mounted upon wheels to run upon the rails. This support will carry several machines, and to remove it out of the way when occasion requires, it will be run back on to a siding; but for ordinary mining purposes, such a support is suitable.

The Stretcher Bar.

—The simplest kind of support is the “stretcher bar.” This consists essentially of a bar so constructed that it may be lengthened or shortened at pleasure, by means of a screw. It is fixed in position by screwing the ends into firm contact with the sides, or with the roof and the floor, of a heading. The machine is fixed to this bar by means of a clamp, which, when loosened, slides along the bar, and allows the drill to be placed in the required position, and to be directed at the required angle. The bar illustrated in [Fig. 26, Plate V.], is that which is used with the Darlington drill; in it, lightness and rigidity are combined in the highest possible degree by the adoption of the hollow section. The mode of setting the bar in a heading is shown in the drawing; the end claws are set against pieces of wood on the floor and the roof, and are tightened by turning the screw with a common bar.

The simple stretcher bar is frequently used in narrow drivings and in shafts of small diameter. But a more satisfactory support in drivings is afforded by a bar suitably mounted upon a carriage designed to run upon rails. The carriage consists simply of a trolly, to the fore part of which the bar is fixed usually by some kind of hinge-joint. It is obvious that the details of the construction of this support may be varied greatly, and numerous designs have been introduced and adopted. In [Figs. 27 and 28, Plate VI.], is shown a support of this character designed by J. Darlington. A single vertical bar is carried on the fore part of the trolly, and fixed, by the usual means, against the centre of the roof. This vertical bar carries an arm, which is capable of turning upon it, as upon a centre, and of sliding up and down it. This arm carries the drill. The central bar having been fixed in position, the arm is slid up to the highest position required, and fixed against the side of the heading. A row of holes are then bored from this arm. When these are completed, the arm is lowered the requisite distance, and another row of holes are bored. This is continued until all the holes are bored over one-half the face. The arm is then swung round, and fixed against the other side of the heading, and the holes are bored over that half the face in like manner. In this way, one-half the heading is kept clear to allow the operations of removing the dislodged rock to be carried on at the same time. If desired, two arms may be used. This arrangement gives undoubtedly great facilities for working the drill, and leaves the heading comparatively unencumbered.

In shaft sinking, the same support, slightly modified, is used without the trolly. The arrangement adopted in this case is shown in [Fig. 29, Plate VII.] The central bar is held firmly in its position by a cross stretcher bar set against the sides of the shaft. The arms are made to revolve upon this bar to allow the holes to be bored in the positions required. When all the holes have been bored, the support, with the machines, is hauled up, by means of a chain attached to the central bar, out of the way of the blast. With this support, the time of fixing, raising, and lowering is reduced to a minimum; while the facility with which the machines may be slid along and fixed to the arm, and the positions of the latter changed, allows the boring to be carried on rapidly.

For open work, as in quarrying, where the stretcher bar cannot be used, the tripod stand is adopted.

The Dubois-François Carriage.

—The support commonly used in France and in Belgium consists of a kind of carriage carrying bars upon which the drills are set. This carriage is used in drivings of all kinds; but it is particularly suitable for tunnelling. It has been adopted, with but slight modification, in the St. Gothard tunnel, and in several other important works of the like character.

A modification of the carriage is shown in [Figs. 30 and 31]. Being designed for ordinary mining operations, it carries but two machines; but it will be readily perceived that, by increasing the number of vertical screws, the same support may be made to carry a larger number. It consists essentially of a vertical frame of flat bar iron a b c d, 8 feet in length, and 4 feet 9 inches in height above the rails, the hinder portion of which rests upon a cast-iron plate e f g h, carried upon two wheels; on this are fixed the two uprights l, l′, which, being bound to the upper part by a transverse bar m m′, form a framing to serve as a support to the two vertical screws p′, q′. The front framing is formed of two longitudinals b c and b′ c′ and the uprights a, a′, and the vertical screws p, q, which are connected to the upper part by the single piece a d. This framing is supported below upon a small trolly with four wheels, connected to the two longitudinals of the framing by a pivot bolt n of T form, the bar of the T being inserted into the elongated openings o cut through the middle of the curved portion of the longitudinals. The cast-iron plate behind, the use of which is only to give stability to the carriage, carries above it, by means of the two curved pieces h, h′, a wrought-iron plate V, upon which the small tools needed for repairs are kept. Two screws, s, s′, carried by lugs cast upon the back of the plate, serve, by turning them down upon the rails, to fix the carriage, the latter being slightly lifted by the screws.

Each machine is supported at two points. Behind, the point of support is given by a cast-iron bracket t, having a projecting eye which enters between the two cheeks formed at the back end of the machine by the continuations of the two longitudinals of the framing. A pin bolt, carried by the machine, allows the latter to be fixed to the bracket, while leaving sufficient freedom of motion to allow of its being directed at the required angle. This bracket, shown in plan in [Fig. 33], is supported by a kind of nut, [Fig. 32], having two handles whereby it may be easily turned. By raising or lowering this, the hinder support of the drill may be brought to the requisite height. To prevent it turning upon the screw, a pin is passed through the hole o, which pin forms a stop for the handles aforementioned. The rotation of the bracket itself is rendered impossible by the form of the vertical screw upon which it is set, as shown in [Fig. 33]. In front, the support is a fork, the shank of which slides along in the piece U, [Figs. 30 and 31]. This support, which is not screwed on the inside, rests upon a nut of the same form as that already described, and the same means are employed to prevent rotation as in the case of the hinder supports.

Section III.—Appliances for Firing Blasting Charges.

In the foregoing sections, the machines and tools used in rock boring have been treated of. It now remains to describe those which are employed in firing the charges after they have been placed in the bore-holes. In this direction, too, great progress has been made in recent times. With the introduction of new explosive agents, arose the necessity for improved means of firing them. Attention being thus directed to the subject, its requirements were investigated and its conditions observed, the outcome being some important modifications of the old appliances and the introduction of others altogether new. Some of the improvements effected are scarcely less remarkable than the substitution of machine for hand boring.

Fig. 28.

Fig. 29.

The means by which the charge of explosive matter placed in the bore-hole is fired constitute a very important part of the set of appliances used in blasting. The conditions which any such means must fulfil are: (1) that it shall fire the charge with certainty; (2) that it shall allow the person whose duty it is to explode the charge to be at a safe distance away when the explosion takes place; (3) that it shall be practically suitable, and applicable to all situations; and (4) that it shall be obtainable at a low cost. To fulfil the second and most essential of these conditions, the means must be either slow in operation, or capable of being acted upon at a distance. The only known means possessing the latter quality is electricity. The application of electricity to this purpose is of recent date, and in consequence of the great advantages which it offers, its use is rapidly extending. The other means in common use are those which are slow in operation, and which allow thereby sufficient time to elapse between their ignition and the explosion of the charge for a person to retire to a safe distance. These means consist generally of a train of gunpowder so placed that the ignition of the particles must necessarily be gradual and slow. The old, and in some parts still employed, mode of constructing this train was as follows: An iron rod of small diameter and terminating in a point, called a “pricker,” was inserted into the charge and left in the bore-hole while the tamping was being rammed down. When this operation was completed, the pricker was withdrawn, leaving a hole through the tamping down to the charge. Into this hole, a straw, rush, quill, or some other like hollow substance filled with gunpowder, was inserted. A piece of slow-match was then attached to the upper end of this train, and lighted.

The combustion of the powder confined in the straw fired the charge, the time allowed by the slow burning of the match being sufficient to enable the man who ignited it to retire to a place of safety. This method of forming the train does not, however, satisfy all the conditions mentioned above. It is not readily applicable to all situations. Moreover, the use of the iron pricker may be a source of danger; the friction of this instrument against silicious substances in the sides of the bore-hole or in the tamping has in some instances occasioned accidental explosions. This danger is, however, very greatly lessened by the employment of copper or phosphor-bronze instead of iron for the prickers. But the method is defective in some other respects. With many kinds of tamping, there is a difficulty in keeping the hole open after the pricker is withdrawn till the straw can be inserted. When the holes are inclined upwards, besides this difficulty, another is occasioned by the liability of the powder constituting the train to run out on being ignited. And in wet situations, special provision has to be made to protect the trains. Moreover, the manufacture of these trains by the workmen is always a source of danger. Many of these defects in the system may, however, be removed by the employment of properly constructed trains. One of these trains or “squibs” is shown full size in [Fig. 28].

Safety Fuse.

—Many of the defects pertaining to the system were removed by the introduction of the fuse invented by W. Bickford, and known as “safety fuse.” The merits of this fuse, which is shown full size in [Fig. 29], are such as to render it one of the most perfect of the slow-action means that have yet been devised. The train of gunpowder is retained in this fuse, but the details of its arrangement are changed so as to fairly satisfy the conditions previously laid down as necessary. It consists of a flexible cord composed of a central core of fine gunpowder, surrounded by hempen yarns twisted up into a tube, and called the countering. An outer casing is made of different materials, according to the circumstances under which it is intended to be used. A central touch thread, or in some cases two threads, passes through the core of gunpowder. This fuse, which in external appearance resembles a piece of plain cord, is tolerably certain in its action; it may be used with equal facility in holes bored in any direction; it is capable of resisting considerable pressure without injury; it may be used without special means of protection in wet ground; and it may be transported from place to place without risk of damage.

In the safety fuse, the conditions of slow burning are fully satisfied, and certainty is in some measure provided for by the touch thread through the centre of the core. As the combustion of the core leaves, in the small space occupied by it, a carbonaceous residue, there is little or no passage left through the tamping by which the gases of the exploding charge may escape, as in the case of the squibs. Hence results an economy of force. Another advantage offered by the safety fuse is, that it may be made to carry the fire into the centre of the bursting charge if it be desired to produce rapid ignition. This fuse can be also very conveniently used for firing charges of compounds other than gunpowder, by fixing a detonating charge at the end of it, and dropping the latter into the charge of the compound. This means is usually adopted in firing the nitro-glycerine compounds, the detonating charge in such cases being generally contained within a metallic cap. In using this fuse, a sufficient length is cut off to reach from the charge to a distance of about an inch, or farther if necessary, beyond the mouth of the hole. One end is then untwisted to a height of about a quarter of an inch, and placed to that depth in the charge. The fuse being placed against the side of the bore-hole with the other end projecting beyond it, the tamping is put in, and the projecting end of the fuse slightly untwisted. The match may then be applied directly to this part. The rate of burning is about two and a half feet a minute.

Safety fuse is sold in coils of 24 feet in length. The price varies according to the quality, and the degree of protection afforded to the train.

Electric Fuses.

—The employment of electricity to fire the charge in blasting rock offers numerous and great advantages. The most important, perhaps, is the greatly increased effect of the explosions when the charges are fired simultaneously. But another advantage, of no small moment, lies in the security from accident which this means of firing gives. When electricity is used, not only may the charge be fired at the moment desired, after the workmen have retired to a place of safety, but the danger due to a misfire is altogether avoided. Further, the facility afforded by electricity for firing charges under water is a feature in this agent of very great practical importance. It would therefore seem, when all these advantages are taken into account, that electricity is destined to become of general application to blasting purposes in this country, as it is already in Germany and in America.

An electric fuse consists of a charge of an explosive compound suitably placed in the circuit of an electric current, which compound is of a character to be acted upon by the current in a manner and in a degree sufficient to produce explosion. The mode in which the current is made to act depends upon the nature of the source of the electricity. That which is generated by a machine is of high tension, but small in quantity; while that which is generated by a battery is, on the contrary, of low tension, but is large in quantity. Electricity of high tension is capable of leaping across a narrow break in the circuit, and advantage is taken of this property to place in the break an explosive compound sufficiently sensitive to be decomposed by the passage of the current. The electricity generated in a battery, though incapable of leaping across a break in the circuit, is in sufficient quantity to develop a high degree of heat. Advantage is taken of this property to fire an explosive compound by reducing the sectional area of the wire composing a portion of the circuit at a certain point, and surrounding this wire with the compound. It is obvious that any explosive compound may be fired in this way; but for the purpose of increasing the efficiency of the battery, preference is given to those compounds which ignite at a low temperature. Hence it will be observed that there are two kinds of electric fuses, namely, those which may be fired by means of a machine, and which are called “tension” fuses, and those which require a battery, and which are known as “quantity” fuses.

In the tension, or machine fuses, the circuit is interrupted within the fuse case, and the priming, as before remarked, is interposed in the break; the current, in leaping across the interval, passes through the priming. In the quantity or battery fuses, the reduction of the sectional area is effected by severing the conducting wire within the fuse case, and again joining the severed ends of the wire by soldering to them a short piece of very fine wire. Platinum wire, on account of its high resistance and low specific heat, is usually employed for this purpose. The priming composition is placed around this fine wire, which is heated to redness by the current as soon as the circuit is closed.

The advantages of high tension lie chiefly in the convenient form and ready action of the machines employed to excite the electricity. Being of small dimensions and weight, simple in construction, and not liable to get quickly out of order, these sources of electricity are particularly suitable for use in mining operations, especially when these operations are entrusted, as they usually are, to men of no scientific knowledge.

Another advantage of high tension is the small effect of line resistance upon the current, a consequence of which is that mines may be fired at long distances from the machine, and through iron wire of very small section. A disadvantage of high tension is the necessity for a perfect insulation of the wires.

When electricity of low tension is employed, the insulation of the wires needs not to be perfect, so that leakages arising from injury to the coating of the wire are not of great importance. In many cases, bare wires may be used. Other advantages of low tension are the ability to test the fuse at any moment by means of a weak current, and an almost absolute certainty of action. For this reason, it is usually preferred for torpedoes and important submarine work. On the other hand, the copper wires used must be of comparatively large section, and the influence of line resistance is so considerable that only a small number of shots can be fired simultaneously when the distance is great.

Fig. 30.

Fig. 31.

Fig. 32.

In [Fig. 30] is shown an external view of an electric tension fuse. It consists of a metal cap containing a detonating composition, upon the top of which is placed the priming to be ignited by the electric spark. The ends of two insulated wires project into this priming, which is fired by the passage of the spark from one of these wires to the other. The insulated wires are sufficiently long to reach a few inches beyond the bore-hole.

Sometimes the fuse is attached to the end of a stick, and the wires are affixed to the latter in the manner shown in [Fig. 31]. The rigidity of the stick allows the fuse to be readily pushed into the bore-hole. When the ground is not very wet, bare wires are, for cheapness, used, and the stick is in that case covered with oiled paper, or some other substance capable of resisting moisture. These “blasting sticks,” as they are called, are extensively used in Germany. When heavy tamping is employed, the stick is not suitable, by reason of the space which it occupies in the bore-hole.

A mode of insulating the wires, less expensive than the guttapercha shown in [Fig. 30], is illustrated in [Fig. 32]. In this case, the wires are cemented between strips of paper, and the whole is dipped into some resinous substance to protect it from water. These “ribbon” wires may be used in ground that is not very wet. They occupy little or no space in the bore-hole, and therefore are suitable for use with tamping.

To connect the fuses with the machine or the battery, two sets of wires are required when a single shot is fired, and three sets may be needed when two or more shots are fired simultaneously. Of these several sets of wires, the first consists of those which are attached to the fuses, and which, by reason of their being placed in the shot-hole, are called the “shot-hole wires.” Two shot-hole wires must be attached to each fuse, and they must be of such a length that, when the fuse has been placed in its proper position in the charge, the ends may project a few inches from the hole. These wires must also be “insulated,” that is, covered with a substance capable of preventing the escape of electricity.

The second set of wires consists of those which are employed to connect the charges one with another, and which, for this reason, are called “connecting wires.” In connecting the charges in single circuit, the end of one of the shot-hole wires of the first charge is left free, and the other wire is connected, by means of a piece of this connecting wire, to one of the shot-hole wires in the second hole; the other wire in this second hole is then connected, in the same manner, to one of the wires in the third hole; and so on till the last hole is reached, one shot-hole wire of which is left free, as in the first. Whenever the connecting wires can be kept from touching the rock, and also from coming into contact one with another—and in most cases this may be done—bare wire may be used, the cost of which is very little. But when this condition cannot be complied with, and, of course, when blasting in water, the connecting wires, like the shot-hole wires, must be insulated. When guttapercha shot-hole wires are used, it is best to have them sufficiently long to allow the ends projecting from one hole to reach those projecting from the next hole. This renders connecting wire unnecessary, and moreover saves one joint for each shot.

Fig. 33.

Fig. 34.

Cables.

—The third set of wires required consists of those used to connect the charges with the machine or the battery. These wires, which are called the “cables,” consist each of three or more strands of copper wire well insulated with guttapercha, or better, indiarubber, the coating of these materials being protected from injury by a sheathing of tape or of galvanized iron wire underlaid with hemp. Two cables are needed to complete the circuit; the one which is attached to the positive pole of the machine, that is, the pole through which the electric current passes out, is distinguished as the “leading cable,” and the other, which is attached to the negative pole, that is, the pole through which the current returns to the machine, is described as the return cable. Sometimes both the leading and the return cables are contained within one covering. When a cable having a metallic sheathing is used, the sheathing may be made to serve as a return cable, care being taken to make good metallic contact with the wires that connect the sheathing to the fuses and to the terminal of the machine. The best kind of unprotected cable consists of a three-strand tinned copper wire, each 0·035 inch in diameter, insulated with three layers of indiarubber to 0·22 inch diameter, and taped with indiarubber-saturated cotton to 0·24 inch diameter, as shown in [Fig. 33]. The best protected cable consists of a similar strand of copper wire, covered with guttapercha and tarred jute, and sheathed with fifteen galvanized iron wires of 0·08 inch diameter each, to a total diameter of 0·48 inch, as shown in [Fig. 34].

Detonators.

—The new explosives of the nitro-cotton and nitro-glycerine class cannot be effectively fired by means of safety or other fuse alone. To bring about their instantaneous decomposition, it is necessary to produce in their midst the explosion of some other substance. The force of this initial explosion causes the charge of gun-cotton, or dynamite, as the case may be, to detonate. It has been found that the explosion of the fulminate of mercury brings about this result most effectively and with the greatest certainty; and this substance is therefore generally used for the purpose. The charge of fulminate is contained in a copper capsule about a quarter of an inch in diameter, and from 1 inch to 1¹⁄₄ inch in length. These caps, with their charge of fulminate, which are now well known to users of the nitro-compounds, are called “detonators.” It is of the highest importance that these detonators should contain a sufficiently strong charge to produce detonation, for if too weak, not only is the whole force of the explosive not developed, but a large quantity of noxious gas is generated. Gun-cotton requires a much stronger charge of fulminate than dynamite.

Fig. 35.

In the electric fuses [illustrated], the metal case shown is the detonator, the fuse being placed inside above the fulminate. When safety fuse is used, the end is cut off clean and inserted into the cap, which is then pressed tightly upon the fuse by means of a pair of nippers, as shown in [Fig. 35]. When water tamping is used, and when, with ordinary tamping, the hole is very wet, a little white-lead or grease must be put round the edge of the cap as a protection. The electric fuses are always made waterproof; consequently, they are ready for use under all circumstances. When the safety fuse burns down into the cap, or when, in the other case, the priming of the electric fuse is fired, the fulminate explodes and causes the detonation of the charge in which it is placed.

Firing Machines and Batteries.

—The electrical machines used for firing tension fuses are of two kinds. In one kind, the electricity is excited by friction, and stored in a condenser to be afterwards discharged by suitable means provided for the purpose. In the other kind, the electricity is excited by the motion of an armature before the poles of a magnet. The former kind are called “frictional electric” exploders; the latter kind are known as “magneto-electric” exploders. When a magneto-electric machine contains an electro-magnet instead of a permanent magnet, it is described as a “dynamo-electric exploder.”

Frictional machines act very well as exploders so long as they are kept in a proper state. But as they are injuriously affected by a moist atmosphere, and weaken rapidly with use by reason of the wearing away of the rubbers, it is necessary to take care that they be in good electrical condition before using them for firing. Unless this care be taken, the quantity of electricity excited by a given number of revolutions of the plate will be very variable, and vexatious failures will ensue. If, however, the proper precautions be observed, very certain and satisfactory results may be obtained. In Germany and in America, frictional exploders are generally used.

Magneto-electric machines possess the very valuable quality of constancy. They are unaffected, in any appreciable degree, by atmospheric changes, and they are not subject to wear. These qualities are of inestimable worth in an exploder used for ordinary blasting operations. Moreover, as they give electricity of a lower tension than the frictional machines, defects of insulation are less important. Of these machines, only the dynamo variety are suitable for industrial blasting. It is of primary importance that an exploder should possess great power. The mistake of using weak machines has done more than anything else to hinder the adoption of electrical firing in this country.

Fig. 36.

The machine most used in Germany is Bornhardt’s frictional exploder, shown in [Fig. 36]. This machine is contained in a wooden case 20 inches in length, 7 inches in breadth, and 14 inches in depth, outside measurement. The weight is about 20 lb.

To fire the charges by means of this exploder, the leading wire is attached to the upper terminal B, and the return to the lower terminal C, the other ends of these wires being connected to the fuses. The handle is then turned briskly from fifteen to thirty times, according to the number of the fuses and the state of the machine, to excite the electricity. The knob A is then pressed suddenly in, and the discharge takes place. To ascertain the condition of the machine, a scale of fifteen brass-headed nails is provided on the outside, which scale may be put in communication with the poles B and C by means of brass chains, as shown in the drawing. If after twelve or fourteen turns, the spark leaps the scale when the knob is pushed in, the machine is in a sufficiently good working condition. To give security to the men engaged, the handle is designed to be taken off when the machine is not in actual use; and the end of the machine into which the cable wires are led is made to close with a lid and lock, the key of which should be always in the possession of the man in charge of the firing operations.

Fig. 37.

Fig. 38.

In America, there are two frictional exploders in common use. One, shown in [Fig. 37], is the invention of H. Julian Smith. The apparatus is enclosed in a wooden case about 1 foot square and 6 inches in depth. The handle is on the top of the case, and is turned horizontally. This handle is removable, as in Bornhardt’s machine. The cable wires having been attached to the terminals, the handle is turned forward a certain number of times to excite the electricity, and then turned a quarter of a revolution backward to discharge the condenser and to fire the blast. By this device, the necessity for a second aperture of communication with the inside is avoided, an important point in frictional machines, which are so readily affected by moisture. The aperture through which the axis of the plate passes, upon which axis the handle is fixed, is tightly closed by a stuffing-box. A leathern strap on one end of the case allows the machine to be easily carried. The weight of this exploder is under 10 lb.

The other exploder used is that designed by G. Mowbray. This machine, which is shown in [Fig. 38], is contained in a wooden barrel-shaped case, and is known as the “powder-keg” exploder, the form and dimensions of the case being those of a powder-keg. The action is similar to that of the machine last described. The cable wires having been attached to the terminals at one end of the keg, the handle at the other end is turned forward to excite the electricity, and the condenser is discharged by making a quarter turn backward, as in Smith’s machine. The handle is in this case also removable. The weight of the powder-keg exploder is about 26 lb.

Both of these machines are very extensively used, and good results are obtained from them. They stand well in a damp atmosphere, and do not quickly get out of order from the wearing of the rubbers. They are also, especially the former, very easily portable.

Fig. 39.

The machine commonly used in England is the dynamo-electric exploder of the Messrs. Siemens. This machine, which is the best of its kind yet introduced for blasting purposes, is not more than half the size of Bornhardt’s frictional exploder; but it greatly exceeds the latter in weight, that of Siemens’ being about 55 lb. The apparatus, which is contained within the casing shown in [Fig. 39], consists of an ordinary Siemens’ armature, which is made, by turning the handle, to revolve between the poles of an electro-magnet. The coils of the electro-magnet are in circuit with the wire of the armature; the residual magnetism of the electro-magnet cores excites, at first, weak currents; these pass into the coils, thereby increasing the magnetism of the cores, and inducing still stronger currents in the armature wire, to the limit of magnetic saturation of the iron cores of the electro-magnets. By the automatic action of the machine, this powerful current is, at every second turn of the handle, sent into the cables leading to the fuses.

To fire this machine, the handle is turned gently till a click is heard from the inside, indicating that the handle is in the right position to start from. The cable wires are then attached to the terminals, and the handle is turned quickly, but steadily. At the completion of the second revolution, the current is sent off into line, as it is termed, that is, the current passes out through the cables and the fuses. As in the case of the frictional machines, the handle is, for safety, made removable. This exploder is practically unaffected by moisture, and it is not liable to get out of order from wear.

Induction coils have been used to fire tension fuses; but it is surprising that they have not been more extensively applied to that purpose. A coil designed for the work required of it is a very effective instrument. If constructed to give a spark not exceeding three inches in length, with comparatively thick wire for quantity, it makes a very powerful exploder. An objection to its use is the necessity for a battery. But a few bichromate of potash cells, provided with spiral springs to hold the zincs out of the liquid, and designed to be set in action by simply pressing down the zincs, give but little trouble, so that the objection is not a serious one. The writer has used an induction exploder in ordinary mining operations without experiencing any difficulty or inconvenience. It is cheap, easily portable, and constant in its action.

Batteries are used to fire what are known as “quantity” or “low tension” fuses. Any cells may be applied to this purpose; but they are not all equally suitable. A firing battery should require but little attention, and should remain in working order for a long time. These conditions are satisfactorily fulfilled by only two cells, namely, the Léclanché and the Bichromate of Potash. The latter is the more powerful, and generally the more suitable. The Léclanché is much used in this country for firing purposes, under the form known as the “Silvertown Firing Battery.” This battery consists of a rectangular teak box, containing ten cells. Two, or more, of these may be joined up together when great power is required. In France, the battery used generally for firing is the Bichromate. This battery is much more powerful than the Léclanché, and as no action goes on when the zincs are lifted out of the liquid, it is equally durable. It is moreover much cheaper. At the suggestion of the writer, Mr. Apps, of the Strand, London, has constructed a bichromate firing battery of very great power. It is contained in a box of smaller dimension than the 10-cell Silvertown. The firing is effected by simply lowering the zincs, which rise again automatically out of the liquid, so that there is no danger of the battery exhausting itself by continuous action in case of neglect. Externally, this battery, like the Silvertown, appears a simple rectangular box, so that no illustration is needed. With either of these, the usual objections urged against the employment of batteries, on the ground of the trouble involved in keeping them in order, and their liability to be injured by ignorant or careless handling, do not apply, or at least apply in only a very unimportant degree.

To guard against misfires, the machine or the battery used should be constructed to give a very powerful current. If this precaution be observed, and the number of fuses in circuit be limited to one-half that which the machine is capable of firing with a fair degree of certainty, perfectly satisfactory results may be obtained. The employment of weak machines and batteries leads inevitably to failure. In the minds of those who have hitherto tried electrical blasting in this country, there seems to be no notion of any relation existing between the work to be done and the force employed to do it. The electrical exploder is regarded as a sort of magic box that needs only to be set in action to produce any required result. Whenever failure ensues, the cause is unhesitatingly attributed to the fuses.

CHAPTER II.
EXPLOSIVE AGENTS USED IN BLASTING ROCKS.

Section I.—Phenomena accompanying an Explosion.

Nature of an Explosion.

—The combination of oxygen with other substances for which it has affinity is called generally “oxidation.” The result of this combination is a new substance, and the process of change is accompanied by the liberation of heat. The quantity of heat set free when two substances combine chemically is constant, that is, it is the same under all conditions. If the change takes place within a short space of time, the heat becomes sensible; but if the change proceeds very slowly, the heat cannot be felt. The same quantity, however, is liberated in both cases. Thus, though the quantity of heat set free by a chemical combination is under all conditions the same, the degree or intensity of the heat is determined by the rapidity with which the change is effected.

When oxidation is sufficiently rapid to cause a sensible degree of heat, the process is described as “combustion.” The oxidation of a lump of coke in the furnace, for example, is effected within a short space of time, and, as the quantity of heat liberated by the oxidation of that weight of carbon is great, a high degree results. And it is well known and obvious that as combustion is quickened, or, in other words, as the time of change is shortened, the intensity of the heat is proportionally increased. So in the case of common illuminating gas, the oxidation of the hydrogen is rapidly effected, and, consequently, a high degree of heat ensues.

When oxidation takes place within a space of time so short as to be inappreciable to the senses, the process is described as “explosion.” The combustion of a charge of gunpowder, for example, proceeds with such rapidity that no interval can be perceived to intervene between the commencement and the termination of the process. Oxidation is in this case, therefore, correctly described as an explosion; but the combustion of a train of gunpowder, or of a piece of quick-match, though exceedingly rapid, yet, as it extends over an appreciable space of time, is not to be so described. By analogy, the sudden change of state which takes place when water is “flashed” into steam, is called an explosion. It may be remarked here that the application of this expression to the bursting of a steam boiler is an abuse of language; as well may we speak of an “explosion” of rock.

From a consideration of the facts stated in the foregoing paragraphs, it will be observed that oxidation by explosion gives the maximum intensity of heat.

Measure of Heat, and specific Heat.

—It is known that if a certain quantity of heat will raise the temperature of a body one degree, twice that quantity will raise its temperature two degrees, three times the quantity, three degrees, and so on. Thus we may obtain a measure of heat by which to determine, either the temperature to which a given quantity of heat is capable of raising a given body, or the quantity of heat which is contained in a given body at a given temperature. The quantity of heat requisite to produce a change of one degree in temperature is different for different bodies, but is practically constant for the same body, and this quantity is called the “specific heat” of the body. The standard which has been adopted whereby to measure the specific heat of bodies is that of water, the unit being the quantity of heat required to raise the temperature of 1 lb. of water through 1° Fahr., say from 32° to 33°. The quantity of heat required to produce this change of temperature in 1 lb. of water is called the “unit of heat,” or the “thermal unit.” Having determined the specific heat of water, that of air may in like manner be ascertained, and expressed in terms of the former. It has been proved by experiments that if air be heated at constant pressure through 1° Fahr., the quantity of heat absorbed is 0·2375 thermal units, whatever the pressure or the temperature of the air may be. Similarly it has been shown that the specific heat of air at constant volume is, in thermal units, 0·1687; that is, if the air be confined so that no expansion can take place, 0·1687 of a thermal unit will be required to increase its temperature one degree.

Heat liberated by an Explosion.

—In the oxidation of carbon, one atom of oxygen may enter into combination with one atom of that substance; the resulting body is a gas known as “carbonic oxide.” As the weight of carbon is to that of oxygen as 12 is to 16, 1 lb. of the former substance will require for its oxidation 1¹⁄₃ lb. of the latter; and since the two enter into combination, the product, carbonic oxide, will weigh 1 + 1¹⁄₃ = 2¹⁄₃ lb. The combining of one atom of oxygen with one of carbon throughout this quantity, that is, 1¹⁄₃ lb. of oxygen, with 1 lb. of carbon, generates 10,100 units of heat. Of this quantity, 5700 units are absorbed in changing the carbon from the solid into the gaseous state, and 4400 are set free. The quantity of heat liberated, namely, the 4400 units, will be expended in raising the temperature of the gas from 32° Fahr., which we will assume to be that of the carbon and the oxygen previous to combustion, to a much higher degree, the value of which may be easily determined. The 4400 units would raise 1 lb. of water from 32° to 32 + 4400 = 4432°; and as the specific heat of carbonic oxide is 0·17 when there is no increase of volume, the same quantity of heat will raise 1 lb. of that gas from 32° to 32 + 4400 0·17 = 25,914°. But in the case under consideration, we have 2¹⁄₃ lb. of the gas, the resulting temperature of which will be 25,9142¹⁄₃ = 9718°.

In the oxidation of carbonic oxide, one atom of oxygen combines with one atom of the gaseous carbon; the resulting body is a gas known as “carbonic acid.” Since 2¹⁄₃ lb. of carbonic oxide contains 1 lb. of carbon, that quantity of the oxide will require 1¹⁄₃ lb. of oxygen to convert it into the acid, that is, to completely oxidize the original pound of solid carbon. By this combination, 10,100 units of heat are generated, as already stated, and since the carbon is now in the gaseous state, the whole of that quantity will be set free. Hence the temperature of the resulting 3²⁄₃ lb. of carbonic acid will be

32 + 4400 + 10,100 0·17 × 3·667 = 23,516°.

It will be seen from the foregoing considerations that if 1 lb. of pure carbon be burned in 2²⁄₃ lb. of pure oxygen, 3²⁄₃ lb. of carbonic acid is produced, and 14,500 units of heat are liberated; and further, that if the gas be confined within the space occupied by the carbon and the oxygen previously to their combination, the temperature of the product may reach 23,516° Fahr.

In the oxidation of hydrogen, one atom of oxygen combines with two atoms of the former substance; the resulting body is water. As the weight of hydrogen is to that of oxygen as 1 is to 16, 1 lb. of the former gas will require for its oxidation 8 lb. of the latter; and since the two substances enter into combination, the product, water, will weigh 1 + 8 = 9 lb. By this union, 62,032 units of heat are generated. Of this quantity, 8694 are absorbed in converting the water into steam, and 53,338 are set free. The specific heat of steam at constant volume being 0·37, the temperature of the product of combustion, estimated as before, will be

32 + 53,338 0·37 × 9 = 16,049°.

Hence it will be observed that if 1 lb. of hydrogen be burned in 8 lb. of oxygen, 9 lb. of steam will be produced, and 53,338 units of heat will be liberated; and further, that the temperature of the product may reach 16,049°.

Gases generated by an Explosion.

—It was shown in the preceding paragraph that in the combustion of carbon, one atom of oxygen may unite with one atom of carbon to form carbonic oxide, or two atoms of oxygen may unite with one atom of carbon to form carbonic acid. When the combination takes place according to the former proportions, the reaction is described as “imperfect combustion,” because the carbon is not fully oxidized; but when the combination is effected in the latter proportions, the combustion is said to be “perfect,” because no more oxygen can be taken up. The products of combustion are in both cases gaseous. Carbonic oxide, the product of imperfect combustion, is an extremely poisonous gas; it is this gas which is so noisome in close headings, and in all ill-ventilated places, after a blast has been fired. A cubic foot of carbonic oxide, the specific gravity of which is 0·975, weighs, at the mean atmospheric pressure, 0·075 lb., so that 1 lb. will occupy a space of 13·5 cubic feet. Thus 1 lb. of carbon imperfectly oxidized will give 2¹⁄₃ lb. of carbonic oxide, which, at the mean atmospheric pressure of 30 inches and the mean temperature of 62° Fahr., will occupy a space of 13·5 × 2¹⁄₃ = 31·5 cubic feet. The product of perfect combustion, carbonic acid, is a far less noxious gas than the oxide, and it is much more easily expelled from confined places, because water possesses the property of absorbing large quantities of it. In an ill-ventilated but wet heading, the gas from a blast is soon taken up. Carbonic acid is a comparatively heavy gas, its specific gravity relatively to that of common air being 1·524. Hence a cubic foot at the ordinary pressure and temperature will weigh 0·116 lb., and 1 lb. of the gas under the same conditions will occupy a space of 8·6 cubic feet. Thus if 1 lb. of carbon be completely oxidized, there will result 3²⁄₃ lb. of carbonic acid, which will fill a space of 8·6 × 3²⁄₃ = 31·5 cubic feet. It will be observed that, though an additional pound of oxygen has been taken up during this reaction, the product occupies the same volume as the oxide. In complete combustion, therefore, a contraction takes place.

In the oxidation of hydrogen, as already pointed out, one atom of oxygen combines with two atoms of the former substance to form water. In this case, the product is liquid. But the heat generated by the combustion converts the water into steam, so that we have to deal with this product also in the gaseous state, in all considerations relating to the effects of an explosion. A cubic foot of steam, at atmospheric pressure and a temperature of 212° Fahr., weighs 0·047 lb.; 1 lb. of steam under these conditions will, therefore, occupy a space of 21·14 cubic feet. Thus the combustion of 1 lb. of hydrogen will produce 9 lb. of steam, which, under the conditions mentioned, will fill a space of 21·14 × 9 = 190·26 cubic feet.

Usually in an explosion a large quantity of nitrogen gas is liberated. This gas, which is not in itself noxious, has a specific gravity of 0·971, so that practically a cubic foot will weigh 0·075 lb., and 1 lb. will occupy a space of 13·5 cubic feet, which are the weight and the volume of carbonic oxide. Other gases are often formed as products of combustion; but the foregoing are the chief, viewed as the results of an explosion, since upon these the force developed almost wholly depends.

Force developed by an Explosion.

—A consideration of the facts enunciated in the foregoing paragraphs will show to what the tremendous energy developed by an explosion is due. It was pointed out that the combustion of 1 lb. of carbon gives rise to 31·5 cubic feet of gas. If this volume of gas be compressed within the space of 1 cubic foot it will obviously have a tension of 31·5 atmospheres; that is, it will exert upon the walls of the containing vessel a pressure of 472 lb. to the square inch. If the same volume be compressed into a space one-eighth of a cubic foot in extent, say a vessel of cubical form and 6 inches side, the tension will be 31·5 × 8 = 252 atmospheres, and the pressure 472 × 8 = 3776 lb. to the square inch. Assuming now the oxygen to exist in the solid state, and the two bodies carbon and oxygen to occupy together a space of one-eighth of a cubic foot, the combustion of the carbon will develop upon the walls of an unyielding containing vessel of that capacity a pressure of 252 atmospheres. Also the combustion of 1 lb. of hydrogen gives rise, as already remarked, to 190·26 cubic feet of steam; and if combustion take place under similar conditions with respect to space, the pressure exerted upon the containing vessel will be 22,830 lb., or nearly 10·5 tons, to the square inch, the tension being 190·26 × 8 = 1522 atmospheres.

The force thus developed is due wholly to the volume of the gas generated, and by no means represents the total amount developed by the explosion. The volume of the gases evolved by an explosion is estimated for a temperature of 62°; but it was shown in a former paragraph that the temperature of the products of combustion at the moment of their generation is far above this. Now it is a well-known law of thermo-dynamics that, the volume remaining the same, the pressure of a gas will vary directly as the temperature; that is, when the temperature is doubled, the pressure is also doubled. By temperature is understood the number of degrees measured by Fahrenheit’s scale on a perfect gas thermometer, from a zero 461°·2 below the zero of Fahrenheit’s scale, that is, 493°·2 below the freezing point of water. Thus the temperature of 62° for which the volume has been estimated is equal to 461·2 + 62 = 523°·2 absolute.

It was shown that the temperature of the product of combustion when carbon is burned to carbonic oxide is 9718° Fahr., which is equivalent to 10179°·2 absolute. Hence it will be observed that the temperature has been increased 10179°·2 523°·2 = 19·45 times. According to the law above enunciated, therefore, the pressure will be increased in a like ratio, that is, it will be, for the volume and the space already given, 3776 × 19·45 = 73,443 lb. = 32·8 tons to the square inch.

When carbon is burned to carbonic acid, the temperature of the product was shown to be 23,516° Fahr., which is equivalent to 23977·2 absolute. In this case, it will be observed that the temperature has been increased 23977·2 523·2 = 45·83 times. Hence the resulting pressure will be 3776 × 45·83 = 173,154 lb. = 77·3 tons to the square inch. It will be seen from these pressures that when combustion is complete, the force developed is 2·36 greater than when combustion is incomplete; and also that the increase of force is due to the larger quantity of heat liberated, since the volume of the gases is the same in both cases. If we suppose the carbon burned to carbonic oxide in the presence of a sufficient quantity of oxygen to make carbonic acid, we shall have 31·5 cubic feet of the oxide + 15·7 cubic feet of free oxygen, or a total volume of 42·7 cubic feet of gases. If this volume be compressed within the space of one-eighth of a cubic foot, it will have a tension of 42·7 × 8 = 341·6 atmospheres, and will exert upon the walls of the containing vessel a pressure of 5124 lb. to the square inch. The temperature of the gases will be 32 + 4400 0·190 × 3·667 = 6347° Fahr. = 6808°·2 absolute, the mean specific heat of the gases being 0·190; whence it will be seen that the temperature has been increased 6808°·2 523·2 = 13·01 times. According to the law of thermo-dynamics, therefore, the pressure under the foregoing conditions will be 5124 × 13·01 = 66,663 lb. = 29·8 tons to square inch. So that, under the conditions assumed in this case, the pressures developed by incomplete and by complete combustion are as 29·8 to 77·3, or as 1 to 2·59.

Similarly, when hydrogen is burned to water, the temperature of the product will be, as shown in a former paragraph, 16,049 Fahr. = 16510·2 absolute; and the pressure will be 22,830 × 16510·2 523·2 = 720,286 lb. = 321·1 tons to the square inch.

It will be observed, from a consideration of the foregoing facts, that a very large proportion of the force developed by an explosion is due to the heat liberated by the chemical reactions which take place. And hence it will plainly appear that, in the practical application of explosive agents to rock blasting, care should be taken to avoid a loss of the heat upon which the effects of the explosion manifestly so largely depend.

Section II.—Nature of Explosive Agents.

Mechanical Mixtures.

—In the preceding section, it was shown that an explosion is simply the rapid oxidation of carbon and hydrogen. To form an explosive agent, the problem is, how to bring together in a convenient form the combustible, carbon or hydrogen, and the oxygen required to oxidize it. Carbon may be obtained pure, or nearly pure, in the solid form. As wood charcoal, for example, that substance may be readily procured in any needful abundance; but pure oxygen does not exist in that state, and it is hardly necessary to point out that only the solid form is available in the composition of an explosive agent. In nature, however, oxygen exists in the solid state in very great abundance in combination with other substances. Silica, for example, which is the chief rock constituent, is a compound of silicon and oxygen, and the common ores of iron are made up chiefly of that metal and oxygen. The elementary constituents of cellulose, or wood fibre, are carbon, hydrogen, and oxygen; and the body known as saltpetre, or nitrate of potash, is compounded of potassium, nitrogen, and oxygen. But though oxygen is thus found in combination with many different substances, it has not the same affinity for all. When it is combined with a substance for which its affinity is strong, as in the silica and the iron oxide, it cannot be separated from that substance without difficulty; but if the affinity be weak, dissociation may be more easily effected. The former combination is said to be “stable,” and the latter is, in contradistinction, described as “unstable.” It will be evident on reflection that only those compounds in which the oxygen exists in unstable combination can be made use of as a constituent part of an explosive agent, since it is necessary that, when required, the oxygen shall be readily given up. Moreover, it will also appear that when one of these unstable oxygen compounds and carbon are brought together the mixture will constitute an explosive agent, since the oxygen which is liberated by the dissociation of the unstable compound will be taken up by the carbon for which it has a stronger affinity. Saltpetre is one of those compounds, and a mixture of this body with charcoal constitutes gunpowder. The means employed to dissociate the elements of saltpetre is heat. It is obvious that other compounds of oxygen might be substituted for the saltpetre, but this body being easily procurable is always employed. The chlorate of potash, for example, is less stable than the nitrate, and therefore an explosive mixture containing the former substance will be more violent than another containing the latter. For the violence of an explosion is in a great measure determined by the readiness with which the oxygen is given up to the combustible. But the chlorate is much more costly than the nitrate. As, however, the force developed is greater, the extra cost would perhaps be compensated by the increased effect of the explosion. But the instability of the chlorate is such that friction or a moderately light blow will produce explosion in a mixture containing that substance, a circumstance that renders it unfit to be the oxidizer in an explosive agent in common use. The nitrate is therefore preferred on the ground of safety. Saltpetre, or nitrate of potash, consists, as already pointed out, of the metal potassium in combination with the substances nitrogen and oxygen. Of these, the last only is directly concerned in the explosion; but the two former, and especially the nitrogen, act indirectly to intensify its effects in a manner that will be explained hereafter.

The chemical formula for nitrate of potash is KNO₃, which signifies that three atoms of oxygen exist in this body in combination with one atom of nitrogen and one atom of kalium or potassium. As the atomic weights of these substances are 16, 14, and 39 respectively, the weight of the molecule is 101, that is, in 101 lb. of nitrate of potash there are 39 lb. of potassium, 14 lb. of nitrogen, and (16 × 3) = 48 lb. of oxygen. Hence the proportion of oxygen in nitrate of potash is by weight 47·5 per cent. It will be seen from this proportion that to obtain 1 lb. of oxygen, 2·1 lb. of the nitrate must be decomposed.

The carbon of gunpowder is obtained from wood charcoal, the light woods, such as alder, being preferred for that purpose. The composition of the charcoal varies somewhat according to the degree to which the burning has been carried, the effect of the burning being to drive out the hydrogen and the oxygen. But, generally, the composition of gunpowder charcoal is about 80 per cent. carbon, 3·25 per cent. hydrogen, 15 per cent. oxygen, and 1·75 per cent. ash. Knowing the composition of the charcoal, it is easy to calculate the proportion of saltpetre required in the explosive mixture.

Thus far we have considered gunpowder as composed of charcoal and saltpetre only. But in this compound, combustion proceeds too slowly to give explosive effects. Were the chlorate of potash used instead of the nitrate, the binary compound would be sufficient. The slowness of combustion in the nitrate mixture is due to the comparatively stable character of that body. To accelerate the breaking up of the nitrate, a quantity of sulphur is mixed up with it in the compound. This substance possesses the property of burning at a low temperature. The proportion of sulphur added varies from 10 per cent. in powder used in fire-arms, to 20 per cent. in that employed for blasting purposes. The larger the proportion of sulphur, the more rapid, within certain limits, is the combustion. Thus ordinary gunpowder is a ternary compound, consisting of charcoal, saltpetre, and sulphur.

As the composition of charcoal varies, it is not practicable to determine with rigorous accuracy the proportion of saltpetre required in every case; a mean value is therefore assumed, the proportions adopted being about—

With these proportions, the carbon should be burned to carbonic acid, and the sulphur should be all taken up by the potassium. Powder of this composition is used for fire-arms. For blasting purposes, as before remarked, the proportion of sulphur is increased at the expense of the saltpetre, in order to quicken combustion and to lessen the cost, to 20 per cent. as a maximum. With such proportions, some of the carbon is burned to carbonic oxide only, and some of the sulphur goes to form sulphurous acid, gases that are particularly noisome to the miner.

It is essential to the regular burning of the mixture that the ingredients be finely pulverized and intimately mixed. The manufacture of gunpowder consists of operations for bringing about these results. The several substances are broken up by mechanical means, and reduced to an impalpable powder. These are then mixed in a revolving drum, and afterwards kneaded into a paste by the addition of a small quantity of water. This paste is subjected to pressure, dried, broken up, and granulated; thus, the mixing being effected by mechanical means, the compound is called a mechanical mixture. It will be observed that in a mechanical mixture the several ingredients are merely in contact, and are not chemically united. They may therefore be separated if need be, or the proportions may be altered in any degree. Mechanical mixtures, provided the bodies in contact have no chemical action one upon another, are stable, that is, they are not liable, being made up of simple bodies, to decompose spontaneously.

Chemical Compounds.

—In a mechanical mixture, as we have seen, the elements which are to react one upon another are brought together in separate bodies. In gunpowder, for example, the carbon is contained in the charcoal, and the oxygen in the saltpetre. But in a chemical compound, these elements are brought together in the same body. In a mechanical mixture, we may put what proportion of oxygen we please. But elements combine chemically only in certain definite proportions, so that in the chemical compound we can introduce only a certain definite proportion of oxygen. The oxygen in saltpetre is in chemical combination with the potassium and the nitrogen, and, as we have already seen, these three substances hold certain definite proportions one to another. That is, to every atom of potassium, there are one atom of nitrogen and three atoms of oxygen. Or, which amounts to the same thing, in 1 lb. of saltpetre, there are 0·386 lb. of potassium, 0·139 lb. of nitrogen, and 0·475 lb. of oxygen. Moreover, these elements occupy definite relative positions in the molecule of saltpetre. But in the mechanical mixture, the molecules of which it is made up have no definite relative positions. Even if the three substances—charcoal, saltpetre, and sulphur—of which gunpowder is composed, could be so finely divided as to be reduced to their constituent molecules, the relative position of these would be determined by the mixing, and it would be impossible so to distribute them that each should find itself in immediate proximity to those with which it was to combine. But so far are we from being able to divide substances into their constituent molecules, that when we have reduced them to an impalpable powder, each particle of that powder contains a large number of molecules. Thus, in a mechanical mixture, we have groups of molecules of one substance mingled irregularly with groups of molecules of another substance, so that the atoms which are to combine are not in close proximity one to another, but, on the contrary, are, many of them, separated by wide intervals. In the chemical compound, however, the atoms are regularly distributed throughout the whole mass of the substance, and are, relatively to one another, in the most favourable position for combining. Viewed from this point, the chemical compound may be regarded as a perfect mixture, the mechanical mixture being a very imperfect one. This difference has an important influence on the effect of an explosion. All the atoms in a chemical compound enter at once into their proper combinations, and these combinations take place in an inconceivably short space of time, while, in a mechanical mixture, the combinations are less direct, and are much less rapidly effected. This is the reason why the former is more violent in its action than the latter. The one is crushing and shattering in its effects, the other rending and projecting. The compound gives a sudden blow; the mixture applies a gradually increasing pressure. It is this sudden action of the compound that allows it to be used effectively without tamping. The air, which rests upon the charge, and which offers an enormous resistance to motion at such inconceivably high velocities, serves as a sufficient tamping.

Gun-cotton may be taken as an example of a chemical compound. The woody or fibrous part of plants is called “cellulose.” Its chemical formula is C₆H₁₀O₅, that is, the molecule of cellulose consists of six atoms of carbon in combination with ten atoms of hydrogen and five atoms of oxygen. If this substance be dipped into concentrated nitric acid, some of the hydrogen is displaced and peroxide of nitrogen is substituted for it. The product is nitro-cellulose, the formula of which is C₆H₇(NO₂)₃O₅. If this formula be compared with the last, it will be seen that three atoms of hydrogen have been eliminated and their place taken by three molecules of the peroxide of nitrogen NO₂; so that we now have a compound molecule, which is naturally unstable. The molecules of the peroxide of nitrogen are introduced into the molecule of cellulose for the purpose of supplying the oxygen needed for the combustion of the carbon and the hydrogen, just as the groups of molecules of saltpetre were introduced into the charcoal of the gunpowder for the combustion of the carbon and the hydrogen of that substance. Only, in the former case, the molecules of the peroxide are in chemical combination, not merely mixed by mechanical means as in the latter. The compound molecule of nitro-cellulose may be written C₆H₇N₃O₁₁, that is, in 297 lb. of the substance, there are (6 × 12) 72 lb. of carbon, (7 × 1) 7 lb. of hydrogen, (3 × 14) 42 lb. of nitrogen, and (11 × 16) 176 lb. of oxygen; or 24·2 per cent. carbon, 2·3 per cent. hydrogen, 14·1 per cent. nitrogen, and 59·4 per cent. oxygen. When the molecule is broken up by the action of heat, the oxygen combines with the carbon and the hydrogen, and sets the nitrogen free. But it will be observed that the quantity of oxygen present is insufficient to completely oxidize the carbon and the hydrogen. This defect, though it does not much affect the volume of gas generated, renders the heat developed, as shown in a former section, considerably less than it would be were the combustion complete, and gives rise to the noxious gas carbonic oxide.

Cotton is one of the purest forms of cellulose, and, as it may be obtained at a cheap rate, it has been adopted for the manufacture of explosives. This variety of nitro-cellulose is known as “gun-cotton.” The raw cotton made use of is waste from the cotton mills, which waste, after being used for cleaning the machinery, is swept from the floors and sent to the bleachers to be cleaned. This is done by boiling in strong alkali and lime. After being picked over by hand to remove all foreign substances, it is torn to pieces in a “teasing” machine, cut up into short lengths, and dried in an atmosphere of 190° F. It is then dipped into a mixture of one part of strong nitric acid and three parts of strong sulphuric acid. The use of the sulphuric acid is, first, to abstract water from the nitric acid, and so to make it stronger; and, second, to take up the water which is formed during the reaction. After the dipping, it is placed in earthenware pots to digest for twenty-four hours, in order to ensure the conversion of the whole of the cotton into gun-cotton. To remove the acid, the gun-cotton is passed through a centrifugal machine, and subsequently washed and boiled. It is then pulped, and again washed with water containing ammonia to neutralize any remaining trace of acid. When rendered perfectly pure, it is compressed into discs and slabs of convenient dimensions for use.

Another important chemical compound is nitro-glycerine. Glycerine is a well-known, sweet, viscous liquid that is separated from oils and fats in the processes of candle-making. Its chemical formula is: C₃H₈O₃; that is, the molecule is composed of three atoms of carbon, in combination with eight atoms of hydrogen, and three atoms of oxygen. In other words, glycerine consists of carbon 39·1 per cent., hydrogen 8·7 per cent., and oxygen 52·2 per cent. When this substance is treated, like cellulose, with strong nitric acid, a portion of the hydrogen is displaced, and peroxide of nitrogen is substituted for it; thus the product is: C₃H₅(NO₂)₃O₃, similar, it will be observed, to nitro-cellulose. This product is known as nitro-glycerine. The formula may be written C₃H₅N₃O₉. Hence, in 227 lb. of nitro-glycerine, there are (3 × 12) 36 lb. of carbon; (5 × 1) 5 lb. of hydrogen; (3 × 14) 42 lb. of nitrogen; and (9 × 16) 144 lb. of oxygen; or 15·8 per cent. is carbon, 2·2 per cent. hydrogen, 18·5 per cent. nitrogen, and 63·5 per cent. oxygen. When the molecule is broken up by the action of heat, the oxygen combines with the carbon and the hydrogen, and sets the nitrogen free. And it will be seen that the quantity of oxygen present is more than sufficient to completely oxidize the carbon and the hydrogen. In this, the nitro-glycerine is superior to the nitro-cotton. In both of these compounds, the products of combustion are wholly gaseous, that is, they give off no smoke, and leave no solid residue.

In the manufacture of nitro-glycerine, the acids, consisting of one part of strong nitric acid and two parts of strong sulphuric acid, are mixed together in an earthenware vessel. When quite cold, the glycerine is run slowly into this mixture, which, during the process, is kept in a state of agitation, as heat is developed in the process; and, as the temperature must not rise above 48° F., the vessels are surrounded with iced water, which is kept in circulation. When a sufficient quantity of glycerine has been run into the mixture, the latter is poured into a tub of water. The nitro-glycerine being much heavier than the dilute acid mixture, sinks to the bottom; the acid liquid is then poured off, and more water added, this process being repeated until the nitro-glycerine is quite free from acid.

Nitro-glycerine is, at ordinary temperatures, a clear, nearly colourless, oily liquid, having a specific gravity of about 1·6. It has a sweet, pungent taste, and if placed upon the tongue, or even if allowed to touch the skin in any part, it causes a violent headache. Below 40° F. it solidifies in crystals.

Dynamite is nitro-glycerine absorbed in a silicious earth called kieselguhr. Usually it consists of about 75 per cent. nitro-glycerine and 25 per cent. kieselguhr. The use of the absorbent is to remove the difficulties and dangers attending the handling of a liquid. Dynamite is a pasty substance of the consistence of putty, and is, for that reason, very safe to handle. It is made up into cartridges, and supplied for use always in that form.

Section III.—Relative Strength of the Common Explosive Agents.

Force developed by Gunpowder.

—In the combustion of gunpowder, the elements of which it is composed, which elements, as we have seen, are carbon, hydrogen, nitrogen, oxygen, potassium, and sulphur, combine to form, as gaseous products, carbonic acid, carbonic oxide, nitrogen, sulphuretted hydrogen, and marsh gas or carburetted hydrogen, and, as solid products, sulphate, hyposulphite, sulphide, and carbonate of potassium. Theoretically, some of these compounds should not be produced; but experiment has shown that they are. It has also been ascertained that the greater the pressure, the higher is the proportion of carbonic acid produced, so that the more work the powder has to do, the more perfect will be the combustion, and, consequently, the greater will be the force developed. This fact shows that overcharging is not only very wasteful of the explosive, but that the atmosphere is more noxiously fouled thereby. The same remark applies even more strongly to gun-cotton and the nitro-glycerine compounds.

The careful experiments of Messrs. Noble and Abel have shown that the explosion of gunpowder produces about 57 per cent. by weight of solid matters, and 43 per cent. of permanent gases. The solid matters are, at the moment of explosion, in a fluid state. When in this state, they occupy 0·6 of the space originally filled by the gunpowder, consequently the gases occupy only 0·4 of that space. These gases would, at atmospheric pressure and 32° F. temperature, occupy a space 280 times that filled by the powder. Hence, as they are compressed into 0·4 of that space, they would give a pressure of 2800·4 × 15 = 10,500 lb., or about 4·68 tons to the square inch. But a great quantity of heat is liberated in the reaction, and, as it was shown in a former section, this heat will enormously increase the tension of the gases. The experiments of Noble and Abel showed that the temperature of the gases at the instant of explosion is about 4000° F. Thus the temperature of 32° + 461°·2 = 493°·2 absolute, has been raised 4000493°·2 = 8·11 times, so that the total pressure of the gases will be 4·68 × 8·11 = 42·6 tons to the square inch. And this pressure was, in the experiments referred to, indicated by the crusher-gauge. When, therefore, gunpowder is exploded in a space which it completely fills, the force developed may be estimated as giving a pressure of about 42 tons to the square inch.

Relative Force developed by Gunpowder, Gun-cotton, and Nitro-glycerine.

—Unfortunately no complete experiments have hitherto been made to determine the absolute force developed by gun-cotton and nitro-glycerine. We are, therefore, unable to estimate the pressure produced by the explosion of those substances, or to make an accurate evaluation of their strength relatively to that of gunpowder. It should, however, be borne in mind that a correct estimate of the pressure produced to the square inch would not enable us to make a full comparison of the effects they were capable of causing. For though, by ascertaining that one explosive gives twice the pressure of another, we learn that one will produce twice the effect of another; yet it by no means follows from that fact that the stronger will produce no more than twice the effect of the weaker. The rending effect of an explosive depends, in a great measure, on the rapidity with which combustion takes place. The force suddenly developed by the decomposition of the chemical compounds acts like a blow, and it is a well-known fact that the same force, when applied in this way, will produce a greater effect than when it is applied as a gradually increasing pressure. But some calculations have been made, and some experiments carried out, which enable us to form an approximate estimate of the relative strength of these explosive substances.

Messrs. Roux and Sarrau give the following as the result of their investigations, derived from a consideration of the weight of the gases generated and of the heat liberated. The substances are simply exploded, and the strength of gunpowder is taken as unity.

The relative strength is that due to the volume of the gases and the heat, no account being taken of the increased effect due to the rapidity of the explosion.

Alfred Noble has essayed to appreciate the effects of these different explosives by means of a mortar loaded with a 32-lb. shot and set at an angle of 10°, the distances traversed by the shot being taken as the results to be compared. Considered, weight for weight, he estimates as follows the relative strengths of the substances compared, gunpowder being again taken as unity:—

The relative strength, bulk for bulk, is, however, of greater importance in rock blasting. This is easily computed from the foregoing table and the specific gravity of the substances, which is 1·00 for gunpowder and compressed gun-cotton, 1·60 for nitro-glycerine, and 1·65 for dynamite. Compared in this way, bulk for bulk, these explosives range as follows:—

Hence, for a given height of charge in a bore-hole, gun-cotton exerts about 2¹⁄₂ times the force of gunpowder, and dynamite about 4¹⁄₄ times that force.

Section IV.—Means of Firing the Common Explosive Agents.

Action of Heat.

—We have seen that the oxygen required for the combustion of the carbon in gunpowder is stored up in the saltpetre. So long as the saltpetre remains below a certain temperature, it will retain its oxygen; but when that temperature is reached, it will part with that element. To fire gunpowder, heat is therefore made use of to liberate the oxygen, which at once seizes upon the carbon with which it is in presence. The means employed to convey heat to an explosive have been described in the preceding chapter. It is necessary to apply heat to one point only of the explosive; it is sufficient if it be applied to only one grain. That portion of the grain which is thus raised in temperature begins to “burn,” as it is commonly expressed, that is, this portion enters at once into a state of combustion, the saltpetre giving up its oxygen, and the liberated oxygen entering into combination with the carbon. The setting up of this action is called “ignition.” The hot gases generated by the combustion set up ignite other grains surrounding the one first ignited; the gases resulting from the combustion of these ignite other grains; and, in this way, ignition is conveyed throughout the mass. Thus the progress of ignition is gradual. But though it takes place, in every case, gradually, if the gases are confined within the space occupied by the powder, it may be extremely rapid. It is easy to see that the gases evolved from a very small number of grains are sufficient to fill all the interstices, and to surround every individual grain of which the charge is composed. But besides this ignition from grain to grain, the same thing goes on from the outside to the inside of each individual grain, the grain burning gradually from the outside to the inside in concentric layers. The successive ignitions in this direction, however, of layer after layer, is usually described as the progress of combustion. Thus the time of an explosion is made up of that necessary for the ignition of all the grains, and of that required for their complete combustion.

The time of ignition is determined in a great measure by the proportion which the interstices, or empty spaces between the grains, bear to the whole space occupied by the powder. If the latter be in the form of an impalpable dust, ignition cannot extend throughout the mass in the manner we have described; but we shall have merely combustion proceeding from grain to grain. If, on the contrary, the powder be in large spherical grains or pellets, the interstices will be large, and the first gases formed will flash through these, and ignite all the grains one after another with such rapidity that ignition may be regarded as simultaneous. Thus the time of ignition is shortened by increasing the size of the grains and approximating the latter to the spherical form.

But the time of combustion is determined by conditions contrary to these. As combustion proceeds gradually from the outside to the inside of a grain, it is obvious that the larger the grain is, the longer will be the time required to burn it in. Also it is evident that if the grain be in the form of a thin flake, it will be burned in a much shorter time than if it be in the spherical form. Thus the conditions of rapid ignition and rapid combustion are antagonistic. The minimum time of explosion is obtained when the grains are irregular in shape and only sufficiently large to allow a fairly free passage to the hot gases. There are other conditions which influence the time of combustion; among them is the density of the grain. This is obvious, since the denser the grain, the greater is the quantity of material to be consumed. But besides this, combustion proceeds more slowly through a dense grain than through an open one. The presence of moisture also tends to retard combustion.

The progress both of ignition and of combustion is accelerated, not uniform. In proportion as the grains are ignited, the gases evolved increase in volume, and as the progress of combustion continues to generate gases, the tension of these increases, until, as we have seen, the pressure rises as high as 42 tons to the square inch. As the pressure increases, the hot gases are forced more and more deeply into the grains, and combustion, consequently, proceeds more and more rapidly.

Detonation.

—By detonation is meant the simultaneous breaking up of all the molecules of which the explosive substance is composed. Properly the term is applicable to the chemical compounds only. But it is applied to gunpowder to denote the simultaneous ignition of all the grains. The mode of firing by detonation is obviously very favourable to the rending effect required of blasting powder, since it reduces to a minimum the time of explosion. It is brought about, in all cases, by means of an initial explosion. The detonator, which produces this initial explosion, consists of an explosive compound, preferably one that is quick in its action, contained within a case sufficiently strong to retain the gases until they have acquired a considerable tension. When the case bursts, this tension forces them instantaneously through the interstices of the powder, and so produces simultaneous ignition. A pellet of gun-cotton, or a cartridge of dynamite, the latter especially, makes a good detonator for gunpowder. Fired in this way, very much better effects may be obtained from gunpowder than when fired in the usual manner. Indeed, in many kinds of rock, more work may be done with it than with gun-cotton or with dynamite.

The action of a detonator upon a chemical compound is different. In this case, the explosion seems to be due more to the vibration caused by the blow than by the heat of the gases from the detonator. Probably both of these causes operate in producing the effect. However this may be, the fact is certain that under the influence of the explosion of the detonator, the molecules of a chemical compound, like nitro-glycerine, are broken up simultaneously, or at least, so nearly simultaneously, that no tamping is needed to obtain the full effect of the explosion. Dynamite is always, and gun-cotton is usually, fired by means of a detonator. A much larger quantity of explosive is needed to detonate gunpowder than is required for dynamite, or gun-cotton, since, for the former explosive, a large volume of gases is requisite. Dynamite detonators usually consist of from six to nine grains of fulminate of mercury contained in a copper cap, as described in the preceding chapter. Gun-cotton detonators are similar, but have a charge of from ten to fifteen grains of the fulminate. An insufficient charge will only scatter the explosive instead of firing it, if it be unconfined, and only explode it without detonation, if it be in a confined space.

Section V.—Some Properties of the Common Explosive Agents.

Gunpowder.

—The combustion of gunpowder, as we have seen, is gradual and comparatively slow. Hence its action is rending and projecting rather than shattering. This constitutes one of its chief merits for certain purposes. In many quarrying operations, for instance, the shattering action of the chemical compounds would be very destructive to the produce. In freeing blocks of slate, or of building stone, a comparatively gentle lifting action is required, and such an action is exerted by gunpowder. Moreover, this action may be modified by using light tamping, or by using no tamping, a mode of employing gunpowder often adopted in slate quarries. The effect of the violent explosives cannot be modified in this way.

Gunpowder is injured by moisture. A high degree of moisture will destroy its explosive properties altogether, so that it cannot be used in water without some protective covering. Even a slight degree of moisture, as little as one per cent. of its weight, materially diminishes its strength. For this reason, it should be used, in damp ground, only in cartridges. This is, indeed, the most convenient and the most economical way of using gunpowder in all circumstances. It is true that there is a slight loss of force occasioned by the empty space around the cartridge, in holes that are far from circular in shape. But at least as much will be lost without the cartridge from the moisture derived from the rock, even if the hole be not wet. But in all downward holes, the empty spaces may be more or less completely filled up with dry loose sand.

The products of the explosion of gunpowder are partly gaseous, partly solid. Of the former, the most important are carbonic acid, carbonic oxide, and nitrogen. The sulphuretted and the carburetted hydrogen are formed in only small quantities. The carbonic oxide is a very noxious gas; but it is not formed in any considerable quantity, except in cases of overcharging. The solid products are compounds of potassium and sulphur, and potassium and carbon. These constitute the smoke, the dense volumes of which characterize the explosion of gunpowder. This smoke prevents the immediate return of the miner to the working face after the blast has taken place.

Gun-cotton.

—The combustion of gun-cotton takes place with extreme rapidity, in consequence of which its action is very violent. Its effect is rather to shatter the rock than to lift it out in large blocks. This quality renders it unsuitable to many quarrying operations. In certain kinds of weak rock, its disruptive effects are inferior to those produced by gunpowder. But in ordinary mining operations, where strong tough rock has to be dealt with, its superior strength and quickness of action, particularly the latter quality, produce much greater disruptive effect than can be obtained from gunpowder. Moreover, its shattering action tends to break up into small pieces the rock dislodged, whereby its removal is greatly facilitated.

Gun-cotton may be detonated when in a wet state by means of a small quantity of the dry material. This is a very important quality, inasmuch as it allows the substance to be used in a wet hole without protection, and conduces greatly to the security of those who handle it. When in the wet state, it is uninflammable, and cannot be exploded by the heaviest blows. Only a powerful detonation will bring about an explosion in it when in the wet state. It is, therefore, for safety, kept and used in that state. Since it is insensible to blows, it may be rammed tightly into the bore-hole, so as to fill up all empty spaces. The primer of dry gun-cotton, however, which is to detonate it, must be kept perfectly dry, and handled with caution, as it readily detonates from a blow. Gun-cotton, when ignited in small quantities in an unconfined space, burns fiercely, but does not explode.

The products of the combustion of gun-cotton are:—carbonic acid, carbonic oxide, water, and a little carburetted hydrogen or marsh-gas. On account of the insufficiency of oxygen, already pointed out, a considerable proportion of carbonic oxide is formed, which vitiates the atmosphere into which it is discharged. Overcharging, as in the case of gunpowder, causes an abnormal quantity of the oxide to be formed.

Dynamite.

—As combustion takes place more rapidly in nitro-glycerine than in gun-cotton, the effects of dynamite are more shattering than those of the latter substance. Gun-cotton holds, indeed, a mean position in this respect between dynamite, on the one hand, and gunpowder on the other. Dynamite is, therefore, even less suitable than gun-cotton for those uses which are required to give the produce in large blocks. But in very hard and tough rock, it is considerably more effective than gun-cotton, and, under some conditions, it will bring out rock which gun-cotton fails to loosen.

Dynamite is unaffected by water, so that it may be used in wet holes; indeed, water is commonly used as tamping, with this explosive. In upward holes, where water cannot, of course, be used, dynamite is generally fired without tamping, its quick action rendering tamping unnecessary.

The pasty form of dynamite constitutes a great practical advantage, inasmuch as it allows the explosive to be rammed tightly into the bore-hole so as to fill up all empty spaces and crevices. This is important, for it is obvious that the more compactly the charge is placed in the hole, the greater will be the effect of the explosion. Moreover, this plastic character renders it very safe to handle, as blows can hardly produce sufficient heat in it to cause explosion. If a small quantity of dynamite be placed upon an anvil and struck with a hammer, it explodes readily; but a larger quantity so struck does not explode, because the blow is cushioned by the kieselguhr. If ignited in small quantities in an unconfined space, it burns quietly without explosion.

If dynamite be much handled out of the cartridges, it causes violent headaches; and the same effect is produced by being in a close room in which there is dynamite in the unfrozen state.

Dynamite possesses one quality which places it at a disadvantage with respect to other explosives, namely, that of freezing at a comparatively high temperature. At about 40° F. the nitro-glycerine solidifies, and the dynamite becomes chalky in appearance. In this state, it is exploded with difficulty, and, consequently, it has to be thawed before being used. This may be safely done with hot water; performed in any other way the operation is dangerous.

The products of the combustion of dynamite are carbonic acid, carbonic oxide, water, and nitrogen. As, however, there is more than a sufficiency of oxygen in the compound, but little of the oxide is formed when the charge is not excessive. If, therefore, dynamite be properly detonated, and overcharging be avoided, its explosion will not greatly vitiate the atmosphere. But if it be only partially detonated hypo-nitric fumes are given off, which have a very deleterious effect upon the health. It is, thus, of the highest importance that complete detonation should be effected, not merely to obtain the full effect of the explosive, but to avoid the formation of this noxious gas. This may be done by using a detonator of sufficient strength, and placing it well into the primer.

Firing Points of the Common Explosive Compounds.

—The following table shows the temperatures at which the commonly used compounds explode:—