Transcriber's Notes:

1. Inconsistent hyphenation of words preserved.
2. Several misprints corrected. Hover over underlined word in the text to see the corrections made. A full list of corrections can be found at [the end] of the text.
3. On [page 19], Aristophanes's period is noted as (488-385 B. C.); Aristophanes was born circa 448 B. C.

LIGHT AND LIBERTY

The Century Books of Useful Science

Artificial Light

ITS INFLUENCE UPON CIVILIZATION

BY

M. LUCKIESH

DIRECTOR OF APPLIED SCIENCE. NELA RESEARCH LABORATORY, NATIONAL LAMP WORKS OF GENERAL ELECTRIC COMPANY
Author of "Color and Its Applications," "Light and Shade and Their Applications," "The Lighting Art," "The Language of Color," etc.

ILLUSTRATED WITH PHOTOGRAPHS

NEW YORK
THE CENTURY CO.
1920

Copyright, 1920, by
The Century Co.

DEDICATED
TO THOSE WHO HAVE ENCOURAGED
ORGANIZED SCIENTIFIC RESEARCH FOR
THE ADVANCEMENT OF CIVILIZATION

PREFACE

In the following pages I have endeavored to discuss artificial light for the general reader, in a manner as devoid as possible of intricate details. The early chapters deal particularly with primitive artificial light and their contents are generally historical. The science of light-production may be considered to have been born in the latter part of the eighteenth century and beginning with that period a few chapters treat of the development of artificial light up to the present time. Until the middle of the nineteenth century mere light was available, but as the century progressed, the light-sources through the application of science became more powerful and efficient. Gradually mere light grew to more light and in the dawn of the twentieth century adequate light became available. In a single century, after the development of artificial light began in earnest, the efficiency of light-production increased fifty-fold and the cost diminished correspondingly. The next group of chapters deals with various economic influences of artificial light and with some of the byways in which artificial light is serving mankind. On passing through the spectacular aspects of lighting we finally emerge into the esthetics of light and lighting.

The aim has been to show that artificial light has become intricately interwoven with human activities and that it has been a powerful influence upon the progress of civilization. The subject is too extensive to be treated in detail in a single volume, but an effort has been made to present a discussion fairly complete in scope. It is hoped that the reader will gain a greater appreciation of artificial light as an economic factor, as an artistic medium, and as a mighty influence upon the safety, efficiency, health, happiness, and general progress of mankind.

M. Luckiesh.

ACKNOWLEDGMENTS

It is a pleasant duty to acknowledge the coöperation of various companies in obtaining the photographs which illustrate this book. With the exception of Plates 2 and 7, which are reproduced from the excellent works of Benesch and Allegemane respectively, the illustrations of early lighting devices are taken from an historical collection in the possession of the National Lamp Works of the General Electric Co. To this company the author is indebted for Plates 1, 3, 4, 5, 6, 9, 11, 15, 18b, 20, 21, 29; to Dr. McFarlan Moore for Plate 10; to Macbeth Evans Glass Co. for Plate 12; to the Corps of Engineers, U. S. Army, for Plate 13; to Lynn Works of G. E. Co. for Plates 14, 16; to Edison Lamp Works of G. E. Co. for Plates 17, 24; to Cooper Hewitt Co. for Plate 18a; to R. U. V. Co. for Plate 19; to New York Edison Co. for Plates 22, 26, 30; to W. D'A. Ryan and the Schenectady Works of G. E. Co. for Plates 23, 25, 31; to National X-Ray Reflector Co. for Plate 28. Besides the companies and the individuals particularly involved in the foregoing, the author is glad to acknowledge his appreciation of the assistance of others during the preparation of this volume.

CONTENTS

LIST OF ILLUSTRATIONS

ARTIFICIAL LIGHT

I

LIGHT AND PROGRESS

The human race was born in slavery, totally subservient to nature. The earliest primitive beings feasted or starved according to nature's bounty and sweltered or shivered according to the weather. When night fell they sought shelter with animal instinct, for not only were activities almost completely curtailed by darkness but beyond its screen lurked many dangers. It is interesting to philosophize upon a distinction between a human being and the animal just below him in the scale, but it may serve the present purpose to distinguish the human being as that animal in whom there is an unquenchable and insatiable desire for independence. The effort to escape from the bondage of nature is not solely a human instinct; animals burrow or build retreats through the instinct of self-preservation. But this instinct in animals is soon satisfied, whereas in human beings it has been leading ever onward toward complete emancipation.

The progress of civilization is a long chain of countless achievements each one of which has increased man's independence. Early man perhaps did not conceive the idea of fire and then set out to produce it. His infant mind did not operate in this manner. But when he accidentally struck a spark, produced fire by friction, or discovered it in some other manner, he saw its possibility. It is thrilling to picture primitive man at his first bonfire, enjoying the warmth, or at least interested in it. But how wonderful it must have become as twilight's curtain was drawn across the heavens! This controllable fire emitted light. It is easy to imagine primitive man pondering over this phenomenon with his sluggish mind. Doubtless he cautiously picked up a flaming stick and timidly explored the crowding darkness. Perhaps he carried it into his cave and behold! night had retreated from his abode! No longer was it necessary for him to retire to his bed of leaves when daylight failed. The fire not only banished the chill of night but was a power over darkness. Viewed from the standpoint of civilization, its discovery was one of the greatest strides along the highway of human progress. The activities of man were no longer bounded by sunrise and sunset. The march of civilization had begun.

In the present age of abundant artificial light, with its manifold light-sources and accessories which have made possible countless applications of light, mankind does not realize the importance of this comfort. Its wonderful convenience and omnipresence have resulted in indifference toward it by mankind in general, notwithstanding the fact that it is essential to man's most important and educative sense. By extinguishing the light and pondering upon his helplessness in the resulting darkness, man may gain an idea of its overwhelming importance. Those unfortunate persons who suffer the terrible calamity of blindness after years of dependence upon sight will testify in heartrending terms to the importance of light. Milton, whose eyesight had failed, laments,

O first created beam and thou great Word

"Let there be light," and light was over all,

Why am I thus bereaved thy prime decree?

Perhaps only through a similar loss would one fully appreciate the tremendous importance of light to him, but imagination should be capable of convincing him that it is one of the most essential and pleasure-giving phenomena known to mankind.

A retrospective view down the vista of centuries reveals by contrast the complexity with which artificial light is woven into human activities of the present time. Written history fails long before the primitive races are reached, but it is safe to trust the imagination to penetrate the fog of unwritten history and find early man huddled in his cave as daylight wanes. Impelled by the restless spirit of progress, this primitive being grasped the opportunity which fire afforded to extend his activities beyond the boundaries of daylight. The crude art upon the walls of his cave was executed by the flame of a smoking fagot. The fire on the ledge at the entrance to his abode became a symbol of home, as the fire on the hearth has symbolized home and hospitality throughout succeeding ages. The accompanying light and the protection from cold combined to establish the home circle. The ties of mated animals expanded through these influences to the bonds of family. Thus light was woven early into family life and has been throughout the ages a moralizing and civilizing influence. To-day the residence functions as a home mainly under artificial light, for owing to the conditions of living and working, the family group gathers chiefly after daylight has failed.

From the pine knot of primitive man to the wonderfully convenient light-sources of to-day there is a great interval, consisting, as appears retrospectively, of small and simple steps long periods apart. Measured by present standards and achievements, development was slow at first and modern man may be inclined to impatience as he views the history of light and human progress. But the achievements of early centuries, which appear so simple at the present time, were really great accomplishments when considered in the light of the knowledge of those remote periods. Science as it exists to-day is founded upon proved facts. The scientist, equipped with a knowledge of physical and chemical laws, is led by his imagination into the darkness of the unexplored unknown. This knowledge illuminates the pathway so that hypotheses are intelligently formed. These evolve into theories which are gradually altered to fit the accumulating facts, for along the battle area of progress there are innumerable scouting-parties gaining secrets from nature. These are supported by individuals and by groups, who verify, amplify, and organize the facts, and they in turn are followed by inventors who apply them. Liaison is maintained at all points, but the attack varies from time to time. It may be intense at certain places and other sectors may be quiet for a time. There are occasional reverses, but the whole line in general progresses. Each year witnesses the acquirement of new territory. It is seen that through the centuries there is an ever-growing momentum as knowledge, efficiency, and organization increase the strength of this invading army of scientists and inventors.

The burning fagot rescued mankind from the shackles of darkness, and the grease-lamp and tallow-candle have done their part. Progress was slow in those early centuries because the great minds of those ages philosophized without a basis of established facts: scientific progress resulted more from an accumulation of accidental discoveries than by a directed attack of philosophy supported by the facts established by experiment. It was not until comparatively recent times, at most three centuries ago, that the great intellects turned to systematically organized scientific research. Such men as Newton laid the foundation for the tremendous strides of to-day. The store of facts increased and as the attitude changed from philosophizing to investigating, the organized knowledge grew apace. All of this paved the way for the momentous successes of the present time.

The end is not in sight and perhaps never will be. The unexplored region extends to infinity and, judged by the past, the momentum of discovery will continue to increase for ages to come, unless the human race decays through the comfort and ease gained from utilizing the magic secrets which are constantly being wrested from nature. Among the achievements of science and invention, the production and application of artificial light ranks high. As an influence upon civilization, no single achievement surpasses it.

Without artificial light, mankind would be comparatively inactive about one half its lifetime. To-day it has been fairly well established that the human organism can flourish on eight hours' sleep in a period of twenty-four hours. Another eight hours spent in work should settle man's obligation to the world. The remaining hours should be his own. Artificial light has made such a distribution of time possible. The working-periods in many cases may be arranged in the interests of economy, which often means continuous operations. The sun need not be considered when these operations are confined to interiors or localized outdoors.

Thus, artificial light has been an important factor in the great industrial development of the present time. Man now burrows into the earth, navigates under water, travels upon the surface of land and sea, and soars among the clouds piloted by light of his own making. Progress does not halt at sunset but continues twenty-four hours each day. Building, printing, manufacturing, commerce, and other activities are prosecuted continuously, the working-shifts changing at certain periods regardless of the rising or setting sun. Adequate artificial lighting decreases spoilage, increases production, and is a powerful factor in the prevention of industrial accidents.

It has ever been true since the advent of artificial light that the intellect has been largely nourished after the completion of the day's work. The highly developed artificial lighting of the present time may account for much of the vast industry of publication. Books, magazines, and newspapers owe much to convenient and inexpensive artificial light, for without it fewer hours would be available for recreation and advancement through reading. Schools, libraries, and art museums may be attended at night for the betterment of the human race. The immortal Lincoln, it is said, gained his early education largely by the light of the fireplace. But all were not endowed with the persistence of Lincoln, so that illiteracy was more common in his day than in the present age of adequate illumination.

The theatrical stage not only depends for its effectiveness upon artificial light but owes its existence and development largely to this agency. In the moving-picture theater, pictures are projected upon the screen by means of it and even the production of the pictures is independent of daylight. These and a vast number of recreational activities owe much, and in some cases their existence, to artificial light.

Not many centuries ago the streets at night were overrun by thieves and to venture outdoors after dark was to court robbery and even bodily harm. In these days of comparative safety it is difficult to realize the influence that abundant illumination has had in increasing the safety of life and property. Maeterlinck in his poetical drama, "The Bluebird," appropriately has made Light the faithful companion of mankind. The Palace of Night, into which Light is not permitted to enter, is the abode of many evils. Thus the poet has played upon the primitive instincts of the impressiveness of light and darkness.

By combining the symbolism of light, color, and darkness with the instincts which have been inherited by mankind from its superstitious ancestry of the age of mythology, another field of application of artificial light is opened. Light has gradually assumed such attributes as truth, knowledge, progress, enlightenment. Throughout the early ages light was more or less worshiped and thus artificial lights became woven in many religious ceremonies. Some of these have persisted to the present time. The great pageants of peace celebrations and world's expositions appropriately feature artificial light. In drawing upon the potentiality of the expressiveness and impressiveness of light and color, artificial light is playing a major part. Doubtless the future generations will be entertained by gorgeous symphonies of light. Experiments are performed in this direction now and then, and it is reasonable to expect that after many centuries of cultivation of the appreciation of light-symphonies, these will take a place among the arts. The elaborate and complicated music of the present time is appreciated by civilized nations only after many centuries of slow cultivation of taste and understanding.

Light-therapy is to-day a distinct science and art. The germicidal action of light-rays and of some of the invisible rays which ordinarily accompany the luminous rays is well proved. Wounds are treated effectively and water is sterilized by the ultraviolet radiant energy in modern artificial illuminants.

Thousands of lighthouses, light-ships, and light-buoys are scattered along sea-coasts, rivers, and channels. They guide the wheelman and warn the lookout of shoals and reefs. Some of these send forth flashes of light whose intensities are measured in millions of candle-power. Many are unattended for days and even months. These powerful lights dominated by automatic mechanisms have replaced the wood-fires which were maintained a few centuries ago upon certain prominent points.

Signal-lights now guide the railroad train through the night. A burning flare dropped from the rear of a train keeps the following train at a safe distance. Huge search-lights penetrate the night air for many miles. When these are equipped with shutters, a code may be flashed from one ship to another or between the vessel and land. A code from a powerful search-light has been read a hundred miles away because the flashes were projected upon a layer of high clouds and were thus visible far beyond the horizon.

Artificial light played its part in the recent war. Huge search-light equipments were devised for portability. This mobile apparatus was utilized against enemy aircraft and in various other ways. Small hand-lamps are used to send out a pencil of light as directed by a pair of sights and the code is flashed by means of a trigger. Raiding-parties are no longer concealed by the curtain of darkness, for rockets and star-shells are used to illuminate large areas. Flares sent upward to drift slowly downward supported by parachutes saved and cost many lives during the recent war. Rockets are used by ships in distress and also by beleaguered troops.

Experiments are being prosecuted to ascertain the possibilities of artificial light in the forcing of plant-growth, and even chickens are made to work longer hours by its use.

Artificial light is now modified in color or spectral character to meet many requirements. Daylight has been reproduced in spectral quality so that certain processes requiring accurate discrimination of color are now prosecuted twenty-four hours a day under artificial daylight. Colored light is made of the correct quality which does not affect photographic plates of various sensibilities. Monochromatic light is utilized in photo-micrography for the best rendition of detail. Light-waves have been utilized as standards of length because they are invariable and fundamental. Numerous other interesting adaptations of artificial light are in daily use.

This is in reality the age of artificial light, for mankind has not only become independent of daylight in certain respects, but has improved upon natural light. The controllability of artificial light makes it superior to natural light in many ways. In fact, uses have been made of artificial light which are impossible with natural light. Light-sources may be made of a vast variety of shapes, and those may be transported wherever desired. They may be equipped with reflectors and other optical devices to direct or to diffuse the light as required.

Thus, artificial light to-day has numerous advantages over light which has been furnished by the Creator. It is sometimes stated that it can never compete with daylight in cheapness, inasmuch as the latter costs nothing. But this is not true. Even in the residence, daylight costs something, because windows are more expensive than plain walls. The expense of washing windows is an appreciable percentage of the cost of gas or electricity. And there is window-breakage to be considered.

In the more elaborate buildings of the congested portions of cities, daylight is satisfactory a lesser number of hours than in the outlying districts. In some stores, offices, and factories artificial light is used throughout the day. Still, the daylighting-equipment is installed and maintained. Furthermore, when it is considered that much expensive area is given to light-courts and much valuable wall space to windows, it is seen that the cost of daylight in congested cities is in reality considerable. Of course, the daylighting-equipment has value in ventilating, but ventilation may be taken care of in a very satisfactory manner as a separate problem.

The cost of skylights in museums and other large buildings is far greater than that of ordinary ceilings and walls, and the extra allowance for heating is appreciable. The expense of maintenance of some skylights is considerable. Thus it is seen that the cost and maintenance of daylighting-equipment, the loss of valuable rental space and of wall area, and the increased expense of heating are factors which challenge the statement that daylight costs nothing. In fact, it is not surprising to find that occasionally the elimination of daylighting—the reliance upon artificial light alone—has been seriously contemplated. When the possibilities of the latter are considered, it is reasonable to expect that it will make greater and greater inroads and that many buildings of the future will be equipped solely with artificial-lighting systems.

Naturally, with the tremendous development of artificial light during the present age, a new profession has arisen. The lighting expert is evolving to fill the needs. He is studying the problems of producing and utilizing artificial illumination. He deals with the physics of light-production. His studies of utilization carry him into the vast fields of physiology and psychology. His is a profession which eventually will lead into numerous highways and byways of enterprise, because the possibilities of lighting extend into all those activities which make their appeal to consciousness through the doorway of vision. These possibilities are limited only by the boundaries of human endeavor and in the broadest sense extend even beyond them, for light is one of the most prominent agencies in the scheme of creation. It contributes largely to the safety, the efficiency, and the happiness of civilized beings and beyond all it is a powerful civilizing agency.

II

THE ART OF MAKING FIRE

Scattered over the earth at the present time various stages of civilization are to be found, from the primitive savages to the most highly cultivated peoples. Although it is possible that there are tribes of lowly beings on earth to-day unfamiliar with fire or ignorant of its uses, savages are generally able to make fire. Thus the use of fire may serve the purpose of distinguishing human beings from the lower animals. Surely the savage of to-day who is unable to kindle fire or who possesses a mind as yet insufficiently developed to realize its possibilities, is quite at the mercy of nature's whims. He lives merely by animal prowess and differs little in deeds and needs from the beasts of the jungle. In this imaginary journey to the remote regions beyond the outskirts of civilization it soon becomes evident that the development of artificial light may be a fair measure of civilization.

In viewing the development of artificial light it is seen that preceding the modern electrical age, man depended universally upon burning material. Obviously, the course of civilization has been highly complex and cannot be symbolized adequately by the branching tree. From its obscure beginning far in the impenetrable fog of prehistoric times, it has branched here and there. These various branches have been subjected to many different influences, with the result that some flourished and endured, some retrogressed, some died, some went to seed and fell to take root and to begin again the upward climb. The ultimate result is the varied civilization of the present time, a study of which aids in penetrating the veil that obscures the ages of unrecorded writing. Likewise, material relics of bygone ages supply some threads of the story of human progress and mythology aids in spanning the misty gap between the earliest ages of man and the period when historic writings were begun. Throughout these various stages it becomes manifest that the development of artificial light is associated with the progress of mankind.

PRIMITIVE FIRE-BASKETS

CRUDE SPLINTER-HOLDERS

According to a certain myth, Prometheus stole fire from heaven and brought this blessing to earth. Throughout the mythologies of various races, fire and, as a consequence, light have been associated with divinity. They have been subjects of worship perhaps more generally than anything else, and these early impressions have survived in the ceremonial uses of light and fire even to the present time. The origin of fire as represented in any of the myths of the superstitious beings of early ages is as suitable as any other, inasmuch as definite knowledge is unavailable. Active volcanoes, spontaneous combustion, friction, accidental focusing of the sun's image, and other means may have introduced primitive beings to fire. A study of savage tribes of the present age combined with a survey of past history of mythology, of material relics, and of the absence of lamps or other lighting utensils leads to the conclusion that the earliest source of light was the wood fire.

EARLY OPEN-FLAME OIL AND GREASE LAMPS

Even to-day the savages of remote lands have not advanced further than the wood-fire stage, and they may be found kneeling upon the ground energetically but skilfully rubbing sticks together until the friction kindles a fire. In using these fire-sticks they convert mechanical energy into heat energy. This is a fundamental principle of physics, employed by them as necessity demands, but they are totally ignorant of it as a scientific law. The things which these savages learn are the result of accidental discovery. Until man pondered over such simple facts and coördinated them so that he could extend his knowledge by general reasoning, his progress could not be rapid. But the sluggish mind of primitive man is capable of devising improvements, however slowly, and the art of making fire by means of rubbing fire-sticks gradually became more refined. Mechanical improvements resulted from experience, with the consequence that finally one stick was rubbed to and fro in a groove, or was rapidly twirled between the palms of the hands while one end was pressed firmly into a hole in a piece of wood. In the course of a few seconds or a minute, depending upon skill and other conditions, a fire was obtained. It is interesting to note how civilized man is often compelled by necessity to adopt the methods of primitive beings. The rubbing of sticks is an emergency measure of the master of woodcraft at the present time, and the production of fire in this manner is the proud accomplishment or ambition of every Boy Scout.

Where only such crude means of kindling fire were available it became the custom in some cases to maintain a fire burning continuously in a public place. Around this pyrtaneum the various civil, political, and religious affairs were carried on by the light and warmth of the public fire. Many quaint customs evolved, apparently, from this ancient procedure.

The tinder-box of modern centuries doubtless originated in very early times, for it is inconceivable that the earliest beings did not become aware of the production of sparks when certain stones were struck together. In the stone age, when human beings spent much of their time chiseling implements and utensils from stone by means of tools of the same substance, it appears certain that this means of producing fire was ever apparent. Many of their sharp implements, such as knives and arrow-heads, were made of quartz and similar material and it is likely that the use of two pieces of quartz for producing a spark originated in those remote periods. Alaskan and Aleutian tribes are known to have employed two pieces of quartz covered with native sulphur. When these were struck together with skill, excellent sparks were obtained.

Later, when iron and steel became available, the more modern tinder-box was developed. An early application of the flint-and-steel principle was made by certain Esquimo tribes who obtained fire by striking a piece of quartz against a piece of iron pyrites. The latter is a yellow sulphide of iron, of crystalline form, best known as "fool's gold." Doubtless, the more primitive beings used dried grass, leaves, and moss as inflammable material upon which the sparks were showered. In later centuries the tinder-box was filled with charred grass, linen, and paper. There was a long interval between the development of fire-sticks and that of the tinder-box as measured by the progress of civilization. During recent centuries ordinary brown paper soaked in saltpeter and dried was utilized satisfactorily as an inflammable material. Such devices have been employed in past ages in widely separated regions of the earth. Elaborate specimens of tinder-boxes from Jamaica, Japan, China, Europe, and various other countries are now reposing in the collections in the possession of museums and of individuals.

If the radiant energy from the sun is sufficiently concentrated upon inflammable material, the latter will ignite. Such concentration may be achieved by means of a convex lens or a concave mirror. This method of producing fire does not antedate the more primitive methods such as striking quartz or rubbing wooden sticks, because the materials required are not readily found or prepared, but it is of very remote origin. Aristophanes in his comedy "The Clouds," which is a satire aimed at the science and philosophy of his period (488-385 B. C.), mentions the "burning lens." Nearly every one is familiar with an achievement attributed to Archimedes in which he destroyed the ships at Syracuse by focusing the image of the sun upon them by means of a concave mirror. The ancient Egyptians were proficient in the art of glass-making, so it is likely that the "burning-glass" was employed by them. Even a crude lens of glass will focus an image of the sun sufficiently well to cause inflammable material to ignite.

The energy in sunlight varies enormously, even on clear days, because the water-vapor in the atmosphere absorbs some of the radiant energy emitted by the sun. This absorbed radiation is chiefly known as infra-red energy, which does not arouse the sensation of light. When the water-vapor content of the atmosphere is high, the sun, though it may appear as bright to the eye, in reality is not as hot as it would be if the water-vapor were not present. However, a fire may be kindled by concentrating only the visible rays in sunlight because of the enormous intensity of sunlight. A convex lens fashioned from ice by means of a sharp-edged stone and finally shaped by melting the surfaces as they are rubbed in the palms of the hands, will kindle a fire in highly inflammable material if the sun is high and the atmosphere is fairly clear. Burning-glasses are used to a considerable extent at the present time in certain countries and it is reported that British soldiers were supplied with them during the Boer War. Indicative of the predominant use to which the glass lens was applied in the past is the employment of the term "burning-glass" instead of lens in the scientific writings as late as a century or two ago.

As civilization advanced, leading intellects began to inquire into the mysteries of nature and the periods of pure philosophy gave way to an era of methodical research. Alchemy and superstition began to retire before the attacks of those pioneers who had the temerity to believe that the scheme of creation involved a vast network of invariable laws. In this manner the powerful sciences of physics and chemistry were born a few centuries ago. Among other things the production of fire and light received attention and the "dark ages" were doomed to end. The crude, uncertain, and inconvenient methods of making fire were replaced by steadily improving scientific devices.

Matches were at first cumbersome, dangerous, and expensive, but these gradually evolved into the safety matches of the present time. Although they were primarily intended for lighting fires and various kinds of lamps, billions of them are now used yearly as convenient light-sources. Smoldering hemp or other material treated with niter and other substances was an early form of match used especially for discharging firearms. The modern wax-taper is an evolutionary form of this type of light-source.

Phosphorus has long played a dominant rôle in the preparation of matches. The first attempt at making them in their modern form appears to have occurred about 1680. Small pieces of phosphorus were used in connection with small splints of wood dipped in sulphur. This type of match did not come into general use until after the beginning of the nineteenth century, owing to its danger and expense. White or yellow phosphorus is a deadly poison; therefore the progress of the phosphorus match was inhibited until the discovery of the relatively harmless form known as red phosphorus. The first commercial application of this form was made in about 1850.

An early ingenious device consisted of a piece of phosphorus contained in a tube. A piston fitted snugly into the tube, by means of which the air could be compressed and the phosphorus ignited. Sulphur matches were ignited from the burning tinder, the latter being fired by flint and steel. In 1828 another form of match consisted of a glass tube containing sulphuric acid and surrounded by a mixture of chlorate of potash and sugar. A pair of nippers was supplied with each box of these "matches," by means of which the tip of the glass tube could be broken off. This liberated the acid, which upon mixing with the other ingredients set fire to them. To this contrivance a roll of paper was attached which was ignited by the burning chemicals.

The lucifer or friction matches appeared in about 1827, but successful phosphorus matches were first made in about 1833. The so-called safety match of the present time was invented in the year 1855. To-day, the total daily output of matches reaches millions and perhaps billions. Automatic machinery is employed in preparing the splints of wood and in dipping them into molten paraffin wax and finally into the igniting composition.

During recent years the principle of the tinder-box has been revived in a device in which sparks are produced by rubbing the mineral cerite (a hydrous silicate of cerium and allied metals) against steel. These sparks ignite a gas-jet or a wick soaked in a highly inflammable liquid such as gasolene or alcohol. This device is a tinder-box of the modern scientific age.

Naturally with the advent of electricity, electrical sparks came into use for lighting gas-jets and mantles and in isolated instances they have served as light-sources. Doubtless, every one is familiar with the parlor stunt of igniting a gas-jet from the discharge from the finger-tips of static electricity accumulated by shuffling the feet across the floor-rug.

Although many of these methods and devices have been used primarily for making fire, they have served as emergency or momentary light-sources. In the outskirts of civilization some of them are employed at the present time and various modern light-sources require a method of ignition.

III

PRIMITIVE LIGHT-SOURCES

Many are familiar with the light of the firefly or of its larvæ, the glow-worm, but few persons realize that a vast number of insects and lower organisms are endowed with the superhuman ability of producing light by physiological processes. Apparently the chief function of these lighting-plants within the living bodies is not to provide light in the sense that the human being uses it predominantly. That is, these wonderful light-sources seem to be utilized more for signaling, for luring prey, and for protection than for strictly illuminating-purposes. Much study has been given to the production of light by animals, because the secrets will be extremely valuable to mankind. As one floats over tide-water on a balmy evening after dark and watches the pulsating spots of phosphorescent light emitted by the lowly jellyfishes, his imaginative mood formulates the question, "Why are these lowly organisms endowed with such a wonderful ability?"

Despite his highly developed mind and body and his boasted superiority, man must go forth and learn the secrets of light-production before he may emancipate himself from darkness. If man could emit light in relative proportion to his size as compared with the firefly, he would need no other torch in the coal-mine. How independent he would be in extreme darkness where his adapted eyes need only a feeble light-source! Primitive man, desiring a light-source and having no means of making fire, imprisoned the glowing insects in a perforated gourd or receptacle of clay, and thus invented the first lantern perhaps before he knew how to make fire. The fireflies of the West Indies emit a continuous glow of considerable luminous intensity and the natives have used these imprisoned insects as light-sources. Thus mankind has exhibited his superiority by adapting the facilities at hand to the growing requirements which his independent nature continuously nourished. His insistent demand for independence in turn has nourished his desire to learn nature's secrets and this desire has increased in intensity throughout the ages.

The act of imprisoning a glowing insect was in itself no greater stride along the highway of progress than the act of picking a tasty fruit from its tree. However, the crude lantern perhaps directed his primitive mind to the possibilities of artificial light. The flaming fagot from the fire was the ancestor of the oil-lamp, the candle, the lantern, and the electric flash-light. It is a matter of conjecture how much time elapsed before his feeble intellect became aware that resinous wood afforded a better light-source than woods which were less inflammable. Nevertheless, pine knots and similar resinous pieces of wood eventually were favored as torches and their use has persisted until the present time. In some instances in ancient times resin was extracted from wood and burned in vessels. This was the forerunner of the grease-and the oil-lamp. In the woods to-day the craftsman of the wilds keeps on the lookout for live trees saturated with highly inflammable ingredients.

Viewed from the present age, these smoking, flickering light-sources appear very crude; nevertheless they represent a wide gulf between their users and those primitive beings who were unacquainted with the art of making fire. Although the wood fire prevailed as a light-source throughout uncounted centuries, it was subjected to more or less improvement as civilization advanced. When the wood fire was brought indoors the day was extended and early man began to develop his crude arts. He thought and planned in the comfort and security of his cave or hut. By the firelight he devised implements and even decorated his stone surroundings with pictures which to-day reveal something of the thoughts and activities of mankind during a civilization which existed many thousand years ago.

When it was too warm to have a roaring fire upon the hearth, man devised other means for obtaining light without undue warmth. He placed glowing embers upon ledges in the walls, upon stone slabs, or even upon suspended devices of non-inflammable material. Later he split long splinters of wood from pieces selected for their straightness of grain. These burning splinters emitting a smoking, feeble light were crude but they were refinements of considerable merit. A testimonial of their satisfactoriness is their use throughout many centuries. Until very recent times the burning splinter has been in use in Scotland and in other countries, and it is probable that at present in remote districts of highly civilized countries this crude device serves the meager needs of those whose requirements have been undisturbed by the progress of civilization. Scott, in "The Legend of Montrose," describes a table scene during a feast. Behind each seat a giant Highlander stood, holding a blazing torch of bog-pine. This was also the method of lighting in the Homeric age.

Crude clay relics representing a human head, from the mouth of which the wood-splinters projected, appear to corroborate the report that the flaming splinter was sometimes held in the mouth in order that both hands of a workman would be free. Splinter-holders of many types have survived, but most of them are of the form of a crude pedestal with a notch or spring clip at its upper end. The splinter was held in this clip and burned for a time depending upon its length and the character of the wood. It was the business of certain individuals to prepare bundles of splinters, which in the later stages of civilization were sold at the market-place or from house to house. Those who have observed the frontiersman even among civilized races will be quite certain that the wood for splinters was selected and split with skill, and that the splinters were burned under conditions which would yield the most satisfactory light. It is a characteristic of those who live close to nature, and are thus limited in facilities, to acquire a surprising efficiency in their primitive activities.

An obvious step in the use of burning wood as a light-source was to place such a fire on a shelf or in a cavity in the wall. Later when metal was available, gratings or baskets were suspended from the ceiling or from brackets and glowing embers or flaming chips were placed upon them. Some of these were equipped with crude chimneys to carry away the smoke, and perhaps to increase the draft. In more recent centuries the first attempt at lighting outdoor public places was by means of metal baskets in which flaming wood emitted light. It was the duty of the watchman to keep these baskets supplied with pine knots. In early centuries street-lighting was not attempted, and no serious efforts worthy of consideration as adequate lighting were made earlier than about a century ago. As a consequence the "link-boy" came into existence. With flaming torch he would escort pedestrians to their homes on dark nights. This practice was in vogue so recently that the "link-boy" is remembered by persons still living. In England the profession appears to have existed until about 1840.

Somewhat akin to the wood-splinter, and a forerunner of the candle, was the rushlight. In burning wood man noticed that a resinous or fatty material increased the inflammability and added greatly to the amount of light emitted. It was a logical step to try to reproduce this condition by artificial means. As a consequence rushes were cut and soaked in water. They were then peeled, leaving lengths of pith partially supported by threads of the skin which were not stripped off. These sticks of pith were placed in the sun to bleach and to dry, and after they were thoroughly dry they were dipped in scalding grease, which was saved from cooking operations or was otherwise acquired for the purpose. A reed two or three feet long held in the splinter-holder would burn for about an hour. Thus it is seen that man was beginning to progress in the development of artificial light. In developing the rushlight he was laying the foundation for the invention of the candle. Pliny has mentioned the burning of reeds soaked in oil as a feature of funeral rites. Many crude forerunners of the candle were developed in various parts of the world by different races. For example, the Malays made a torch by wrapping resinous gum in palm leaves, thus devising a crude candle with the wick on the outside.

Many primitive uses of vegetable and animal fats were forerunners of the oil-lamp. In the East Indies the candleberry, which contains oily seeds, has been burned for light by the natives. In many cases burning fish and birds have served as lamps. In the Orkney Islands the carcass of a stormy petrel with a wick in its mouth has been utilized as a light-source, and in Alaska a fish in a split stick has provided a crude torch for the natives. These primitive methods of obtaining artificial light have been employed for centuries and many are in use at the present time among uncivilized tribes and even by civilized beings in the remote outskirts of civilization. Surely progress is limited where a burning fish serves as a torch, or where, at best, the light-sources are feeble, smoking, flickering, and ill-smelling!

Progress insisted upon a light-source which was free from the defects of the crude devices already described and the next developments were improvements to the extent at least that combustion was more thorough. The early oil-lamps and candles did not emit much smoke, but they were still feeble light-sources and not always without noticeable odors. Nevertheless, they marked a tremendous advance in the production of artificial light. Although they were not scientific developments in the modern sense, the early oil-lamp and the candle represented the great possibilities of utilizing knowledge rather than depending upon the raw products of nature in unmodified forms. The advent of these two light-sources in reality marked the beginning of the civilization which was destined to progress and survive.

Although such primitive light-sources as the flaming splinter and the glowing ember have survived until the present age, lamps consisting of a wick dipped into a receptacle containing animal and vegetable oils have been in use among the more advanced peoples since prehistoric times. Oil-lamps are to be seen in the earliest Roman illustrations. During the height of ancient civilization along the eastern shores of the Mediterranean Sea, elaborate lighting was effected by means of the shallow grease-or oil-lamp. It is difficult to estimate the age in which this form of light-source originated, but some lamps in existence in collections at the present time appear to have been made as early as four or five thousand years before the Christian era. It is noteworthy that such lamps did not differ materially in essential details from those in use as late as a few centuries ago.

At first the grease used was the crude fat from animals. Vegetable oils also were burned in the early lamps. The Japanese, for example, extracted oil from nuts. When the demands of civilization increased, extensive efforts were made to obtain the required fats and oils. Amphibious animals of the North and the huge mammals of the sea were slaughtered for their fat, and vegetable sources were cultivated. Later, sperm and colza were the most common oils used by the advanced races. The former is an animal oil obtained from the head cavities of the sperm-whale; the latter is a vegetable oil obtained from rape-seed. Mineral oil was introduced as an illuminant in 1853, and the modern lamp came into use.

The grease-and oil-lamps in general were of such a form that they could be carried with ease and they had flat bottoms so that they would rest securely. The simplest forms had a single wick, but in others many wicks dipped into the same receptacle. The early ones were of stone, but later, lamps were modeled from clay or terra cotta and finally from metals. They were usually covered and the wick projected through a hole in the top near the edge. Large stone vases filled with a hundred pounds of liquid fat are known to have been used in early times. As a part of the setting in the celebration of festivals the ancient nations of Asia and Africa placed along the streets bronze vases filled with liquid fat. The Esquimaux to-day use this form of lamp, in which whale-oil and seal blubber is the fuel. Incidentally, these lamps also supply the only artificial heat for their huts and igloos. The heat from these feeble light-sources and from their bodies keeps these natives of the arctics warm within the icy walls of their abodes.

A TYPICAL METAL MULTIPLE-WICK OPEN-FLAME OIL-LAMP

Very beautiful oil-lamps of brass, bronze, and pewter evolved in such countries as Egypt. Many of these were designed for and used in religious ceremonies. The oil-lamps of China, Scotland, and other countries in later centuries were improved by the addition of a pan beneath the oil-receptacle, to catch drippings from the wick or oil which might run over during the filling. The Chinese lamps were sometimes made of bamboo, but the Scottish lamps were made of metal. A flat metal lamp, called a crusie, was one of the chief products of blacksmiths and was common in Scotland until the middle of the nineteenth century. This type of lamp was used by many nations and has been found in the catacombs of Rome. The crusie was usually suspended by an iron hook and the flow of oil to the wick could be regulated by tilting. The wick in the Scottish lamps consisted of the pith of rushes, cloth, or twisted threads. These early oil-lamps were almost always shallow vessels into which a short wick was dipped, and it was not until the latter part of the eighteenth century that other forms came into general use. The change in form was due chiefly to the introduction of scientific knowledge when mineral oil was introduced. As early as 1781 the burning of naptha obtained by distilling coal at low temperatures was first discussed, but no general applications were made until a later period. This was the beginning of many marked improvements in oil-lamps, and was in reality the birth of the modern science of light-production.

A GROUP OF OIL-LAMPS OF TWO CENTURIES AGO

As the activities of man became more complex he met from his growing store of knowledge the increasing requirements of lighting. In consequence, many ingenious devices for lighting were evolved. For example, in England in the seventeenth century man was already burrowing into the earth for coal and of course encountered coal-gases. These inflammable gases were first known for the direful effects which they so often produced rather than for their useful qualities. Although they were known to miners long before they received scientific attention, the earliest account of them in the Transactions of the Royal Society was presented in the year 1667. A description of early gas-lighting has been reserved for a later chapter, but the foregoing is noted at this point to introduce a novel early method of lighting in coal-mines where inflammable gases were encountered. In discussing this coal-gas another early writer stated that "it will not take fire except by flame" and that "sparks do not affect it." One of the early solutions of the problem of artificial lighting under such conditions is summarized as follows:

Before the invention of Sir Humphrey Davy's Safety Lamp, this property of the gas gave rise to a variety of contrivances for affording the miners sufficient light to pursue their operations; and one of the most useful of these inventions was a mill for producing light by sparks elicited by the collision of flint and steel.

Such a stream of sparks may appear a very crude and unsatisfactory solution as judged by present standards, but it was at least an ingenious application of the facilities available at that time. Various other devices were resorted to in the coal-mines before the introduction of a safety lamp.

In discussing the candle it is necessary again to go back to an early period, for it slowly evolved in the course of many centuries. It is the natural descendant of the rushlight, the grease-lamp, and various primitive devices. Until the advent of the more scientific age of artificial lighting, the candle stood preëminent among early light-sources. It did not emit appreciable smoke or odor and it was conveniently portable and less fragile than the oil-lamp. Candles have been used throughout the Christian era and some authorities are inclined to attribute their origin to the Phœnicians. It is known that the Romans used them, especially the wax-candles, in religious ceremonies. The Phœnicians introduced them into Byzantium, but they disappeared under the Turkish rule and did not come into use again until the twelfth century.

The wax-candle was very much more expensive than the tallow-candle until the fifteenth century, when its relative cost was somewhat reduced, bringing it within the means of a greater proportion of the people. Nevertheless it has long been used, chiefly by the wealthy; the departing guest of the early Victorian inn would be likely to find an item on his bill such as this: "For a gentleman who called himself a gentleman, wax-lights, 5/." Poor men used tallow dips or went to bed in the dark. It is interesting to note the importance of the candle in the household budget of early times in various sayings. For example, "The game is not worth the candle," implies that the cost of candle-light was not ignored. In these days little attention is given to the cost of artificial light under similar conditions. If a person "burns a candle at both ends" he is wasteful and oblivious to the consequences of extravagance whether in material goods or in human energy.

With the rise of the Christian church, candles came to be used in religious ceremonies and many of the symbolisms, meanings, and customs survive to the present time. Some of the finest art of past centuries is found in the old candlesticks. Many of these antiques, which ofttimes were gifts to the church, have been preserved to posterity by the church. The influence of these lighting accessories is often noted in modern lighting-fixtures, but unfortunately early art often suffers from adaptation to the requirements of modern light-sources, or the eyesight suffers from a senseless devotion to art which results in the use of modern light-sources, unshaded and glaring, in places where it was unnecessary to shade the feeble candle.

The oldest materials employed for making candles are beeswax and tallow. The beeswax was bleached before use. The tallow was melted and strained and then cotton or flax fibers were dipped into it repeatedly, until the desired thickness was obtained. In early centuries the pith of rushes was used for wicks. Tallow is now used only as a source of stearine. Spermaceti, a fatty substance obtained from the sperm-whale, was introduced into candle-making in about 1750 and great numbers of men searched the sea to fill the growing demands. Paraffin wax, a mixture of solid hydrocarbons obtained from petroleum, came into use in 1854 and stearine is now used with it. The latter increases the rigidity and decreases the brittleness of the candle. Some of the modern candles are made of a mixture of stearine and the hard fat extracted from cocoanut-oil. Modern candles vary in composition, but all are the product of much experience and of the application of scientific knowledge. The wicks are now made chiefly of cotton yarn, braided or plaited by machinery and chemically treated to aid in complete combustion when the candle is burned. Their structure is the result of long experience and they are now made so that they bend and dip into the molten fuel and are wholly consumed. This eliminates the necessity of trimming.

Candles have been made in various ways, including dipping, pouring, drawing, and molding. Wax-candles are made by pouring, because wax cannot be molded satisfactorily. Drawing is somewhat similar to dipping, except that the process is more or less continuous and is carried out by machinery. Molding, as the term implies, involves the use of molds, of the size and shape desired.

The candlestick evolved from the most primitive wooden objects to elaborately designed and decorated works of art. The primitive candlestick was crude and was no more than a holder of some kind for keeping the candle upright. Later a form of cup was attached to the stem of the holder, to catch the dripping wax or fat. The latter improvement has persisted throughout the centuries. The modern candle is by no means an unsatisfactory light-source. Those who have had experience with it in the outskirts of civilization will testify that it possesses several desirable characteristics. Supplies of candles are transported without difficulty; the lighted candle is easily carried about; and the light in a quiescent atmosphere is quite satisfactory, if common sense is used in shading and placing the candle. Although in a sense a primitive light-source, it is a blessing in many cases and, incidentally, it is extensively used to-day in industries, in religious ceremonies, as a decorative element at banquets, and in the outposts of civilization.

This account of the evolution of light-sources has crossed the threshold of what may be termed modern scientific light-production in the case of the candle and the oil-lamp. There is a period of a century or more during which scientific progress was slow, but those years paved the way for the extraordinary developments of the last few decades.

IV

THE CEREMONIAL USE OF LIGHT

Inasmuch as the symbolisms and ceremonial uses of light originated in the childhood of the human race and were nourished throughout the age of mythology, the early light-sources are associated more with this phase of artificial light than modern ones. For this reason it appears appropriate to present this discussion before entering into the later stages of the development and utilization of artificial light. Furthermore, many of the traditions of lighting at the present time are survivors of the early ages. Lighting-fixtures show the influence of this byway of lighting, and in those cases where the ceremonial use of light has survived to the present time, modern light-sources cannot be employed wisely in replacing more primitive ones without consideration of the origin and existence of the customs. In fact, candles are likely to be used for hundreds of years to come, owing to the sentiment connected with them and to the established customs founded upon centuries of traditional use.

Doubtless, the sun as a source of heat and light and of the blessings which these bring to earth, is responsible largely for the divine significance bestowed upon light. Darkness very deservingly acquired many uncomplimentary attributes, for danger lurked behind its veil and it was the suitable abode of evil spirits. It harbored all that was the antithesis of goodness, happiness, and security. Light naturally became sacred, life-giving, and symbolic of divine presence. Fire was to primitive beings the most impressive phenomenon over which they had any control, and it was sufficiently mysterious in its operation to warrant a connection with the supernatural. Thus it was very natural that these earlier beings worshiped it as representing divine presence. The sun, as Ra, was one of the chief gods of the ancient Egyptians; and the Assyrians, the Babylonians, the ancient Greeks, and many other early peoples gave a high place to this deity. Among simpler races the sun was often the sole object of worship, and those peoples who worship Light as the god of all, in a sense are not far afield. Fire-worshipers generally considered fire as the purest representation of heavenly fire, the origin of everything that lives.

Light was considered such a blessing that lamps were buried with the dead in order that spirits should be able to have it in the next world. This custom has prevailed widely but the fact that the lamps were unlighted indicates that only the material aspect was considered. It is interesting to note that the lamps and other light-sources in pagan temples and religious processions were not symbolical but were offerings to the gods. In later centuries a deeper symbolical meaning became attached to light and burning lamps were placed upon the tombs of important personages. The burying of lamps with the dead appears to have originated in Asia. The Phœnicians and Romans apparently continued the custom, but no traces of it have been found in Greece and Egypt.

Fire and light have been closely associated in various religious creeds and their ceremonies. The Hindu festival in honor of the goddess of prosperity is attended by the burning of many lamps in the temples and homes. The Jewish synagogues have their eternal lamps and in their rituals fire and light have played prominent rôles. The devout Brahman maintains a fire on the hearth and worships it as omniscient and divine. He expects a brand from this to be used to light his funeral pyre, whose fire and light will make his spirit fit to enter his heavenly abode. He keeps a fire burning on the altar, worships Agni, the god of fire, and makes fire sacrifices on various occasions such as betrothals and marriages. To the Mohammedans lighted lamps symbolize holy places, and the Kaaba at Mecca, which contains a black stone supposed to have been brought from heaven, is illuminated by thousands of lamps. Many of the uses to which light was put in ancient times indicate its rarity and sacred nature. Doubtless, the increasing use of artificial light at festivals and celebrations of the present time is partly the result of lingering customs of bygone centuries and partly due to a recognition of an innate appeal or attribute of light. Certainly nothing is more generally appropriate in representing joy and prosperity.

Throughout all countries ancient races had woven natural light and fire into their rites and customs, so it became a natural step to utilize artificial light and fire in the same manner. It would be tedious and monotonous to survey the vast field of ancient worship of light, for the underlying ideas are generally similar. The mythology of the Greeks is illustrative of the importance attached to fire and light by the cultivated peoples of ancient times. The myth of Prometheus emphasizes the fact that in those remote periods fire and light were regarded as of prime importance. According to this myth, fire and light were contained in heaven and great cunning and daring were necessary in order to obtain it. Prometheus stole this heavenly fire, for which act he was chained to the mountain and made to suffer. The Greeks mark this event as the beginning of human civilization. All arts are traced to Prometheus, and all earthly woe likewise. As past history is surveyed it appears natural to think of scientific men who have become martyrs to the quest of hidden secrets. They have made great sacrifices for the future benefit of civilization and not a few of them have endured persecution even in recent times. The Greeks recognized that a new era began with the acquisition of artificial light. Its divine nature was recognized and it became a phenomenon for worship and a means for representing divine presence. The origin of fire and light made them holy. The fire on the altar took its place in religious rites and there evolved many ceremonial uses of lamps, candles, and fire.

The Greeks and Romans burned sacred lamps in the temples and utilized light and fire in many ceremonies. The torch-race, in which young men ran with lighted torches, the winner being the one who reached the goal first with his torch still alight, originated in a Grecian ceremony of lighting the sacred fire. There are many references in ancient Roman and Grecian literature to sacred lamps burning day and night in sanctuaries and before statues of gods and heroes. On birthdays and festivals the houses of the Romans were specially ornamented with burning lamps. The Vestal Virgins in Rome maintained the sacred fire which had been brought by fugitives from Troy. In ancient Rome when the fire in the Temple of Vesta became extinguished, it was rekindled by the rubbing of a piece of wood upon another until fire was obtained. This was carried into the temple by the Vestal Virgin and the sacred fire was rekindled. The fire produced in this manner, for some reason, was considered holy.

The early peoples displayed many lamps on feast-days and an example of extravagance in this respect is an occasion when King Constantine commanded that the entire city of Constantinople be illuminated by wax-candles on Christmas Eve. Candelabra, of the form of the branching tree, were commonly in use in the Roman temples.

The ceremonial use of light in the Christian church evolved both from adaptations of pagan customs and of the natural symbolisms of fire and light. However, these acquired a deeper meaning in Christianity than in early times because they were primarily visible representations or manifestations of the divine presence. The Bible contains many references to the importance and symbolisms of light and fire. According to the First Book of Moses, the achievement of the Creator immediately following the creation of "the heavens and the earth" was the creation of light. The word "light" is the forty-sixth word in Genesis. Christ is "the true light" and Christians are "children of light" in war against the evil "powers of darkness." When St. Paul was converted "there shined about him a great light from heaven." The impressiveness and symbolism of fire and light are testified to in many biblical expressions. Christ stands "in the midst of seven candle-sticks" with "his eyes as a flame of fire." When the Holy Ghost appeared before the apostles "there appeared unto them cloven tongues of fire." When St. Paul was preaching the gospel of Christ at Alexandria "there were many lights" suggesting a festive illumination.

According to the Bible, the perpetual fire which came originally from heaven was to be kept burning on the altar. It was holy and those whose duty it was to keep it burning were guilty of a grave offense if they allowed it to be extinguished. If human hands were permitted to kindle it, punishment was meted out. The two sons of Aaron who "offered strange fire before the Lord" were devoured by "fire from the Lord." The seven-branched candlestick was lighted eternally and these burning light-sources were necessary accompaniments of worship.

The countless ceremonial uses of fire and light which had evolved in the past centuries were bound to influence the rites and customs of the Christian church. The festive illumination of pagan temples in honor of gods was carried over into the Christian era. The Christmas tree of to-day is incomplete without its many lights. Its illumination is a homage of light to the source of light. The celebration of Easter in the Church of the Holy Sepulchre in Jerusalem is a typical example of fire-worship retained from ancient times. At the climax of the services comes the descent of the Holy Fire. The central candelabra suddenly becomes ablaze and the worshipers, each of whom carries a wax taper, light their candles therefrom and rush through the streets. The fire is considered to be of divine origin and is a symbol of resurrection. The custom is similar in meaning to the light which in older times was maintained before gods.

During the first two or three centuries of the Christian era the ceremonial use of light does not appear to have been very extensive. Writings of the period contain statements which appear to ridicule this use to some extent. For example, one writer of the second century states that "On days of rejoicing ... we do not encroach upon daylight with lamps." Another, in the fourth century, refers with sarcasm to the "heathen practice" in this manner: "They kindle lights as though to one who is in darkness. Can he be thought sane who offers the light of lamps and candles to the Author and Giver of all light?"

That candles were lighted in cemeteries is evidenced by an edict which forbade their use during the day. Lamps of the early centuries of the Christian era have been found in the catacombs of Rome which are thought to have been ceremonial lamps, for they were not buried with the dead. They were found only in niches in the walls. During these same centuries elaborate candelabra containing hundreds of candles were kept burning before the tombs of saints. Notwithstanding the doubt that exists as to the extent of ceremonial lighting in the early centuries of the Christian era, it is certain that by the beginning of the fifth century the ceremonial use of light in the Christian church had become very extensive and firmly established. That this is true and that there were still some objections is indicated by many controversies. Some thought that lamps before tombs were ensigns of idolatry and others felt that no harm was done if religious people thus tried to honor martyrs and saints. Some early writings convey the idea that the ritualistic use of lights in the church arose from the retention of lights necessary at nocturnal services after the hours of worship had been changed to daytime.

Passing beyond the early controversial period, the ceremonial use of light is everywhere in evidence at ordinary church services. On special occasions such as funerals, baptisms, and marriages, elaborate altar-lighting was customary. The gorgeous candelabra and the eternal lamp are noted in many writings. Early in the fifth century the pope ordered that candles be blessed and provided rituals for this ceremony. Shortly after this the Feast of Purification of the Virgin was inaugurated and it became known as Candlemas because on this day the candles for the entire year were blessed. However, it appears that the blessing of candles was not carried out in all churches. Altar lights were not generally used until the thirteenth century. They were originally the seven candles carried by church officials and placed near the altar.

The custom of placing lighted lamps before the tombs of martyrs was gradually extended to the placing of such lamps before various objects of a sacred or divine relation. Finally certain light-sources themselves became objects of worship and were surrounded by other lamps, and the symbolisms of light grew apace. A bishop in the sixth century heralded the triple offering to God represented by the burning wax-candle. He pointed out that the rush-wick developed from pure water; that the wax was the product of virgin bees; and that the flame was sent from heaven. Each of these, he was certain, was an offering acceptable to God. Wax-candles became associated chiefly with religious ceremonies. The wax later became symbolic of the Blessed Virgin and of the body of Christ. The wick was symbolical of Christ's soul, the flame represented his divine character, and the burning candle thus became symbolical of his death. The lamp, lantern, and taper are frequently symbols of piety, heavenly wisdom, or spiritual light. Fire and flames are emblems of zeal and fervor or of the sufferings of martyrdom and the flaming heart symbolizes fervent piety and spiritual or divine love.

By the time the Middle Ages were reached the ceremonial uses of light became very complex, but for the Roman Catholic Church they may be divided into three general groups: (1) They were symbolical of God's presence or of the effect of his presence; of Christ or of "the children of light"; or of joy and content at festivals. (2) They may be offered in fulfillment of a religious vow; that is, as an act of worship. (3) They may possess certain divine power because of their being blessed by the church, and therefore may be helpful to soul and body. The three conceptions are indicated in the prayers offered at the blessing of the candles on Candlemas as follows: (1) "O holy Lord ... who ... by thy command didst cause this liquid to come by the labor of bees to the perfection of wax, ... we beseech thee ... to bless and sanctify these candles for the use of men, and the health of bodies and souls...." (2) "...these candles, which we thy servants desire to carry lighted to magnify thy name; that by offering them to thee, being worthily inflamed with the holy fire of thy most sweet charity, we may deserve...." (3) "O Lord Jesus Christ, the true light, ... mercifully grant, that as these lights enkindled with visible fire dispel nocturnal darkness, so our hearts illuminated by visible fire," etc.

In general, the ceremonial uses of lights in this church were originated as a forceful representation of Christ and of salvation. On the eve of Easter a new fire, emblematic of the arisen Christ, is kindled, and all candles throughout the year are lighted from this. During the service of Holy Week thirteen lighted candles are placed before the altar and as the penitential songs are sung they are extinguished one by one. When but one remains burning it is carried behind the altar, thus symbolizing the last days of Christ on earth. It is said that this ceremony has been traced to the eighth century. On Easter Eve, after the new fire is lighted and blessed, certain ceremonies of light symbolize the resurrection of Christ. From this new fire three candles are lighted and from these the Paschal Candle. The origin of the latter is uncertain, but it symbolizes a victorious Christ. From it all the ceremonial lights of the church are lighted and they thereby are emblematic of the presence of the light of Christ.

Many interesting ceremonial uses may be traced out, but space permits a glimpse of only a few. At baptismal services the paschal candle is dipped into the water so that the latter will be effective as a regenerative element. The baptized child is reborn as a child of light. Lighted candles are placed in the hands of the baptized persons or of their god-parents. Those about to take vows carry lights before the church official and the same idea is attached to the custom of carrying or of holding lights on other occasions such as weddings and first communion. Lights are placed around the bodies of the dead and are carried at the funeral. They not only protect the dead from the powers of darkness but they symbolize the dead as still living in the light of Christ. The use of lighted candles around bodies of the dead still survives to some extent among Protestants, but their significance has been lost sight of. Even in the eighteenth century funerals in England were accompanied by lighted tapers, but the carrying of lights in other processions appears to have ceased with the Reformation. In some parts of Scotland it is still the custom to place two lighted candles on a table beside a corpse on the day of the funeral.

With the importance of light in the ritual of the church it is not surprising that the extinction of lights is a part of the ceremony of excommunication. Such a ceremony is described in an early writing thus: "Twelve priests should stand about the bishop, holding in their hands lighted torches, which at the conclusion of the anathema or excommunication they should cast down and trample under foot." When the excommunicant is reinstated, a lighted candle is placed in his hands as a symbol of reconciliation. These and many other ceremonial uses of light have been and are practised, but they are not always mandatory. Furthermore, the customs have varied from time to time, but the few which have been touched upon illustrate the impressive part that light has played in religious services.

During the Reformation the ceremonial use of lights was greatly altered and was abolished in the Protestant churches as a relic of superstition and papal authority. In the Lutheran churches ceremonial lights were largely retained, in the Church of England they have been subjected to many changes largely through the edicts of the rulers. In the latter church many controversies were waged over ceremonial lights and their use has been among the indictments of a number of officials of the church in impeachment cases before the House of Commons. Many uses of light in religious ceremonies were revived in cathedrals after the Restoration and they became wide-spread in England in the nineteenth century. As late as 1889 the Archbishop of Canterbury ruled that certain ceremonial candles were lawful according to the Prayer-Book of Edward VI, but the whole question was left open and unsettled.

These byways of artificial light are complex and fascinating because their study leads into many channels and far into the obscurity of the childhood of the human race. A glimpse of them is important in a survey of the influence of artificial light upon the progress of civilization because in these usages the innate and acquired impressiveness of light is encountered. Although many ceremonial uses of light remain, it is doubtful if their significance and especially their origin are appreciated by most persons. Nevertheless, no more interesting phase of artificial light is encountered than this, which reaches to the foundation of civilization.

V

OIL-LAMPS OF THE NINETEENTH CENTURY

It will be noted that the light-sources throughout the early ages were flames, the result of burning material. This principle of light-production has persisted until the present time, but in the latter part of the nineteenth century certain departures revolutionized artificial lighting. However, it is not the intention to enter the modern period in this chapter except in following the progress of the oil-lamp through its period of scientific development. The oil-lamp and the candle were the mainstays of artificial lighting throughout many centuries. The fats and waxes which these light-sources burned were many but in the later centuries they were chiefly tallow, sperm-oil, spermaceti, lard-oil, olive-oil, colza-oil, bees-wax and vegetable waxes. Those fuels which are not liquid are melted to liquid form by the heat of the flame before they are actually consumed. The candle is of the latter type and despite its present lowly place and its primitive character, it is really an ingenious device. Its fuel remains conveniently solid so that it is readily shipped and stored; there is nothing to spill or to break beyond easy repair; but when it is lighted the heat of its flame melts the solid fuel and thus it becomes an "oil-lamp." Animal and vegetable oils were mainly used until the middle of the nineteenth century, when petroleum was produced in sufficient quantities to introduce mineral oils. This marked the beginning of an era of developments in oil-lamps, but these were generally the natural offspring of early developments by Ami Argand.

Before man discovered that nature had stored a tremendous supply of mineral oil in the earth he was obliged to hunt broadcast for fats and waxes to supply him with artificial light. He also was obliged to endure unpleasant odors from the crude fuels and in early experiments with fats and waxes the odor was carefully noted as an important factor. Tallow was a by-product of the kitchen or of the butcher. Stearine, a constituent of tallow, is a compound of glyceryl and stearic acid. It is obtained by breaking up chemically the glycerides of animal fats and separating the fatty acids from glycerin. Fats are glycerides; that is, combinations of oleic, palmetic, and stearic acids. Inasmuch as the former is liquid at ordinary temperatures and the others are solid, it follows that the consistency or solidity of fats depend upon the relative proportions of the three constituents. The sperm-whale, which lives in the warmer parts of all the oceans, has been hunted relentlessly for fuels for artificial lighting. In its head cavities sperm-oil in liquid form is found with the white waxy substance known as spermaceti. Colza-oil is yielded by rape-seed and olive-oil is extracted from ripe olives. The waxes are combinations of allied acids with bases somewhat related to glycerin but of complex composition. Fats and waxes are more or less related, but to distinguish them carefully would lead far afield into the complexities of organic chemistry. All these animal and vegetable products which were used as fuels for light-sources are rich in carbon, which accounts for the light-value of their flames. The brightness of such a flame is due to incandescent carbon particles, but this phase of light-production is discussed in another chapter. These oils, fats, and waxes are composed by weight of about 75 to 80 per cent. carbon; 10 to 15 per cent. hydrogen; and 5 to 10 per cent. oxygen.

Until the middle of the eighteenth century the oil-lamps were shallow vessels filled with animal or vegetable oil and from these reservoirs short wicks projected. The flame was feeble and smoky and the odors were sometimes very repugnant. Viewing such light-sources from the present age in which light is plentiful, convenient, and free from the great disadvantages of these early oil-lamps, it is difficult to imagine the possibility of the present civilization emerging from that period without being accompanied by progress in light-production. The improvements made in the eighteenth century paved the way for greater progress in the following century. This is the case throughout the ages, but there are special reasons for the tremendous impetus which light-production has experienced in the past half-century. These are the acquirement of scientific knowledge from systematic research and the application of this knowledge by organized development.

The first and most notable improvement in the oil-lamp was made by Argand in 1784. Our nation was just organizing after its successful struggle for independence at the time when the production of light as a science was born. Argand produced the tubular wick and contributed the greatest improvement by being the first to perform the apparently simple act of placing a glass chimney upon the lamp. His burner consisted of two concentric metal tubes between which the wick was located. The inner tube was open, so that air could reach the inner surface of the wick as well as the outer surface. The lamp chimney not only protected the flame from drafts but also improved combustion by increasing the supply of air. It rested upon a perforated flange below the burner. If the glass chimney of a modern kerosene lamp be lifted, it will be noted that the flame flickers and smokes and that it becomes steady and smokeless when the chimney is replaced. The advantages of such a chimney are obvious now, but Argand for his achievements is entitled to a place among the great men who have borne the torch of civilization. He took the first step toward adequate artificial light and opened a new era in lighting.

The various improvements of the oil-lamp achieved by Argand combined to effect complete combustion, with the result that a steady, smokeless lamp of considerable luminous intensity was for the first time available. Many developments followed, among which was a combination of reservoir and gravity feed which maintained the oil at a constant level. In later lamps, upon the adoption of mineral oil, this was found unnecessary, perhaps owing to the construction of the wick and to the physical characteristics of the oil which favored capillary action in the wick. However, the height of the oil in the reservoir of modern oil-lamps makes some difference in the amount of light emitted.

The Carcel lamp, which appeared in 1800, consisted of a double piston operated by clockwork. This forced the oil through a tube to the burner. Franchot invented the moderator lamp in 1836, which, because of its simplicity and efficiency soon superseded many other lamps designed for burning animal and vegetable oils. The chief feature of the moderator lamp is a spiral spring which forces the oil upward through a vertical tube to the burner. These are still used to some extent in France, but owing to the fact that "mechanical" lamps eventually were very generally replaced by more simple ones, it does not appear necessary to describe these complex mechanisms in detail.

When coal is distilled at moderate temperatures, volatile liquids are obtained. These hydrocarbons, being inflammable, naturally attracted attention when first known, and in 1781 their use as fuel for lamps was suggested. However, it was not until 1820 that the light oils obtained by distilling coal-tar, a by-product of the coal-gas industry which was then in its early stage of development, were burned to some extent in the Holliday lamp. In this lamp the oil is contained in a reservoir from the bottom of which a fine metal tube carries the oil down to a rose-burner. The oil is heated by the flame and the vaporized mineral oil which escapes through small orifices is burned. This type of lamp has undergone many physical changes, but its principle survives to the present time in the gasolene and kerosene burners hanging on a pole by the side of the street-peddler's stand.

Although petroleum products were not used to any appreciable extent for illuminating-purposes until after the middle of the nineteenth century, mineral oil is mentioned by Herodotus and other early writers. In 1847 petroleum was discovered in a coal-mine in England, but the supply failed in a short time. However, the discoverer, James Young, had found that this oil was valuable as a lubricant and upon the failure of this source he began experiments in distilling oil from shale found in coal deposits. These were destined to form the corner-stone of the oil industry in Scotland. In 1850 he began producing petroleum in this manner, but it was not seriously considered for illuminating-purposes. However, in Germany about this time lamps were developed for burning the lighter distillates and these were introduced into several countries. But the price of these lighter oils was so great that little progress was made until, in 1859, Col. E. L. Drake discovered oil in Pennsylvania. By studying the geological formations and concluding that oil should be obtained by boring, Drake gave to the world a means of obtaining petroleum, and in quantities which were destined to reduce the price of mineral oil to a level undreamed of theretofore. To his imagination, which saw vast reservoirs of oil in the depths of the earth, the world owes a great debt. Lamps were imported from Germany to all parts of the civilized world and the kerosene lamp became the prevailing light-source. Hundreds of American patents were allowed for oil-lamps and their improvements in the next decade.

LAMPS OF A CENTURY OR TWO AGO

The crude petroleum, of course, is not fit for illuminating purposes, but it contains components which are satisfactory. The various components are sorted out by fractional distillation and the oil for burning in lamps is selected according to its volatility, viscosity, stability, etc. It must not be so volatile as to have a dangerously low flashing-point, nor so stable as to hinder its burning well. In this fractional distillation a vast variety of products are now obtained. Gasolene is among the lighter products, with a density of about 0.65; kerosene has a density of about 0.80; the lubricating-oils from 0.85 to 0.95; and there are many solids such as vaseline and paraffin which are widely used for many purposes. This process of refining oils is now the source of paraffin for making candles, in which it is usually mixed with substances like stearin in order to raise its melting-point.

ELABORATE FIXTURES OF THE AGE OF CANDLES

Crude petroleum possesses a very repugnant odor; it varies in color from yellow to black; and its specific gravity ranges from about 0.80 to 1.00, but commonly is between 0.80 and 0.90. Its chemical constitution is chiefly of carbon and hydrogen, in the approximate ratio of about six to one respectively. It is a mixture of paraffin hydrocarbons having the general formula of C_nH_2n+2 and the individual members of this series vary from CH₄ (methane) to C₁₅H₃₂ (pentadecane), although the solid hydrocarbons are still more complex. Petroleum is found in many countries and the United States is particularly blessed with great stores of it.

The ordinary lamp consisting of a wick which draws up the mineral oil and feeds it to a flame is efficient and fairly free from danger. It requires care and may cause disaster if it is upset, but it has been blamed unjustly in many accidents. A disadvantage of the kerosene lamp over electric lighting, for example, is the relatively greater possibility of accidents through the carelessness of the user. This point is brought out in statistics of fire-insurance companies, which show that the fires caused by kerosene lamps are much more numerous than those from other methods of lighting. If in a modern lamp of proper construction a close-fitting wick is used and the lamp is extinguished by turning down and blowing across the chimney, there is little danger in its use excepting accidental breakage or overturning.

In oil-lamps at the present time mineral oils are used which possess flashing-points above 75°F. The highly volatile components of petroleum are dangerous because they form very explosive mixtures with air at ordinary temperatures. A mineral oil like kerosene, to be used with safety in lamps, should not be too volatile. It is preferable that an inflammable vapor should not be given off at temperatures under 120°F. The oil must be of such physical characteristics as to be drawn up to the burner by capillarity from the reservoir which is situated below. It is volatilized by the heat of the flame into a mixture of hydrogen and hydrocarbon gases and these are consumed under the heat of the process of consumption by the oxygen in the air. The resulting products of this combustion, if it is complete, are carbon dioxide and water-vapor. For each candle-power of light per hour about 0.24 cubic foot of carbon dioxide and 0.18 cubic foot of water-vapor are formed by a modern oil-lamp. That an open flame devours something from the air is easily demonstrated by enclosing it in an air-tight space. The flame gradually becomes feeble and smoky and finally goes out. It will be noted that a burning lamp will vitiate the atmosphere of a closed room by consuming the oxygen and returning in its place carbon dioxide. This is similar to the vitiation of the atmosphere by breathing persons and tests indicate that for each two candle-power emitted by a kerosene flame the vitiation is equal to that produced by one adult person. Inasmuch as oil-lamps are ordinarily of 10 to 20 candle-power, it is seen that one lamp will consume as much oxygen as several persons.

In order that oil-lamps may produce a brilliant light free from smoke, combustion must be complete. The correct quantity of oil must be fed to the burner and it must be properly vaporized by heat. If insufficient oil is fed, the intensity of the light is diminished and if too much is available at the burner, smoke and other products of incomplete combustion will be emitted. The wick is an important factor, for, through capillarity, it feeds oil forcefully to the burner against the action of gravity. This action of a wick is commonly looked upon with indifference but in reality it is caused by an interesting and really wonderful phenomenon. Wicks are usually made of high-grade cotton fiber loosely spun into coarse threads and these are woven into a loose plait. The wick must be dry before being inserted into the burner; and it is desirable that it be considerably longer than is necessary merely to reach the bottom of the reservoir. A flame burning in the open will smoke because insufficient oxygen is brought in contact with it. The injurious products of this incomplete combustion are carbon monoxide and oil vapors, which are a menace to health.

To supply the necessary amount of oxygen (air) to the flame, a forced draft is produced. Chimneys are simple means of accomplishing this, and this is their function whether on oil-lamps or factories. Other means of forced draft have been used, such as small fans or compressed air. In the railway locomotive the short smoke-stack is insufficient for supplying large quantities of air to the fire-box so the exhausted steam is allowed to escape into the stack. With each noisy puff of smoke a quantity of air is forcibly drawn into the fire-box through the burning fuel. In the modern oil-lamp the rush of air due to the "pull" of the chimney is broken and the air is diffused by the wire gauze or holes at the base of the burner. These metal parts, being hot, also serve to warm the oil before it reaches the burning end of the wick, thus serving to aid vaporization and combustion.

The consumption of oil per candle-power per hour varies considerably with the kind of lamp and with the character of the oil. The average consumption of oil-lamps burning a mineral oil of about 0.80 specific gravity and a rather high flashing-point is about 50 to 60 grams of oil per candle-power per hour for well-designed flame-lamps. Kerosene weighs about 6.6 pounds per gallon; therefore, about 800 candle-power hours per gallon are obtained from modern lamps employing wicks. Kerosene lamps are usually of 10 to 20 candle-power, although they are made up to 100 candle-power. These luminous intensities refer to the maximum horizontal candle-power. The best practice now deals with the total light output, which is expressed in lumens, and on this basis a consumption of one gallon of kerosene per hour would yield about 8000 lumens.

Oil-lamps have been devised in which the oil is burned as a spray ejected by air-pressure. These burn with a large flame; however, a serious feature is the escape of considerable oil which is not burned. These lamps are used in industrial lighting, especially outdoors, and possess the advantage of consuming low-grade oils. They produce about 700 to 800 candle-power hours per gallon of oil. Lamps of this type of the larger sizes burn with vertical flames two or three feet high. The oil is heated as it approaches the nozzle and is fairly well vaporized on emerging into the air. The names of Lucigen, Wells, Doty, and others are associated with this type of lamp or torch, which is a step in the direction of air-gas lighting.

During the latter part of the nineteenth century numerous developments were made which paralleled the progress in gas-lighting. Experiments were conducted which bordered closely upon the next epochal event in light-production—the appearance of the gas mantle. One of these was the use of platinum gauze by Kitson. He produced an apparatus similar to the oil-spray lamp, on a small and more delicate scale. The hot blue flame was not very luminous and he attempted to obtain light by heating a mantle of fine platinum gauze. Although these mantles emitted a brilliant light for a few hours, their light-emissivity was destroyed by carbonization. After the appearance of the Welsbach mantle, Kitson's lamp and others met with success by utilizing it. From this point, attention was centered upon the new wonder, which is discussed in a later chapter after certain scientific principles in light-production have been discussed.

The kerosene or mineral-oil lamp was a boon to lighting in the nineteenth century and even to-day it is a blessing in many homes, especially in villages, in the country, and in the remote districts of civilization. Its extensive use at the present time is shown by the fact that about eight million lamp-chimneys are now being manufactured yearly in this country. It is convenient and safe when carelessness is avoided, and is fairly free from odor. Its vitiation of the atmosphere may be counteracted by proper ventilation and there remains only the disadvantage of keeping it in order and of accidental breakage and overturning. The kerosene lantern is widely used to-day, but the danger due to accident is ever-present. The consequences of such accidents are often serious and are exemplified in the terrible conflagration in Chicago in 1871, when Mrs. O'Leary's cow kicked over a lantern and started a fire which burned the city. Modern developments in lighting are gradually encroaching upon the territory in which the oil-lamp has reigned supreme for many years. Acetylene plants were introduced to a considerable extent some time ago and to-day the self-contained home-lighting electric plant is being installed in large numbers in the country homes of the land.

VI

EARLY GAS-LIGHTING

Owing to the fact that the smoky, flickering oil-lamp persisted throughout the centuries and until the magic touch of Argand in the latter part of the eighteenth century transformed it into a commendable light-source, the reader is prepared to suppose that gas-lighting is of recent origin. Apparently William Murdock in England was the first to install pipes for the conveyance of gas for lighting purposes. In an article in the "Philosophical Transactions of the Royal Society of London" dated February 25, 1808, in which he gives an account of the first industrial gas-lighting, he states:

It is now nearly sixteen years, since, in a course of experiments I was making at Redruth in Cornwall, upon the quantities and qualities of the gases produced by distillation from different mineral and vegetable substances, I was induced by some observation I had previously made upon the burning of coal, to try the combustible property of the gases produced from it....

Inasmuch as he is credited with having lighted his home by means of piped gas, this experimental installation may be considered to have been made in 1792. In his first trial he burned the gas at the open ends of the pipes; but finding this wasteful, he closed the ends and in each bored three small holes from which the gas-flames diverged. It is said that he once used his wife's thimble in an emergency to close the end of the pipe; and, the thimble being much worn and consequently containing a number of small holes, tiny gas-jets emerged from the holes. This incident is said to have led to the use of small holes in his burners. He also lighted a street lamp and had bladders filled with gas "to carry at night, with which, and his little steam carriage running on the road, he used to astonish the people." Apparently unknown to Murdock, previous observations had been made as to the inflammability of gas from coal. Long before this Dr. Clayton described some observations on coal-gas, which he called "the spirit of coals." He filled bladders with this gas and kept them for some time. Upon his pricking one of them with a pin and applying a candle, the gas burned at the hole. Thus Clayton had a portable gas-light. He was led to experiment with distillation of coal from some experiences with gas from a natural coal bed, and he thus describes his initial laboratory experiment:

I got some coal, and distilled it in a retort in an open fire. At first there came over only phlegm, afterwards a black oil, and then likewise, a spirit arose which I could no ways condense; but it forced my lute and broke my glasses. Once when it had forced my lute, coming close thereto, in order to try to repair it, I observed that the spirit which issued out caught fire at the flame of the candle, and continued burning with violence as it issued out in a stream, which I blew out, and lighted again alternately several times.

He then turned his attention to saving some of the gas and hit upon the use of bladders. He was surprised at the amount of gas which was obtained from a small amount of coal; for, as he stated, "the spirit continued to rise for several hours, and filled the bladders almost as fast as a man could have blown them with his mouth; and yet the quantity of coals distilled was inconsiderable."

Although this account appeared in the Transactions of the Royal Society in 1739, there is strong evidence that Dr. Clayton had written it many years before, at least prior to 1691.

But before entering further into the early history of gas-lighting, it is interesting to inquire into the knowledge possessed in the seventeenth century pertaining to natural and artificial gas. Doubtless there are isolated instances throughout history of encounters with natural gas. Surely observant persons of bygone ages have noted a small flame emanating from the end of a stick whose other end was burning in a bonfire or in the fireplace. This is a gas-plant on a small scale; for the gas is formed at the burning end of the wooden stick and is conducted through its hollow center to the cold end, where it will burn if lighted. If a piece of paper be rolled into the form of a tube and inclined somewhat from a horizontal position, inflammable gas will emanate from the upper end if the lower end is burning. By applying a match near the upper end, we can ignite this jet of gas. However, it is certain that little was known of gas for illuminating purposes before the eighteenth century.

The literature of an ancient nation is often referred to as revealing the civilization of the period. Surely the scientific literature which deals with concrete facts is an exact indicator of the technical knowledge of a period! That little was known of natural gas and doubtless of artificial gas in the seventeenth century is shown by a brief report entitled "A Well and Earth in Lancashire taking Fire at a Candle," by Tho. Shirley in the Transactions of the Royal Society in 1667. Much of the quaint charm of the original is lost by inability to present the text in its original form, but it is reproduced as closely as practicable. The report was as follows:

About the latter End of Feb. 1659, returning from a Journey to my House in Wigan, I was entertained with the Relation of an odd Spring situated in one Mr. Hawkley's Ground (if I mistake not) about a Mile from the Town, in that Road which leads to Warrington and Chester: The People of this Town did confidently affirm, That the Water of this Spring did burn like Oil.

When we came to the said Spring (being 5 or 6 in Company together) and applied a lighted Candle to the Surface of the Water; there was 'tis true, a large Flame suddenly produced, which burnt the Foot of a Tree, growing on the Top of a neighbouring Bank, the Water of which Spring filled a Ditch that was there, and covered the Burning-place; I applied the lighted Candle to divers Parts of the Water contained in the said Ditch, and found, as I expected, that upon the Touch of the Candle and the Water the Flame was extinct.

Again, having taken up a Dish full of water at the flaming Place, and held the lighted Candle to it, it went out. Yet I observed that the Water, at the Burning-place, did boil, and heave, like Water in a Pot upon the Fire, tho' by putting my Hand into it, I could not perceive it so much as warm.

This Boiling I conceived to proceed from the Eruption of some bituminous or sulphureous Fumes; considering this Place was not above 30 or 40 Yards distant from the Mouth of a Coal-Pit there: And indeed Wigan, Ashton, and the whole Country, for many Miles compass, is underlaid with Coal. Then, applying my Hand to the Surface of the Burning-place of the Water, I found a strong Breath, as it were a Wind, to bear against my Hand.

When the Water was drained away, I applied the Candle to the Surface of the dry Earth, at the same Point where the Water burned before; the Fumes took fire, and burned very bright and vigorous. The Cone of the Flame ascended a Foot and a half from the Superficies of the Earth; and the Basis of it was of the Compass of a Man's Hat about the Brims. I then caused a Bucket full of Water to be pour'd on the Fire, by which it was presently quenched. I did not perceive the Flame to be discoloured like that of sulphurous Bodies, nor to have any manifest Scent with it. The Fumes, when they broke out of the Earth, and press'd against my Hand, were not, to my best Remembrance, at all hot.

Turning again to Dr. Clayton's experiments, we see that he pointed out striking and valuable properties of coal-gas but apparently gave no attention to its useful purposes. Furthermore, his account appears to have attracted no particular notice at the time of its publication in 1739. Dr. Richard Watson in 1767 described the results of experiments which he had been making with the products arising from the distillation of coal. In his process he permitted the gas to ascend through curved tubes, and he particularly noted "its great inflammability as well as elasticity." He also observed that "it retained the former property after it had passed through a great quantity of water." His published account dealt with a variety of facts and computations pertaining to the quantities of coke, tar, etc., produced from different kinds of coal and was a scientific work of value, but apparently the usefulness of the property of inflammability of coal-gas did not occur to him.

It is usually the habit of the scientific explorer of nature to return from excursions into her unfrequented recesses with new knowledge, to place it upon exhibition, and to return for more. The inventor passes by and sees applications for some of these scientific trophies which are productive of momentous consequences to mankind. Sir Humphrey Davy described his primitive arc-lamp three quarters of a century before Brush developed an arc-lamp for practical purposes. Maxwell and Hertz respectively predicted and produced electromagnetic waves long before Marconi applied this knowledge and developed "wireless" telegraphy. In a similar manner scientific accounts of the production and properties of coal-gas antedated by many years the initial applications made by Murdock to illuminating purposes.

Up to the beginning of the nineteenth century the civilized world had only a faint glimpse of the illuminating property of gas, but practicable gas-lighting was destined soon to be an epochal event in the progress of lighting. The dawn of modern science was coincident with the dawn of a luminous era.

At Soho foundry in 1798 Murdock constructed an apparatus which enabled him to exhibit his lighting-plan on a larger scale and to experiment on purifying and burning the gas so as to eliminate odor and smoke. Soho was an unique institution described as a place

to which men of genius were invited and resorted from every civilized country, to exercise and to display their talents. The perfection of the manufacturing arts was the great and constant aim of its liberal and enlightened proprietors, Messrs. Boulton and Watt; and whoever resided there was surrounded by a circle of scientific, ingenious, and skilful men, at all times ready to carry into effect the inventions of each other.

The Treaty of Amiens, which gave to England the peace she was sorely in need of, afforded Murdock an opportunity in 1802 favorable for making a public display of gas-lighting. The illumination of the Soho works on this occasion is described as "one of extraordinary splendour." The fronts of the extensive range of buildings were ornamented with a large number of devices which displayed the variety of forms of gas-lights. At that time this was a luminous spectacle of great novelty and the populace came from far and wide "to gaze at, and to admire, this wonderful display of the combined effects of science and art."

Naturally, Murdock had many difficulties to overcome in these early days, but he possessed skill and perseverance. His first retorts for distilling coal were similar to the common glass retort of the chemist. Next he tried cast-iron cylinders placed perpendicularly in a common furnace, and in each were put about fifteen pounds of coal. In 1804 he constructed them with doors at each end, for feeding coal and extracting coke respectively, but these were found inconvenient. In his first lighting installation in the factory of Phillips and Lee in 1805 he used a large retort of the form of a bucket with a cover on it. Inside he installed a loose cage of grating to hold the coal. When carbonization was complete the coke could be removed as a whole by extracting this cage. This retort had a capacity of fifteen hundred pounds of coal. He labored with mechanical details, varied the size and shape of the retorts, and experimented with different temperatures, with the result that he laid a solid foundation for coal-gas lighting. For his achievements he is entitled to an honorable place among the torch-bearers of civilization.

The epochal feature of the development of gas-lighting is that here was a possibility for the first time of providing lighting as a public utility. In the early years of the nineteenth century the foundation was laid for the great public-utility organizations of the present time. Furthermore, gas-lighting was an improvement over candles and oil-lamps from the standpoints of convenience, safety, and cost. The latter points are emphasized by Murdock in his paper presented before the Royal Society in 1808, in which he describes the first industrial installation of gas-lighting. He used two types of burners, the Argand and the cockspur. The former resembled the Argand lamp in some respects and the latter was a three-flame burner suggesting a fleur-de-lis. In this installation there were 271 Argand burners and 636 cockspurs. Each of the former "gave a light equal to that of four candles; and each of the latter, a light equal to two and a quarter of the same candles; making therefore the total of the gas light a little more than 2500 candles." The candle to which he refers was a mold candle "of six in the pound" and its light was considered a standard of luminous intensity when it was consuming tallow at the rate of 0.4 oz. (175 grains) per hour. Thus the candle became very early a standard light-source and has persisted as such (with certain variations in the specifications) until the present time. However, during recent years other standard light-sources have been devised.

According to Murdock, the yearly cost of gas-lighting in this initial case was 600 pounds sterling after allowing generously for interest on capital invested and depreciation of the apparatus. The cost of furnishing the same amount of light by means of candles he computed to be 2000 pounds sterling. This comparison was on the basis of an average of two hours of artificial lighting per day. On the basis of three hours of artificial lighting per day, the relative cost of gas-and candle-lighting was about one to five. Murdock was characteristically modest in discussing his achievements and his following statement should be read with the conditions of the year 1808 in mind:

The peculiar softness and clearness of this light with its almost unvarying intensity, have brought it into great favour with the work people. And its being free from the inconvenience and danger, resulting from sparks and frequent snuffing of candles, is a circumstance of material importance, as tending to diminish the hazard of fire, to which cotton mills are known to be exposed.

Although this installation in the mill of Phillips and Lee is the first one described by Murdock, in reality it is not the first industrial gas-lighting installation. During the development of gas apparatus at the Soho works and after his luminous display in 1802, he gradually extended gas-lighting to all the principal shops. However, this in a sense was experimental work. Others were applying their knowledge and ingenuity to the problem of making gas-lighting practicable, but Murdock has been aptly termed "the father of gas-lighting." Among the pioneers was Le Bon in France, Becher in Munich, and Winzler or Winsor, a German who was attracted to the possibilities of gas-lighting by an exhibition which Le Bon gave in Paris in 1802. Winsor learned that Le Bon had been granted a patent in Paris in 1799 for making an illuminating gas from wood and tried to obtain the rights for Germany. Being unsuccessful in this, he set about to learn the secrets of Le Bon's process, which he did, perhaps largely owing to an accumulation of information directly from the inventor during the negotiations. Winsor then turned to England as a fertile field for the exploitation of gas-lighting and after conducting experiments in London for some time he made plans to organize the National Heat and Light Co.

Winsor was primarily a promoter, with little or no technical knowledge; for in his claims and advertisements he disregarded facts with a facility possessed only by the ignorant. He boasted of his inventions and discoveries in the most hyperbolical language, which was bound to provoke a controversy. Nevertheless, he was clever and in 1803 he publicly exhibited his plan of lighting by means of coal-gas at the Lyceum Theatre in London. He gave lectures accompanied by interesting and instructive experiments and in this manner attracted the public to his exhibition. All this time he was promoting his company, but his promoting instinct caused his representations to be extravagant and deceptive, which exposed him to the ridicule and suspicion of learned men. His attempt to obtain certain exclusive rights by Act of Parliament failed because of opposition of scientific men toward his claims and of the stand which Murdock justly made in self-protection. These years of controversy yield entertaining literature for those who choose to read it, but unfortunately space does not permit dwelling upon it. The investigations by committees of Parliament also afford amusing side-lights. Throughout all this Murdock appeared modest and conservative and had the support of reputable scientific men, but Winsor maintained extravagant claims.

During one of these investigations Sir Humphrey Davy was examined by a committee from the House of Commons in 1809. He refuted Winsor's claims for a superior coke as a by-product and stated that the production of gas by the distillation of coal had been well known for thirty or forty years and the production of tar as long. He stated that it was the opinion of the Council of the Royal Society that Murdock was the first person to apply coal-gas to lighting in actual practice. As secretary of the Society, Sir Humphrey Davy stated that at the last session it had bestowed the Count Rumford medal upon Murdock for "his economical application of the gas light."

Winsor proceeded to float his company without awaiting the Act of Parliament and in 1807 lighted a street in Pall Mall. Through the opposition which he aroused, and owing to the just claims of priority on the part of Murdock, the bill to incorporate the National Heat and Light Co. with a capital of 200,000 pounds sterling was thrown out. However, he succeeded in 1812 in receiving a charter very much modified in form, for the Chartered Gas Light and Coke Co. which was the forerunner of the present London Gas Light and Coke Co.

The conditions imposed upon this company as presented in an early treatise on gas-lighting (by Accum in 1818) were as follows:

The power and authorities granted to this corporate body are very restricted and moderate. The individuals composing it have no exclusive privilege; their charter does not prevent other persons from entering into competition with them. Their operations are confined to the metropolis, where they are bound to furnish not only a stronger and better light to such streets and parishes as chuse to be lighted with gas, but also at a cheaper price than shall be paid for lighting the said streets with oil in the usual manner. The corporation is not permitted to traffic in machinery for manufacturing or conveying the gas into private houses, their capital or joint stock is limited to £200,000, and his Majesty has the power of declaring the gas-light charter void if the company fail to fulfil the terms of it.

The progress of this early company was slow at first, but with the appointment of Samuel Clegg as engineer in 1813 an era of technical developments began. New stations were built and many improvements were introduced. By improving the methods of purifying the gas a great advance was made. The utility of gas-lighting grew apace as the prejudices disappeared, but for a long time the stock of the company sold at a price far below par. About this time the first gas explosion took place and the members of the Royal Society set a precedent which has lived and thrived: they appointed a committee to make an inquiry. But apparently the inquiry was of some value, for it led "to some useful alterations and new modifications in its apparatus and machinery."

Many improvements were being introduced during these years and one of them in 1816 increased the gaseous product from coal by distilling the tar which was obtained during the first distillation. In 1816 Clegg obtained a patent for a horizontal rotating retort; for an apparatus for purifying coal-gas with "cream of lime"; and for a rotative gas-meter.

Before progressing too far, we must mention the early work of William Henry. In 1804 he described publicly a method of producing coal-gas. Besides making experiments on production and utilization of coal-gas for lighting, he devoted his knowledge of chemistry to the analysis of the gas. He also made analytical studies of the relative value of wood, peat, oil, wax, and different kinds of coal for the distillation of gas. His chemical analyses showed to a considerable extent the properties of carbureted hydrogen upon which illuminating value depended. The results of his work were published in various English journals between 1805 and 1825 and they contributed much to the advancement of gas-lighting.

Although Clegg's original gas-meter was complicated and cumbersome, it proved to be a useful device. In fact, it appears to have been the most original and beneficial invention occasioned by early gas-lighting. Later Samuel Crosley greatly improved it, with the result that it was introduced to a considerable extent; but by no means was it universally adopted. Another improvement made by Clegg at this time was a device which maintained the pressure of gas approximately constant regardless of the pressure in the gasometer or tank. Clegg retired from the service of the gas company in 1817 after a record of accomplishments which glorifies his name in the annals of gas-lighting. Murdock is undoubtedly entitled to the distinction of having been the first person who applied gas-lighting to large private establishments, but Clegg overcame many difficulties and was the first to illuminate a whole town by this means.

In London in 1817 over 300,000 cubic feet of coal-gas was being manufactured daily, an amount sufficient to operate 76,500 Argand burners yielding 6 candle-power each. Gas-lighting was now exciting great interest and was firmly established. Westminster Bridge was lighted by gas in 1813, and the streets of Westminster during the following year. Gas-lighting became popular in London by 1816 and in the course of the next few years it was adopted by the chief cities and towns in the United Kingdom and on the Continent. It found its way into the houses rather slowly at first, owing to apprehension of the attendant dangers, to the lack of purification of the gas, and to the indifferent service. It was not until the latter half of the nineteenth century that it was generally used in residences.

The gas-burner first employed by Murdock received the name "cockspur" from the shape of the flame. This had an illuminating value equivalent to about one candle for each cubic foot of gas burned per hour. The next step was to flatten the welded end of the gas-pipe and to bore a series of holes in a line. From the shape of the flames this form of burner received the name "cockscomb." It was somewhat more efficient than the cockspur burner. The next obvious step was to slit the end of the pipe by means of a fine saw. From this slit the gas was burned as a sheet of flame called the "bats-wing." In 1820 Nielson made a burner which allowed two small jets to collide and thus form a flat flame. The efficiency of this "fish-tail" burner was somewhat higher than that of the earlier ones. Its flame was steadier because it was less influenced by drafts of air. In 1853 Frankland showed an Argand burner consisting of a metal ring containing a series of holes from which jets of gas issued. The glass chimney surrounded these, another chimney, extending somewhat lower, surrounded the whole, and a glass plate closed the bottom. The air to be fed to the gas-jets came downward between the two chimneys and was heated before it reached the burner. This increased the efficiency by reducing the amount of cooling at the burner by the air required for combustion. This improvement was in reality the forerunner of the regenerative lamps which were developed later.

In 1854 Bowditch brought out a regenerative lamp and, owing to the excessive publicity which this lamp obtained, he is generally credited with the inception of the regenerative burner. This principle was adopted in several lamps which came into use later. They were all based upon the principle of heating both the gas and the air required for combustion prior to their reaching the burner. The burner is something like an inverted Argand arranged to produce a circular flame projecting downward with a central cusp. The air- and gas-passages are directly above the flame and are heated by it. In 1879 Friedrich Siemens brought out a lamp of this type which was adapted from a device originally designed for heating purposes, owing to the superior light which was produced. This was the best gas-lamp up to that time. Later, Wenham, Cromartie, and others patented lamps operating on this same principle.

Murdock early modified the Argand burner to meet the requirements of burning gas and by using the chimney obtained better combustion and a steadier flame than from the open burners. He and others recognized that the temperature of the flame had a considerable effect upon the amount of light emitted and non-conducting material such as steatite was substituted for the metal, which cooled the flame by conducting heat from it. These were the early steps which led finally to the regenerative burner.

The increasing efficiency of the various gas-burners is indicated by the following, which are approximately the candle-power based upon equal rates of consumption, namely, one cubic foot of gas per hour:

It is seen that the possibilities of gas lighting were recognized in several countries, all of which contributed to its development. Some of the earlier accounts have been drawn chiefly from England, but these are intended merely to serve as examples of the difficulties encountered. Doubtless, similar controversies arose in other countries in which pioneers were also nursing gas-lighting to maturity. However, it is certain that much of the early progress of lighting of this character was fathered in England. Gas-lighting was destined to become a thriving industry, and is of such importance in lighting that another chapter is given its modern developments.

VII

THE SCIENCE OF LIGHT-PRODUCTION

In previous chapters much of the historical development of artificial lighting has been presented and several subjects have been traced to the modern period which marks the beginning of an intensive attack by scientists upon the problems pertaining to the production of efficient and adequate light-sources. Many historical events remain to be touched upon in later chapters, but it is necessary at this point for the reader to become acquainted with certain general physical principles in order that he may read with greater interest some of the chapters which follow. It is seen that from a standpoint of artificial lighting, the "dark age" extended well into the nineteenth century. Oil-lamps and gas-lighting began to be seriously developed at the beginning of the last century, but the pioneers gave attention chiefly to mechanical details and somewhat to the chemistry of the fuels. It was not until the science of physics was applied to light-sources that rapid progress was made.

All the light-sources used throughout the ages, and nearly all modern ones, radiate light by virtue of the incandescence of solids or of solid particles and it is an interesting fact that carbon is generally the solid which emits light. This is due to various physical characteristics of carbon, the chief one being its extremely high melting-point. However, most practicable light-sources of the past and present may be divided into two general classes: (1) Those in which solids or solid particles are heated by their own combustion, and (2) those in which the solids are heated by some other means. Some light-sources include both principles and some perhaps cannot be included under either principle without qualification. The luminous flames of burning material such as those of wood-splinters, candles, oil-lamps, and gas-jets, and the glowing embers of burning material appear in the first class; and incandescent gas-mantles, electric filaments, and arc-lamps to some extent are representative of the second class. Certain "flaming" arcs involve both principles, but the light of the firefly, phosphorescence, and incandescent gas in "vacuum" tubes cannot be included in this simplified classification. The status of these will become clear later.

It has been seen that flames have been prominent sources of artificial light; and although of low luminous efficiency, they still have much to commend them from the standpoints of portability, convenience, and subdivision. The materials which have been burned for light, whether solid or liquid, are rich in carbon, and the solid particles of carbon by virtue of their incandescence are responsible for the brightness of a flame. A jet of pure hydrogen gas will burn very hot but with so low a brightness as to be barely visible. If solid particles are injected into the flame, much more light usually will be emitted. A gas-burner of the Bunsen type, in which complete combustion is obtained by mixing air in proper proportions with the gas, gives a hot flame which is of a pale blue color. Upon the closing of the orifice through which air is admitted, the flame becomes bright and smoky. The flame is now less hot, as indicated by the presence of smoke or carbon particles, and combustion is not complete. However, it is brighter because the solid particles of carbon in passing upward through the flame become heated to temperatures at which they glow and each becomes a miniature source of light.

A close observer will notice that the flame from a match, a candle, or a gas-jet, is not uniformly bright. The reader may verify this by lighting a match and observing the flame. There is always a bluish or darker portion near the bottom. In this less luminous part the air is combining with the hydrogen of the hydrocarbon which is being vaporized and disintegrated. Even the flame of a candle or of a burning splinter is a miniature gas-plant, for the solid or liquid hydrocarbons are vaporized before being burned. Owing to the incoming colder air at this point, the flame is not hot enough for complete combustion. The unburned carbon particles rise in its draft and become heated to incandescence, thus accounting for the brighter portion. In cases of complete combustion they are eventually oxidized into carbon dioxide before they are able to escape. If a piece of metal be held in the flame, it immediately becomes covered with soot or carbon, because it has reduced the temperature below the point at which the chemical reaction—the uniting of carbon with oxygen—will continue. An ordinary flat gas-flame of the "bats-wing" type may vary in temperature in its central portion from 300°F. at the bottom to about 3000°F. at the top. The central portion lies between two hotter layers in which the vertical variation is not so great. The brightness of the upper portion is due to incandescent carbon formed in the lower part.

When scientists learned by exploring flames that brightness was due to the radiation of light by incandescent solid matter, the way was open for many experiments. In the early days of gas-lighting investigations were made to determine the relation of illuminating value to the chemical constitution of the gas. The results combined with a knowledge of the necessity for solid carbon in the flame led to improvements in the gas for lighting purposes. Gas rich in hydrocarbons which in turn are rich in carbon is high in illuminating value. Heating-effect depends upon heat-units, so the rating of gas in calories or other heat-units per cubic foot is wholly satisfactory only for gas used for heating. The chemical constitution is a better indicator of illuminating value.

As scientific knowledge increased, efforts were made to get solid matter into the flames of light-sources. Instead of confining efforts to the carbon content of the gas, solid materials were actually placed in the flame, and in this manner various incandescent burners were developed. A piece of lime placed in a hydrogen flame or that of a Bunsen burner is seen to become hot and to glow brilliantly. By producing a hotter flame by means of the blowpipe, in which hydrogen and oxygen are consumed, the piece of lime was raised to a higher temperature and a more intense light was obtained. In Paris there was a serious attempt at street-lighting by the use of buttons of zirconia heated in an oxygen-coal-gas flame, but it proved unsuccessful owing to the rapid deterioration of the buttons. This was the line of experimentation which led to the development of the lime-light. The incandescent burner was widely employed, and until the use of electricity became common the lime-light was the mainstay for the stage and for the projection of lantern slides. It is in use even to-day for some purposes. The origin of the phrase "in the lime-light" is obvious. The luminous intensity of the oxyhydrogen lime-light as used in practice was generally from 200 to 400 candle-power. The light decreases rapidly as the burner is used, if a new surface of lime is not presented to the flame from time to time. At the high temperatures the lime is somewhat volatile and the surface seems to change in radiating power. Zirconium oxide has been found to serve better than lime.

Improvements were made in gas-burners in order to obtain hotter flames into which solid matter could be introduced to obtain bright light. Many materials were used, but obviously they were limited to those of a fairly high melting-point. Lime, magnesia, zirconia, and similar oxides were used successfully. If the reader would care to try an experiment in verification of this simple principle, let him take a piece of magnesium ribbon such as is used in lighting for photography and ignite it in a Bunsen flame. If it is held carefully while burning, a ribbon of ash (magnesia) will be obtained intact. Placing this in the faintly luminous flame, he will be surprised at the brilliance of its incandescence when it has become heated. The simple experiment indicates the possibilities of light-production in this direction. Naturally, metals of high melting-point such as platinum were tried and a network of platinum wire, in reality a platinum mantle, came into practical use in about 1880. The town of Nantes was lighted by gas-burners using these platinum-gauze mantles, but the mantles were unsuccessful owing to their rapid deterioration. This line of experimentation finally bore fruit of immense value for from it the gas-mantle evolved.

A group of so-called "rare-earths," among which are zirconia, thoria, ceria, erbia, and yttria (these are oxides of zirconium, etc.) possess a number of interesting chemical properties some of which have been utilized to advantage in the development of modern artificial light. They are white or yellowish-white oxides of a highly refractory character found in certain rare minerals. Most of them are very brilliant when heated to a high temperature. This latter feature is easily explained if the nature of light and the radiating properties of substances are considered. Suppose pieces of different substances, for example, glass and lime, are heated in a Bunsen flame to the same temperature which is sufficiently great to cause both of them to glow. Notwithstanding the identical conditions of heating, the glass will be only faintly luminous, while the piece of lime will glow brilliantly. The former is a poor radiator; furthermore, the lime radiates a relatively greater percentage of its total energy in the form of luminous energy.

The latter point will become clearer if the reader will refresh his memory regarding the nature of light. Any luminous source such as the sun, a candle flame, or an incandescent lamp is sending forth electromagnetic waves not unlike those used in wireless telegraphy excepting that they are of much shorter wave-length. The eye is capable of recording some of these waves as light just as a receiving station is tuned to record a range of wave-lengths of electromagnetic energy. The electromagnetic waves sent forth by a light-source like the sun are not all visible, that is, all of them do not arouse a sensation of light. Those that do comprise the visible spectrum and the different wave-lengths of visible radiant energy manifest themselves by arousing the sensations of the various spectral colors. The radiant energy of shortest wave-length perceptible by the visual apparatus excites the sensation of violet and the longest ones the sensation of deep red. Between these two extremes of the visible spectrum, the chief spectral colors are blue, green, yellow, orange, and red in the order of increasing wave-lengths. Electromagnetic energy radiated by a light-source in waves of too great wave-length to be perceived by the eye as light is termed as a class "infra-red radiant energy." Those too short to be perceived as light are termed as a class "ultraviolet radiant energy." A solid body at a high temperature emits electro-magnetic energy of all wave-lengths, from the shortest ultra-violet to the longest infra-red.

Another complication arises in the variation in visibility or luminosity of energy of wave-lengths within the range of the visible spectrum. Obviously, no amount of energy incapable of exciting the sensation of light will be visible. The energy of those wave-lengths near the ends of the visible spectrum will be inefficient in producing light. That energy which excites the sensation of yellow-green produces the greatest luminosity per unit of energy and is the most efficient light. The visibility or luminous efficiency of radiant energy may be ranged approximately in this manner according to the colors aroused: yellow-green, yellow, green, orange, blue-green, red, blue, deep red, violet.

Newton, an English scientist, first described the discovery of the visible spectrum and this is of such fundamental importance in the science of light that the first paragraph of his original paper in the "Transactions of the Royal Society of London" is quoted as follows:

In the Year 1666 (at which time I applied my self to the Grinding of Optick Glasses of other Figures than Spherical) I procured me a Triangular Glass-Prism, to try therewith the celebrated Phaenomena of Colours. And in order thereto, having darkened my Chamber, and made a small Hole in my Window-Shuts, to let in a convenient Quantity of the Sun's Light, I placed my Prism at its Entrance, that it might be thereby refracted to the opposite Wall. It was at first a very pleasing Divertisement, to view the vivid and intense Colours produced thereby; but after a while applying my self to consider them more circumspectly, I became surprised to see them in an oblong Form; which, according to the receiv'd Law of Refractions, I expected should have been circular. They were terminated at the Sides with streight Lines, but at the Ends the Decay of Light was so gradual, that it was difficult to determine justly what was the Figure, yet they seemed Semicircular.

Even Newton could not have had the faintest idea of the great developments which were to be based upon the spectrum.

Now to return to the peculiar property of rare-earth oxides—namely, their unusual brilliance when heated in a flame—it is easy to understand the reason for this. For example, when a number of substances are heated to the same temperature they may radiate the same amount of energy and still differ considerably in brightness. Many substances are "selective" in their absorbing and radiating properties. One may radiate more luminous energy and less infra-red energy, and for another the reverse may be true. The former would appear brighter than the latter. The scientific worker in light-production has been searching for such "selective" radiators whose other properties are satisfactory. The rare-earths possess the property of selectivity and are fortunately highly refractory. Welsbach used these in his mantle, whose efficiency is due partly to this selective property. Recent work indicates that much higher efficiencies of light-production are still attainable by the principles involved in the gas-mantle.

Turning again to flames, another interesting physical phenomenon is seen on placing solutions of different chemical salts in the flame. For example, if a piece of asbestos is soaked in sodium chloride (common salt) and is placed in a Bunsen flame, the pale-blue flame suddenly becomes luminous and of a yellow color. If this is repeated with other salts, a characteristic color will be noted in each case. The yellow flame is characteristic of sodium and if it is examined by means of a spectroscope, a brilliant yellow line (in fact, a double line) will be seen. This forms the basis of spectrum analysis as applied in chemistry.

Every element has its characteristic spectrum consisting usually of lines, but the complexity varies with the elements. The spectra of elements also exhibit lines in the ultra-violet region which may be studied with a photographic plate, with a photo-electric cell, and by other means. Their spectral lines or bands also extend into the infra-red region and here they are studied by means of the bolometer or other apparatus for detecting radiant energy by the heat which it produces upon being absorbed. Spectrum analysis is far more sensitive than the finest weighing balance, for if a grain of salt be dissolved in a barrel of water and an asbestos strip be soaked in the water and held in a Bunsen flame, the yellow color characteristic of sodium will be detectable. A wonderful example of the possibilities of this method is the discovery of helium in the sun before it was found on earth! Its spectral lines were detected in the sun's spectrum and could not be accounted for by any known element. However, it should be stated that the spectrum of an element differs generally with the manner obtained. The electric spark, the arc, the electric discharge in a vacuum tube, and the flame are the means usually employed.

The spectrum has been dwelt upon at some length because it is of great importance in light-production and probably will figure strongly in future developments. Although in lighting little use has been made of the injection of chemical salts into ordinary flames, it appears certain that such developments would have risen if electric illuminants had not entered the field. However, the principle has been applied with great success in arc-lamps. In the first arc-lamps plain carbon electrodes were used, but in some of the latest carbon-arcs, electrodes of carbon impregnated with various salts are employed. For example, calcium fluoride gives a brilliant yellow light when used in the carbons of the "flame" arc. These are described in detail later.

Following this principle of light-production the vacuum tubes were developed. Crookes studied the light from various gases by enclosing them in a tube which was pumped out until a low vacuum was produced. On connecting a high voltage to electrodes in each end, an electrical discharge passed through the residual gas making it luminous. The different gases show their characteristic spectra and their desirability as light-producers is at once evident.

However, the most general principle of light-production at the present time is the radiation of bodies by virtue of their temperature. If a piece of wire be heated by electricity, it will become very hot before it becomes luminous. At this temperature it is emitting only invisible infra-red energy and has an efficiency of zero as a producer of light. As it becomes hotter it begins to appear red, but as its temperature is raised it appears orange, until if it could be heated to the temperature of the sun, about 10,000°F., it would appear white. All this time its luminous efficiency is increasing, because it is radiating not only an increasing percentage of visible radiant energy but an increasing amount of the most effective luminous energy. But even when it appears white, a large amount of the energy which it radiates is invisible infra-red and ultra-violet, which are ineffective in producing light, so at best the substance at this high temperature is inefficient as a light-producer.

In this branch of the science of light-production substances are sought not only for their high melting-point, but for their ability to radiate selectively as much visible energy as possible and of the most luminous character. However, at best the present method of utilizing the temperature radiation of hot bodies has limitations.

The luminous efficiencies of light-sources to-day are still very low, but great advances have been made in the past half-century. There must be some radical departures if the efficiency of light-production is to reach a much higher figure. A good deal has been said of the firefly and of phosphorescence. These light-sources appear to emit only visible energy and, therefore, are efficient as radiators of luminous radiant energy. But much remains to be unearthed concerning them before they will be generally applicable to lighting. If ultra-violet radiation is allowed to impinge upon a phosphorescent material, it will glow with a considerable brightness but will be cool to the touch. A substance of the same brightness by virtue of its temperature would be hot; hence phosphorescence is said to be "cold" light.

An acquaintance with certain terms is necessary if the reader is to understand certain parts of the text. The early candle gradually became a standard, and uniform candles are still satisfactory standards where high accuracy is not required. Their luminous intensity and illuminating value became units just as the foot was arbitrarily adopted as a unit of length. The intensity of other light-sources was represented in terms of the number of candles or fraction of a candle which gave the same amount of light. But the luminous intensity of the candle was taken only in the horizontal direction. In the same manner the luminous intensities of light-sources until a short time ago were expressed in candles as measured in a certain direction. Incandescent lamps were rated in terms of mean horizontal candles, which would be satisfactory if the luminous intensity were the same in all directions, but it is not. Therefore, the candle-power in one direction does not give a measure of the total light-output.

If a source of light has a luminous intensity of one candle in all directions, the illumination at a distance of one foot in any direction is said to be a foot-candle. This is the unit of illumination intensity. A lumen is the quantity of light which falls on one square foot if the intensity of illumination is one foot-candle. It is seen that the area of a sphere with a radius of one foot is 4p or 12.57 square feet; therefore, a light-source having a luminous intensity of one candle in all directions emits 12.57 lumens. This is the satisfactory unit, for it measures total quantity of light, and luminous efficiencies may be expressed in terms of lumens per watt, lumens per cubic foot of gas per hour, etc.

Of course, the efficiencies of light-sources are usually of interest to the consumer if they are expressed in terms of cost. But from a practical point of view there are many elements which combine to make another important factor, namely, satisfactoriness. Therefore, the efficiency of artificial lighting from the standpoint of the consumer should be the ratio of satisfactoriness to cost. However, the scientist is interested chiefly in the efficiency of the light-source which may be expressed in lumens per watt, or the amount of light obtained from a given rate of consumption or of emission of energy. This method of rating light-sources penalizes those radiating considerable energy which does not produce the sensation of light or which at best is of wave-lengths that are inefficient in this respect. That radiant energy which is wholly of a wave-length of maximum visibility, or, in other words, excites the sensation of yellow-green, is the most efficient in producing luminous sensation. Of course, no illuminants are available which approach this theoretical ideal and it is not likely that this would be a practical ideal. Under monochromatic yellow-green light the magical drapery of color would disappear and the surroundings would be a monochrome of shades of this hue. Having no colors with which to contrast this color, the world would be colorless. This should be obvious when it is considered that an object which is red under an illuminant containing all colors such as sunlight would be black or dark gray under monochromatic yellow-green light. The red under present conditions is kept alive by contrast with other colors, because the latter live by virtue of the fact that most of our present illuminants contain their hues. It is assumed that the reader knows that a red object, for example, appears red because it reflects (or transmits) red rays and absorbs the other rays in the illuminant. In other words, color is due to selective absorption reflection, or transmission.

Perhaps the ideal illuminant, which is most generally satisfactory for general activities, is a white light corresponding to noon sunlight. If this is chosen as the scientific ideal, the illuminants of the present time are much more "efficient" than if the most efficient light is the ideal.

The luminous efficiency of the radiant energy most efficient in producing the sensation of light (yellow-green) is about 625 lumens per watt. That is, if energy of this wave-length alone were radiated by a hypothetical light-source, each watt would produce 625 lumens. The luminous efficiency of the most efficient white light is about 265 lumens per watt; in other words, if a hypothetical light-source radiated energy of only the visible wave-lengths and in proportions to produce the sensation of white, each watt would produce 265 lumens. If such a white light were obtained by pure temperature radiation—that is, by a normal radiator at a temperature of 10,000°F., which is impracticable at present—the luminous efficiency would be about 100 lumens per watt. The normal radiator which emits energy by virtue of its temperature without selectively radiating more or less energy in any part of the spectrum than indicated by the theoretical radiation laws is called a "black-body" or normal radiator. Modern illuminants have luminous efficiencies ranging from 5 to 30 lumens per watt, so it is seen that much is to be done before the limiting efficiencies are reached.

The amount of light obtained from various gas-burners for each cubic foot of gas consumed per hour varies for open gas-flames from 5 to 30 lumens; for Argand burners from 35 to 40 lumens; for regenerative lamps from 50 to 75 lumens; and for gas-mantles from 200 to 250 lumens.

In the development of light-sources, of course, any harmful effects of gases formed by burning or chemical action must be avoided. Some of the fumes from arcs are harmful, but no commercial arc appears to be dangerous when used as it is intended to be used. Gas-burners rob the atmosphere of oxygen and vitiate it with gases, which, however, are harmless if combustion is complete. That adequate ventilation is necessary where oxygen is being consumed is evident from the data presented by authorities on hygiene. A standard candle when burning vitiates the air in a room almost as much as an adult person. An ordinary kerosene lamp vitiates the atmosphere as much as a half-dozen persons. An ordinary single mantle burner causes as much vitiation as two or three persons.

In order to obtain a bird's-eye view of progress in light-production, the following table of relative luminous efficiencies of several light-sources is given in round numbers. These efficiencies are in terms of the most efficient (yellow-green) light.

The luminous efficiency of a light-source is distinguished from that of a lamp. The former is the ratio of the light produced to the amount of energy radiated by the light-source. The latter is the ratio of the light produced to the total amount of energy consumed by the device. In other words, the luminous efficiency of a lamp is less than that of the light-source because the consumption of energy in other parts of the lamp besides the light-source are taken into account. These additional losses are appreciable in the mechanisms of arc-lamps but are almost negligible in vacuum incandescent filament lamps. They are unknown for the firefly, so that its luminous efficiency only as a light-source can be determined. Its efficiency as a lighting-plant may be and perhaps is rather low.

VIII

MODERN GAS-LIGHTING

As has been seen, the lighting industry, as a public service, was born in London about a century ago and companies to serve the public were organized on the Continent shortly after. From this early beginning gas-light remained for a long time the only illuminant supplied by a public-service company. It has been seen that throughout the ages little advance was made in lighting until oil-lamps were improved by Argand in the eighteenth century. Candles and open-flame oil-lamps were in use when the Pyramids were built and these were common until the approach of the nineteenth century. In fact, several decades passed after the first gas-lighting was installed before this form of lighting began to displace the improved oil-lamps and candles. It was not until about 1850 that it began to invade the homes of the middle and poorer classes. During the first half of the nineteenth century the total light in an average home was less than is now obtained from a single light-source used in residences; still, the total cost of lighting a residence has decreased considerably. If the social and industrial activities of mankind are visualized for these various periods in parallel with the development of artificial lighting, a close relation is evident. Did artificial light advance merely hand in hand with science, invention, commerce, and industry, or did it illuminate the pathway?

Although gas-lighting was born in England it soon began to receive attention elsewhere. In 1815 the first attempt to provide a gas-works in America was made in Philadelphia; but progress was slow, with the result that Baltimore and New York led in the erection of gas-works. There are on record many protests against proposals which meant progress in lighting. These are amusing now, but they indicate the inertia of the people in such matters. When Bollman was projecting a plan for lighting Philadelphia by means of piped gas, a group of prominent citizens submitted a protest in 1833 which aimed to show that the consequences of the use of gas were appalling. But this protest failed and in 1835 a gas-plant was founded in Philadelphia. Thus gas-lighting, which to Sir Walter Scott was a "pestilential innovation" projected by a madman, weathered its early difficulties and grew to be a mighty industry. Continued improvements and increasing output not only altered the course of civilization by increased and adequate lighting but they reduced the cost of lighting over the span of the nineteenth century to a small fraction of its initial cost.

Think of the city of Philadelphia in 1800, with a population of about fifty thousand, dependent for its lighting wholly upon candles and oil-lamps! Washington's birthday anniversary was celebrated in 1817 with a grand ball attended by five hundred of the élite. An old report of the occasion states that the room was lighted by two thousand wax-candles. The cost of this lighting was a hundred times the cost of as much light for a similar occasion at the present time. Can one imagine the present complex activities of a city like Philadelphia with nearly two million inhabitants to exist under the lighting conditions of a century ago? To-day there are more than fifty thousand street lamps in the city—one for each inhabitant of a century ago. Of these street lamps about twenty-five thousand burn gas. This single instance is representative of gas-lighting which initiated the "light age" and nursed it through the vicissitudes of youth. The consumption of gas has grown in the United States during this time to three billion cubic feet per day. For strictly illuminating purposes in 1910 nearly one hundred billion cubic feet were used. This country has been blessed with large supplies of natural gas; but as this fails new oil-fields are constantly being discovered, so that as far as raw materials are concerned the future of gas-lighting is assured for a long time to come.

The advent of the gas-mantle is responsible for the survival of gas-lighting, because when it appeared electric lamps had already been invented. These were destined to become the formidable light-sources of the approaching century and without the gas-mantle gas-lighting would not have prospered. Auer von Welsbach was conducting a spectroscopic study of the rare-earths when he was confronted with the problem of heating these substances. He immersed cotton in solutions of these salts as a variation of the regular means for studying elements by injecting them into flames. After burning the cotton he found that he had a replica of the original fabric composed of the oxide of the metal, and this glowed brilliantly when left in the flame.

This gave him the idea of producing a mantle for illuminating purposes and in 1885 he placed such a mantle in commercial use. His first mantles were unsatisfactory, but they were improved in 1886 by the use of thoria, an oxide of thorium, in conjunction with other rare-earth oxides. His mantle was now not only stronger but it gave more light. Later he greatly improved the mantles by purifying the oxides and finally achieved his great triumph by adding a slight amount of ceria, an oxide of cerium. Welsbach is deserving of a great deal of credit for his extensive work, which overcame many difficulties and finally gave to the world a durable mantle that greatly increased the amount of light previously obtainable from gas.

The physical characteristics of a mantle depend upon the fabric and upon the rare-earths used. It must not shrink unduly when burned, and the ash should remain porous. It has been found that a mantle in which thoria is used alone is a poor light-source, but that when a small amount of ceria is added the mantle glows brilliantly. By experiment it was determined that the best proportions for the rare-earth content are one part of ceria and ninety-nine parts of thoria. Greater or less proportions of ceria decreased the light-output. The actual percentage of these oxides in the ash of the mantle is about 10 per cent., making the content of ceria about one part in one thousand.

Mantles are made by knitting cylinders of cotton or of other fiber and soaking these in a solution of the nitrates of cerium and thorium. One end of the cylinder is then sewed together with asbestos thread, which also provides the loop for supporting the mantle over the burner. After the mantle has dried in proper form, it is burned; the organic matter disappears and the nitrates are converted into oxides. After this "burning off" has been accomplished and any residual blackening is removed, the mantle is dipped into collodion, which strengthens it for shipping and handling. The collodion is a solution of gun-cotton in alcohol and ether to which an oil such as castor-oil has been added to prevent excessive shrinkage on drying.

The materials and structure of the fabric of mantles have been subjected to much study. Cotton was first used; then ramie fibers were introduced. The ramie mantle was found to possess a greater life than the cotton mantle. Later the mantles were mercerized by immersion in ammonia-water and this process yielded a stronger material. The latest development is the use of an artificial silk as the base fabric, which results in a mantle superior to previous mantles in strength, flexibility, permanence of form, and permanence of luminous property. This artificial silk mantle will permit of handling even after it has been in use for several hundred hours. This great advance appears to be due to the fact that after the artificial-silk fibers have been burned off, the fibers are solid and continuous instead of porous as in previous mantles.

The color-value of the light from mantles may be varied considerably by altering the proportions of the rare-earths. The yellowness of the light has been traced to ceria, so by varying the proportions of ceria, the color of the light may be influenced.

The inverted mantle introduced greater possibilities into gas-lighting. The light could be directed downward with ease and many units such as inverted bowls were developed. In fact, the lighting-fixtures and the lighting-effects obtainable kept pace with those of electric lighting, notwithstanding the greater difficulties encountered by the designer of gas-lighting fixtures. Many problems were encountered in designing an inverted burner operating on the Bunsen principle, but they were finally satisfactorily solved. In recent years a great deal of study has been given to the efficiency of gas-burners, with the result that a high level of development has been reached.

Several methods of electrical ignition have been evolved which in general employ the electric spark. Electrical ignition and developments of remote control have added great improvements especially to street-lighting by means of gas. Gas-valves for remote control are actuated by gas pressure and by electromagnets. In general, the gas-lighting engineers have kept pace marvelously with electric lighting, when their handicaps are considered.

Various types of burners have appeared which aimed to burn more gas in a given time under a mantle and thereby to increase the output of light. These led to the development of the pressure system in which the pressure of gas was at first several times greater than usual. The gas is fed into the mixing tube under this higher pressure in a manner which also draws in an adequate amount of air. In this way the combustion at the burner is forced beyond the point reached with the usual pressure. Ordinary gas pressure is equal to that of a few inches of water, but high-pressure systems employ pressures as great as sixty inches of water. Under this high-pressure system, mantle-burners yield as high as 500 lumens per cubic foot of gas per hour.

The fuels for gas-lighting are natural gas, carbureted water-gas, and coal-gas obtained by distilling coal, but there are different methods of producing the artificial gases. Coal-gas is produced analytically by distilling certain kinds of coal, but water-gas and producer-gas are made synthetically by the action of several constituents upon one another. Carbureted water-gas is made from fixed carbon, steam, and oil and also from steam and oil. Producer-gas is made by the action of steam or air or both upon fixed carbon. Water-gas made from steam and oil is usually limited to those places where the raw materials are readily available. The composition of a gas determines its heating and illuminating values, and constituents favorable to one are not necessarily favorable to the other. Coal-gas usually is of lower illuminating value than carbureted water-gas. It contains more hydrogen, for example, than water-gas and it is well known that hydrogen gives little light on burning.

It has been seen in a previous chapter that the distillation of gas from coal for illuminating purposes began in the latter part of the eighteenth century. From this beginning the manufacture of coal-gas has been developed to a great and complex industry. The method is essentially destructive distillation. The coal is placed in a retort and when it reaches a temperature of about 700°F. through heating by an outside fire, the coal begins to fuse and hydrocarbon vapors begin to emanate. These are generally paraffins and olefins. As the temperature increases, these hydrocarbons begin to be affected. The chemical combinations which have long existed are broken up and there are rearrangements of the atoms of carbon and hydrogen. The actual chemical reactions become very complex and are somewhat shrouded in uncertainty. In this last stage the illuminating and heating values of the gas are determined. Usually about four hours are allowed for the complete distillation of the gaseous and liquid products from a charge of coal. Many interesting chemical problems arise in this process and the influences of temperature and time cannot be discussed within the scope of this book. Besides the coal-gas, various by-products are obtained depending upon the raw materials, upon the procedure, and upon the market.

After the coal-gas is produced it must be purified and the sulphureted hydrogen at least must be removed. One method of accomplishing this is by washing the gas with water and ammonia, which also removes some of the carbon dioxide and hydrocyanic acid. Various other undesirable constituents are removed by chemical means, depending upon the conditions. The purified gas is now delivered to the gas-holder; but, of course, all this time the pressure is governed, in order that the pressure in the mains will be maintained constant.

Much attention has been given to the enrichment of gas for illuminating purposes; that is, to produce a gas of high illuminating value from cheap fuel or by inexpensive processes. This has been done by decomposing the tar obtained during the distillation of coal and adding these gases to the coal-gas; by mixing carbureted water-gas with coal-gas; by carbureting inferior coal-gases; and by mixing oil-gas with inferior coal-gas.

Water-gas is of low illuminating value, but after it is carbureted it burns with a brilliant flame. The water-gas is made by raising the temperature of the fuel bed of hard coal or coke by forced air, which is then cut off, while steam is passed through the incandescent fuel. This yields hydrogen and carbon monoxide. To make carbureted water-gas, oil-gas is mixed with it, the latter being made by heating oil in retorts.

A great many kinds of gas are made which are determined by the requirements and the raw materials available. The amount of illuminating gas yielded by a ton of fuel, of course, varies with the method of manufacture, with the raw material, and with the use to which the fuel is to be put. The production of coal-gas per ton of coal is of the order of magnitude of 10,000 cubic feet. A typical yield by weight of a coal-gas retort is,

The coke is not pure carbon but contains the non-volatile minerals which will remain as ash when the coke is burned, just as if the original coal had been burned. On the crown of the retort used in coal-gas production, pure carbon is deposited. This is used for electric-arc carbons and for other purposes. From the tar many products are derived such as aniline dyes, benzene, carbolic acid, picric acid, napthalene, pitch, anthracene, and saccharin.

A typical analysis of the gas distilled from coal is very approximately as follows,

It is seen that illuminating gas is not a definite compound but a mixture of a number of gases. The proportion of these is controlled in so far as possible in order to obtain illuminating value and some of them are reduced to very small percentages because they are valueless as illuminants or even harmful. The constituents are seen to consist of light-giving hydrocarbons, of gases which yield chiefly heat, and of impurities. The chief hydrocarbons found in illuminating gas are,

A gas which has played a prominent part in lighting is acetylene, produced by the interaction of water and calcium carbide. No other gas easily produced upon a commercial scale yields as much light, volume for volume, as acetylene. It has the great advantage of being easily prepared from raw material whose yield of gas is considerably greater for a given amount than the raw materials which are used in making other illuminating gases. The simplicity of the manufacture of acetylene from calcium carbide and water gives to this gas a great advantage in some cases. It has served for individual lighting in houses and in other places where gas or electric service was unavailable. Where space is limited it also had an advantage and was adopted to some extent on automobiles, motor-boats, ships, lighthouses, and railway cars before electric lighting was developed for these purposes.

The color of the acetylene flame is satisfactory and it is extremely brilliant compared with most flames. An interesting experiment is found in placing a spark-gap in the flame and sending a series of sparks across it. If the conditions are proper the flame will became very much brighter. When the gas issues from a proper jet under sufficient pressure, the flame is quite steady. Its luminous efficiency gives it an advantage over other open gas-flames in lighting rooms, because for the same amount of light it vitiates the air and exhausts the oxygen to a less degree than the others. Of course, in these respects the gas-mantle is superior.

The reaction which takes place when water and calcium carbide are brought together is a double decomposition and is represented by,

CaC₂ + H₂O = C₂H₂ + CaO

It will be seen that the products are acetylene gas and calcium oxide or lime. The lime, being hydroscopic and being in the presence of water or water-vapor in the acetylene generator, really becomes calcium hydroxide Ca(OH)₂, commonly called slaked lime. If there are impurities in the calcium carbide, it is sometimes necessary to purify the gas before it may be safely used for interior lighting.

The burners and mantles used in acetylene lighting are essentially the same as those for other gas-lighting, excepting, of course, that they are especially adapted for it in minor details.

The chief source of calcium carbide in this country is the electric furnace. Cheap electrical energy from hydro-electric developments, such as the Niagara plants, have done much to make the earth yield its elements. Aluminum is very prevalent in the soil of the earth's surface, because its oxide, alumina, is a chief constituent of ordinary clay. But the elements, aluminum and oxygen, cling tenaciously to each other and only the electric furnace with its excessively high temperatures has been able to separate them on a large commercial scale. Similarly, calcium is found in various compounds over the earth's surface. Limestone abounds widely, hence the oxide and carbonate of lime are wide-spread. But calcium clings tightly to the other elements of its compounds and it has taken the electric furnace to bring it to submission. The cheapness of calcium carbide is due to the development of cheap electric power. It is said that calcium carbide was discovered as a by-product of the electric furnace by accidentally throwing water upon the waste materials of a furnace process. The discovery of a commercial scale of manufacture of calcium carbide has been a boon to isolated lighting. Electric lighting has usurped its place on the automobile and is making inroads in country-home lighting. Doubtless, acetylene will continue to serve for many years, but its future does not appear as bright as it did many years ago.

The Pintsch gas, used to some extent in railroad passenger-cars in this country, is an oil-gas produced by the destructive distillation of petroleum or other mineral oil in retorts heated externally. The product consists chiefly of methane and heavy hydrocarbons with a small amount of hydrogen. In the early days of railways, some trains were not run after dark and those which were operated were not always lighted. At first attempts were made at lighting railway cars with compressed coal-gas, but the disadvantage of this was the large tank required. Obviously, a gas of higher illuminating-value per volume was desired where limited storage space was available, and Pintsch turned his attention to oil-gas. Gas suffers in illuminating-value upon being compressed, but oil-gas suffers only about half the loss that coal-gas does. In about 1880 Pintsch developed a method of welding cylinders and buoys which satisfied lighthouse authorities and he was enabled to furnish these filled with compressed gas. Thus the buoy was its own gas-tank. He devised lanterns which would remain lighted regardless of wind and waves and thus gained a start with his compressed-gas systems. He compressed the gas to a pressure of about one hundred and fifty pounds per square inch and was obliged to devise a reducer which would deliver the gas to the burner at about one pound per square inch. This regulator served well throughout many years of exacting service. The system began to be adopted on ships and railroads in 1880 and for many years it has served well.

Although gas-lighting has affected the activities of mankind considerably by intensifying commerce and industry and by advancing social progress, the illuminants which eventually took the lead have extended the possibilities and influences of artificial light. In the brief span of a century civilized man is almost totally independent of natural light in those fields over which he has control. What another century will bring can be predicted only from the accomplishments of the past. These indicate possibilities beyond the powers of imagination.

IX

THE ELECTRIC ARCS

Early in 1800 Volta wrote a letter to the President of the Royal Society of London announcing the epochal discovery of a device now known as the voltaic pile. This letter was published in the Transactions and it created great excitement among scientific men, who immediately began active investigations of certain electrical phenomena. Volta showed that all metals could be arranged in a series so that each one would indicate a positive electric potential when in contact with any metal following it in the series. He constructed a pile of metal disks consisting of zinc and copper alternated and separated by wet cloths. At first he believed that mere contact was sufficient, but when, later, it was shown that chemical action took place, rapid progress was made in the construction of voltaic cells. The next step after his pile was constructed was to place pairs of strips of copper and zinc in cups containing water or dilute acid. Volta received many honors for his discovery, which contributed so much to the development of electrical science and art—among them a call to Paris by Bonaparte to exhibit his electrical experiments, and to receive a medal struck in his honor.

While Volta was being showered with honors, various scientific men with great enthusiasm were entering new fields of research, among which was the heating value of electric current and particularly of electric sparks made by breaking a circuit. Late in 1800 Sir Humphrey Davy was the first to use charcoal for the sparking points. In a lecture before the Royal Society in the following year he described and demonstrated that the "spark" passing between two pieces of charcoal was larger and more brilliant than between brass spheres. Apparently, he was producing a feeble arc, rather than a pure spark. In the years which immediately followed many scientific men in England, France, and Germany were publishing the results of their studies of electrical phenomena bordering upon the arc.

By subscription among the members of the Royal Society, a voltaic battery of two thousand cells was obtained and in 1808 Davy exhibited the electric arc on a large scale. It is difficult to judge from the reports of these early investigations who was the first to recognize the difference between the spark and the arc. Certainly the descriptions indicate that the simple spark was not being experimented with, but the source of electric current available at that time was of such high resistance that only feeble arcs could have been produced. In 1809 Davy demonstrated publicly an arc obtained by a current from a Volta pile of one thousand plates. This he described as "a most brilliant flame, of from half an inch to one and a quarter inches in length."

In the library of the Royal Society, Davy's notes made during the years of 1805 and 1812 are available in two large volumes. These were arranged and paged by Faraday, who was destined to contribute greatly to the future development of the science and art of electricity. In one of these volumes is found an account of a lecture-experiment by Davy which certainly is a description of the electric arc. An extract of this account is as follows:

The spark [presumably the arc], the light of which was so intense as to resemble that of the sun, ... produced a discharge through heated air nearly three inches in length, and of a dazzling splendor. Several bodies which had not been fused before were fused by this flame.... Charcoal was made to evaporate, and plumbago appeared to fuse in vacuo. Charcoal was ignited to intense whiteness by it in oxymuriatic acid, and volatilized by it, but without being decomposed.

From a consideration of his source of electricity, a voltaic pile of two thousand plates, it is certain that this could not have been an electric spark. Later in his notes Davy continued:

...the charcoal became ignited to whitness, and by withdrawing the points from each other, a constant discharge took place through the heated air, in a space at least equal to four inches, producing a most brilliant ascending arch of light, broad and conical in form in the middle.

This is surely a description of the electric arc. Apparently the electrodes were in a horizontal position and the arc therefore was horizontal. Owing to the rise of the heated air, the arc tended to rise in the form of an arch. From this appearance the term "arc" evolved and Davy himself in 1820 definitely named the electric flame, the "arc." This name was continued in use even after the two carbons were arranged in a vertical co-axial position and the arc no more "arched." An interesting scientific event of 1820 was the discovery by Arago and by Davy independently that the arc could be deflected by a magnet and that it was similar to a wire carrying current in that there was a magnetic field around it. This has been taken advantage of in certain modern arc-lamps in which inclined carbons are used. In these arcs a magnet keeps the arc in place, for without the magnet the arc would tend to climb up the carbons and go out.