COl U>JlVkiJv^U\ LIERAMEr; ITHACA. N. t. i4853 ~-^:j jjOHN M. OLIN LIBRARY X-1,'*K ^ --^ DATEJ^UE ri!^ ' V H^ iiau 'itM'A 0am Wtm^ Iavuord FHtNTUDlNU.S.A. ( TP715 .Lm""" ""'"""'y "-Ibrary Artiricial liaht, olin 3 1924 030 702 512 Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924030702512 ARTIFICIAL LIGHT Ube (Eenturi? Boofts of mseful Science Artificial Light ITS INFLXJENCE 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 ^r-Vri ^^ }' ^/ xVC Copyright, 1920, by Thb Centuky Co. 4^ Jjtc^n fjcaCo^^ K> DEDICATED TO THOSE WHO HAVE ENCOURAGED ORGANIZED SCIENTIFIC RESEARCH FOR THE ADVANCEMENT OF CIVILIZATION PEEFACE 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 he considered to have been bom 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 be- came more powerful and efficient. Gradually mere light grew to more light and in the dawn of the twen- tieth century adequate light became available. In a single century, after the development of artificial light began in earnest, the efficiency of light-production in- creased fifty-fold and the cost diminished correspond- ingly. 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 man- kind. 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 be- come intricately interwoven with human activities and X PREFACE 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 cooperation 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 AUegemane 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 E. 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 CKAKnSK PAGE I Light and Progress 3 II The Art of Making Fire 15 III Primitive Light-Sources 24 IV The Ceremonial Use op Light 38 V Oil-Lamps op the Nineteenth Century . . 51 VI Early Gas-Lighting 63 VII The Science op Light-Production .... 80 VIII Modern Gas-Lighting 97 IX The Electric Arcs Ill X The Electric Incandescent Filament Lamps 127 XI The Light op the Future 143 XII Lighting the Streets 152 XIII Lighthouses 163 XIV Artificial Light in Warfare 178 XV Signaling 194 XVI The Cost of Light 208 XVII Light and Safety 225 XVIII The Cost of Living 238 XIX Artificial Light and Chemistry .... 256 XX Light and Health 269 ' XXI Modifying Artificial Light 284 XXII Spectacular Lighting 298 XXIII The Expressiveness of Light 310 XXIV Lighting the Home 325 XXV Lighting— A Fine Art? 341 Reading References 357 Index 359 LIST OF ILLUSTRATIONS Light and Liberty Frontispiece FACING PAGE Primitive fire-baskets 16 Crude splinter-holders 16 Early open-flame oil and grease lamps 17 A typical metal multiple-wick open-flame oil-lamp . . 32 A group of oil-lamps of two centuries ago 33 Lamps of a century or two ago 56 Elaborate fixtures of the age of candles 57 Flame arc 128 Direct current arc 128 On the testing-racks of the manufacturer of incandescent filament lamps 129 Carbon-dioxide tube for accurate color-matching . . . 160 The Moore nitrogen tube 160 Modern street lighting 161 A completed lighthouse lens 176 Torro Point Lighthouse, Panama Canal 176 American search-light position on Western Front in 1919 177 American standard field search-light and power unit . . 177 Signal-light for airplane 232 Trench light-signaling outfit 232 Aviation field light-signal projector 232 Signal search-light for airplane 232 Unsafe, unproductive lighting worthy of the dark ages . 233 The same factory made safe, cheerful, and more produc- tive by modem lighting 233 Locomotive electric headlight 240 Search-light on a fire-boat 240 xiii xiv ILLUSTRATIONS FACINO PAQE Building ships under artificial light at Hog Island Ship- yard 241 Artificial light in photography 256 Sterilizing water with radiant energy from quartz mer- cury-arcs 257 Judging color under artificial daylight 272 Artificial daylight 273 Fireworks and illuminated battle-fleet at Hudson-Pulton Celebration 288 Fireworks exhibition on May Day at Panama-Pacific Ex- position 289 The new flood lighting contrasted with the old outline lighting 304 Niagara Falls flooded with light 305 Artificial light honoring those who fell and those who returned 320 The expressiveness of light in churches 321 Obtaining two different moods in a room by a portable lamp which supplies direct and indirect components of light 336 The lights of New York City 337 Artificial light in community affairs 352 Panama-Pacific Exposition 353 ARTIFICIAL LIGHT ARTIFICIAL LIGHT LIGHT AND PROGRESS The human race was bom in slavery, totally sub- servient 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 comBl||;ely curtailed by darkness but beyond its screen lurfed many dangers. It is interesting to philosophize upon a distinction be- tween a human being and the animal just below him in the scale, but it may serve the present purpose to dis- tinguish the human being as that animal in whom there is an unquenchable and insatiable desire for independ- ence. 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 Siatisfied, whereas in human beings it has been leading ever onward toward complete emancipation. The progress of civilization is a long chain of count- less achievements each one of which has increased man's independence. Early man perhaps did not con- ceive the idea of fire and then set out to produce it. 3 4 ARTIFICIAL LIGHT His infant mind did not operate dn 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 be- come as twilight 's curtain was drawn across the heav- ens ! This controllable fire emitted light. It is easy to imagine primitive man pondering over this phenome- non 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 civil- ization 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, not- withstanding 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 over- whelming importance. Those unfortunate persons who LIGHT AND PEOGRESS 5 suffer the terrible calamity of bliadness after years of dependence upon sight will testify m heartrending terms to the importance of light. Milton, whose eye- sight had failed, laments, 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 re- veals 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 hospi- tality 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 6 AETIFICIAL LIGHT throughout the ages a moralizing and civilizing influ- ence. 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 wonder- fully 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 modem 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 sci- entist, equipped with a knowledge of physical and chemical laws, is led by his imagination into the dark- ness of the unexplored unknown. This knowledge illuminates the pathway so that hypotheses are intelli- gently formed. These evolve into theories which are gradually altered to fit the accumulating facts, for along the battle area of progress there are innumerable scout- ing-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 occa- sional reverses, but the whole line in general pro- LIGHT AND PROGRESS 7 gresses. 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 philoso- phized without a basis of established facts: scientific progress resulted more from an accumulation of acci- dental discoveries than by a directed attack of philoso- phy 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 investigat- ing, the organized knowledge grew apace. All of this paved the way for the momentous successes of the pres- ent time. The end is not in sight and pefhaps 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. 8 ARTIFICIAL LIGHT Without artificial light, mankind would be compara- tively inactive about one half its lifetime. To-day it has been fairly well established that the human organ- ism 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 re- maining hours should be his own. Artificial light has made such a distribution of time possible. The work- ing-periods in many cases may be arranged in the in- terests of economy, which often means continuous operations. The sun need not be considered when these operations are confined to interiors or localized out- doors. 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 mak- ing. Progress does not halt at sunset but continues twenty-four hours each day. Building, printing, manu- facturing, commerce, and other activities are prose- cuted continuously, the working-shifts changing at cer- tain periods regardless of the rising or setting sun. Adequate artificial lighting decreases spoilage, in- creases production, and is a powerful factor in the pre- vention 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 de- veloped artificial lighting of the present time may ac- count for much of the vast industry of publication. Books, magazines, and newspapers owe much to con- LIGHT AND PEOGEESS 9 venient and inexpensive artificial light, for -without it fewer hours would be available for recreation and ad- vancement through reading. Schools, libraries, and art museums may be attended at night for the better- ment 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 per- sistence of Lincoln, so that illiteracy was more common in his day than in the present age of adequate illumina- tion. The theatrical stage not only depends for its effect- iveness 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 increas- ing 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 impressive- ness of light and darkness. By combining the symbolism of light, color, and darkness with the instincts which have been inherited 10 ARTIFICIAL LIGHT 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 wor- shiped and thus artificial lights became woven in many religious ceremonies. Some of these have persisted to the present time. The great pageants of peace cele- brations 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 chan- nels. 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 LIGHT AND PROGEESS 11 candle-power. Many are unattended for days and even months. These powerful lights dominated by auto- matic 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 porta- bility. 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. Kaiding-parties are no longer con- cealed 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 para- chutes saved and cost many lives during the recent war. Eockets 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. 12 ARTIFICIAL LIGHT 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 be- cause 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 man- kind 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 nat- ural light. Light-sources may be made of a vast vari- ety of shapes, and these 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 LIGHT AND PEOGRESS 13 gas or electricity. And there is window-breakage to be considered. In the more elaborate buildings of the congested por- tions 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 con- siderable. 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 ap- preciable. The expense of maintenance of some sky- lights is considerable. Thus it is seen that i;he cost and maintenance of daylighting-equipment, the loss of valuable rental space and of waU area, and the in- creased 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 possi- bilities 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 arti- ficial light during the present age, a new profession has 14 ARTIFICIAL LIGHT 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 numer- ous 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 broad- est 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 efificiency, 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 primi- tive savages to the most highly cultivated peoples. Al- though it is possible that there are tribes of lowly be- ings 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 distin- guishing 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 be- comes 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 de- pended 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 impene- trable fog of prehistoric times, it has branched here and there. These various branches have been sub- jected to many different influences, with the result that 15 16 AETIFICIAL LIGHT some flourished and endured, some retrogressed, some died, some went to seed and fell to take root and to be- gin again the upward chmb. 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. Through- out these various stages it becomes manifest that the development of artificial light is associated with the progress of mankind. 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 divin- ity. They have been subjects of worship perhaps more generally than anything else, and these early impres- sions 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, inas- much as definite knowledge is unavailable. Active volcanoes, spontaneous combustion, friction, acciden- tal 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 sur- vey 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. PRIMITIVE PIBE-BASKETS CBUDE SPLINTER-HOLDERS THE AET OF MAKING FIRE 17 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 funda- mental principle of physics, employed by them as neces- sity 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 pon- dered over such simple facts and coordinated them so that he could extend his knowledge by general reason- ing, his progress could not be rapid. But the sluggish mind of primitive man is capable of devising improve- ments, however slowly, and the art of making fire by means of rubbing fire-sticks gradually became more re- fined. Mechanical improvements resulted from expe- rience, 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 pro- duction of fire in this manner is the proud accomplish- ment or ambition of every Boy Scout. Where only such crude means of kindling fire were available it became the custom in some cases to main- tain a fire burning continuously in a public place. 18 ARTIFICIAL LIGHT 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 modem centuries doubtless orig- inated in very early times, for it is inconceivable that the earliest beings did not become aware of the pro- duction of sparks when certain stones were struck to- gether. 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 similiar 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 kno^vn to have employed two pieces of quartz cov- ered with native sulphur. When these were struck to- gether with skill, excellent sparks were obtained. Later, when iron and steel became available, the more modern tinder-box was developed. An early applica- tion 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 primi- tive beings used dried grass, leaves, and moss as in- flammable material upon which the sparks were show- ered. 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 THE ART OF MAKING FIEE 19 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 util- ized 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 indi- viduals. 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 at- tributed 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 dear days, because the water-vapor in the atmosphere 20 AETIFICIAL LIGHT 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 kin- dled by concentrating only the visible rays in sunlight because of the enormous intensity of sunlight, A con- vex 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 sol- diers 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 re- search. Alchemy and superstition began to retire be- fore 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 pow- erful sciences of physics and chemistry were bom a few centuries ago. Among other things the produc- tion of fire and light received attention and the "dark ages" were doomed to end. The crude, uncertain, and THE ART OF MAKING FIRE 21 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 pri- marily intended for lighting fires and various kinds of lamps, billions of them are now used yearly as con- venient light-sources. Smoldering hemp or other ma- terial 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 role 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 sul- phur. This type of match did not come into general use until after the beginning of the nineteenth cen- tury, owing to its danger and expense. White or yel- low phosphorus is a deadly poison ; therefore the prog- ress 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 lat- ter being fired by flint and steel. In 1828 another form of match consisted of a glass tube containing sulphuric 22 ARTIFICIAL LIGHT 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 em- ployed in preparing the splints of wood and in dipping them into molten paraffin wax and finally into the ig- niting composition. During recent years the principle of the tinder-box has been revived in a device in which sparks are pro- duced 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 in- flammable liquid such as gasolene or alcohol. This device is a tinder-box of the modem 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 THE ART OF MAKING FIRE 23 been used primarily for making fire, they have served as emergency or momentary light-sources. In the out- skirts of civilization some of them are employed at the present time and various modem Kght-sources re- quire a method of ignition. Ill PEIMITIVE LIGHT-SOUECES Many are familiar with the light of the firefly or of its larvae, the glow-worm, but few persons realize that a vast number of insects and lower organisms are en- dowed with the superhuman ability of producing light by physiological processes. Apparently the chief func- tion 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, forlur- ing prey, and for protection than for strictly illuminat- ing-purposes. Much study has been given to the pro- duction 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 en- dowed 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 rel- ative proportion to his size as compared with the fire- fly, he would need no other torch in the coal-mine. How independent he would be in extreme darkness 24 PRIMITIVE LIGHT-SOURCES 25 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 su- periority by adapting the facilities at hand to the growing requirements which his independent nature continuously nourished. His insistent demand for in- dependence in turn has nourished his desire to learn nature's secrets and this desire has increased in in- tensity throughout the ages. The act of imprisoning a glowing insect was in it- self 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 primi- tive 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 26 AETIFICIAL LIGHT keeps on the lookout for live trees saturated with highly inflainmable ingredients. Viewed from the present age, these smoking, flicker- ing 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 com- fort and security of his oave or hut. By the firelight he devised implements and even decorated his stone surroundings with pictures which to-day reveal some- thing 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 splin- ters emitting a smoking, feeble light were crude but they were refinements of considerable merit. A testi- monial 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 coun- tries, and it is probable that at present in remote dis- tricts of highly civilized countries this crude device serves the meager needs of those whose requirements PRIMITIVE LIGHT-SOURCES 27 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 High- lander 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, ap- pear 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 facili- ties, 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 28 ARTIFICIAL LIGHT with crude chimneys to carry away the smoke, and per- haps 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 ap- pears to have existed until about 1840. Somewhat akin to the wood-splinter, and a forerun- ner 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 repro- duce this condition by artificial means. As a conse- quence rushes were cut and soaked in water. They were then peeled, leaving lengths of pith partially sup- ported 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 ac- quired 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 pro- gress in the development of artificial light. In devel- PRIMITIVE LIGHT-SOURCES 29 oping 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 de- veloped 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 burn- ing fish and birds have served as lamps. In the Ork- ney 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 ob- taining artificial light have been employed for cen- turies 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-smeUing ! 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 ar- 30 ARTIFICIAL LIGHT tificial light. Although they were not scientific devel- opments in the modern sense, the early oil-lamp and the candle represented the great possibilities of utiliz- ing knowledge rather than depending upon the raw products of nature in unmodified forms. The advent of these two light-sources in reality marked the begin- ning of the civihzation 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-la-mps are to be seen in the earliest Roman illustrations. During the height of an- cient civilization along the eastern shores of the Med- iterranean Sea, elaborate lighting was effected by means of the shallow grease- or oil-lamp. It is diffi- cult to estimate the age in which this form of light- source originated, but some lamps in existence in col- lections 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 ani- mals. Vegetable oils also were burned in the early lamps. The Japanese, for example, extracted oil from nuts. When the demands of civilization increased, ex- tensive 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 PRIMITIVE LIGHT-SOUECES 31 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 ob- tained from the head cavities of the sperm-whale ; the latter is a vegetable oil obtained from rape-seed. Min- eral oil was introduced as an illuminant in 1853, and the modem 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. Very beautiful oil-lamps of brass, bronze, and pew- ter 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 weTe improved by the addition of a 32 ARTIFICIAL LIGHT 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 prod- ucts 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 Eome. 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 twisited threads. These early oil-lamps were al- most 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 gen- eral 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 nap- tha 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 modem science of light-production. As the activities of man became more complex he met from his growing store of knowledge the increas- ing requirements of lighting. In consequence, many ingenious devices for lighting were evolved. For ex- ample, 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 A TYPICAL METAL MULTIPLE-WICK OPEN-FLAME OIL-LAMP PRIMITIVE LIGHT-SOURCES 33 produced rather than for their useful qualities. Al- though they were known to miners long before they received scientific attention, the earliest account of them iu the Transaotions 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 inflam- mable 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 af- fect it." One of the early solutions of the problem of artificial lighting under such conditions is summar- ized 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 suffi- cient Mght to pursue their operations; and one of the most useful of these inventions was a mill for pro- ducing 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 stand- ards, 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 intro- duction 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 descend- ant of the rushlight, the grease-lamp, and various prim- itive devices. Until the advent of the more scientific 34 AETIFICIAL LIGHT age of artificial ligMing, the candle stood preeminent 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 Phoenicians. It is known that the Eomans used them, especially the wax-candles, in religious ceremonies. The Phoenicians 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. Nev- ertheless 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 condi- tions. If a person "burns a candle at both ends" he is wasteful and oblivious to the consequences of ex- travagance whether in material goods or in human energy. With the rise of the Christian church, candlesi came to be used in religious ceremonies and many of the PRIMITIVE LIGHT-SOUECES 35 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 an- tiques, which ofttimes were gifts to the church, have been preserved to posterity by the church. The influ- ence of these lighting accessories is often noted in mod- em lighting-fixtures, but unfortunately early art often suffers from adaptation to the requirements of modem light-sources, or the eyesight suffers from a senseless devotion to art which results in the use of modem 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 repeat- edly, 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. Sper- maceti, 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 ap- plication of scientific knowledge. The wicks are now made chiefly of cotton yarn, braided or plaited by ma- 36 ARTIFICIAL LIGHT chinery and chemically treated to aid in complete com- bustion when the candle is bnmed. 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-can- dles are made by pouring, because wax cannot be molded satisfactorily. Drawing is somewhat similar to dipping, except that the process is more or less con- tinuous 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 keep- ing the candle upright. Later a form of cup was at- tached to the stem of the holder, to catch the dripping wax or fat. The latter improvement has persisted throughout the centuries. The modem candle is by no means an unsatisfactory light-source. Those who have had experience with it in the outskirts of civilization wUl testify that it possesses several desirable char- acteristics. Supplies of candles are transported with- out difficulty ; the lighted candle is easily carried about ; and the light in a quiescent atmosphere is quite satis- factory, if common sense is used in shading and plac- ing 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 PRIMITIVE LIGHT-SOUECES 37 ceremonies, as a decorative element at banquets, and ia the outposts of civilization. This account of the evolution of light-sources has crossed the threshold of what may be termed modem scientific light-productioai 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 develop- ments 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 be- fore 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 sur- vivors 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 con- sideration 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 cen- turies of traditional use. Doubtless, the sun as a source of heat and light and of the blessings which these bring to earth, is responsi- ble largely for the divine significance bestowed upon light. Darkness very deservingly acquired many un- complimentary attributes, for danger lurked behind its veil and it was the suitable abode of evil spirits. It 38 THE CEREMONIAL USE OF LIGHT 39 harbored all that was the antithesis of goodness, hap- piness, 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 Ka, was one of the chief gods of the ancient Egyptians ; and the Assyrians, the Baby- lonians, the ancient Greeks, and many other early peo- ples 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 un- lighted 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 mean- ing 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 orig- inated in Asia. The Phoenicians and Romans appar- ently continued the custom, but no traces of it have been found in Greece and Egypt. 40 AETIFICIAL LIGHT Fire and light have been closely associated in va- rious 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 tem- ples and homes. The Jewish synagogues have their eternal lamps and in their rituals fire and light have played prominent roles. The devout Brahman main- tains a fire on the hearth and worships it as omnis- cient 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 illum- inated 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 pres- ent 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 noth- ing 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 mo- notonous to survey the vast field of ancient worship of light, for the underlying ideas are generally simi- THE CEREMONIAL USE OF LIGHT 41 lar. The mythology of the Greeks is illustrative of the importance attached to fire and light by the culti- vated peoples of ancient times. The myth of Prome- theus emphasizes the fact that in those remote pe- riods fire and light were regarded as of prime impor- tance. According to this myth, fire and light were con- tained 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 moun- tain 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 acqui- sition of artificial light. Its divine nature was recog- nized 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 al- tar 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 cere- monies. 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 lit- erature to sacred lamps burning day and night in sane- 42 AETIFICIAL LIGHT tuaries and before statues of gods and heroes. On birthdays and festivals the houses of the Eomans were specially ornamented with burning lamps. The Vestal Virgins in Eome maintained the sacred fire which had been brought by fugitives from Troy. In ancient Eome when the fire in the Temple of Vesta became extin- guished, 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 Eoman 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 rep- resentations 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 THE CEREMONIAL USE OF LIGHT 43 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 al- lowed it to be extinguished. If human hands were per- mitted 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 accom- paniments of worship. The countless ceremonial uses of fire and light which had evolved in the past centuries were bound to influ- ence 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 typi- cal example of fire-worship retained from ancient times. At the climax of the services comes the descent of the 44 AETIFICIAL LIGHT 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 ori- gin 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 Chris- tian era the ceremonial use of light does not appear to have been very extensive. Writings of the period con- tain 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 Eome 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 THE CEEEMONIAL USE OF LIGHT 45 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 writ- ings 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 Vir- gin was inaugurated and it became known as Candle- mas 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 cen- tury. They were originally the seven candles carried by church ofl&cials 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 di- vine relation. Finally certain light-sources themselves became objects of worship and were surrounded by other lamps, and the symbolisms of light grew apace. 46 ARTIFICIAL LIGHT A bishop in the sixth century heralded the triple offer- ing 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 re- ligious 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 repre- sented 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 em- blems of zeal and fervor or of the sufferings of martyr- dom and the flaming heart symbolizes fervent piety and spiritual or divine love. By the time the Middle Ages were reached the cere- monial uses of light became very complex, but for the Eoman 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 be- ing blessed by the church, and therefore may be help- ful to soul and body. The three conceptions are indi- cated in the prayers offered at the blessing of the can- dles on Candlemas as follows : (1) "0 holy Lord . . . who ... by thy command didst cause this liquid to come by the labor of bees to the perfection of wax, . . . THE CEREMONIAL USE OF LIGHT 47 we beseech thee ... to bless and sanctify these can- dles 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) "0 Lord Jesus Christ, the true light, . . . mercifully grant, that as these lights en- kindled 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 can- dles 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 sym- bolize 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 cere- monial 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 bap- tismal services the paschal candle is dipped into the 48 ARTIFICIAL LIGHT water so that the latter will be effective as a regenera- tive 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 oflS- cial 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 ta- pers, but the carrying of lights in other processions ap- pears 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 excommuni- cant is reinstated, a lighted candle is placed in his hands as a symbol of reconciliation. These and many THE CEREMONIAL USE OF LIGHT 49 other ceremonial uses of light have been and are prac- tised, but they are not always mandatory. Further- more, 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 contro- versies 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 Res- toration and they became wide-spread in England in the nineteenth century. As late as 1889 the Arch- bishop 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 fas- cinating because their study leads into many channels and far into the obscurity of the childhood of the hu- man race. A glimpse of them is important in a sur- vey of the influence of artificial light upon the progress of civilization because in these usages the innate and acquired impressiveness of light is encountered. Al- 50 AETIFICIAL LIGHT though many ceremonial uses of light remain, it is doubtful if their significance and especially their ori- gin are appreciated by most persons. Nevertheless, no more interesting phase of artificial light is encount- ered than this, which reaches to the foundation of civilization. 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 scien- tific 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 con- veniently solid so that it is readily shipped and stored ; there is nothing to spill or to break beyond easy re- pair ; but when it is lighted the heat of its flame melts the solid fuel and thus it becomes an ' ' oil-lamp. ' ' Ani- mal and vegetable oils were mainly used until the mid- dle of the nineteenth century, when petroleum was pro- si 52 ARTIFICIAL LIGHT duced 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 off- spring of early developments by Ami Argand. Before man discovered that nature had stored a tre- mendous supply of mineral oil in the earth he was obliged to hunt broadcast for fats and waxes to sup- ply 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, com- binations of oleic, palmetic, and stearic acids. Inas- much as the former is liquid at ordinary temperatures and the others are solid, it follows that the consistency or solidity of fats depends upon the relative propor- tions 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 light- ing. In its head cavities sperm-oil in liquid form is found with the white waxy substance known as sper- maceti. Colza-oil is yielded by rape-seed and olive-oil is extracted from ripe olives. The waxes are combina.- tions 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 care- fully would lead far afield into the complexities of or- ganic chemistry. All these animal and vegetable prod- OIL-LAMPS 53 ucts 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 veg- etable oil and from these reservoirs short wicks pro- jected. 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 possi- bility of the present civilization emerging from that period without being accompanied by progress in light- production. The improvements made in the eight- eenth 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 tre- mendous impetus which light-production has expe- rienced in the past half-century. These are the ac- quirement of scientific knowledge from systematic re- search and the application of this knowledge by or- ganized 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 inde- pendence at the time when the production of light as a science was born. Argand produced the tubular wick 54 AETIFICIAL LIGHT 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 per- forated 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 Ms 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 con- siderable luminous intensity was for the first time avail- able. 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 un- necessary, 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 amonnt of light emitted. The Carcel lamp, which appeared in 1800, consisted OIL-LAMPS 55 of a double piston operated by clockwork. TMs forced the oil through a tube to the burner. Franchot in- vented the moderator lamp in 1836, which, because of its simplicity and eflficiency 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 ex- tent in France, but owing to the fact that "mechani- cal" lamps eventually were very generally replaced by more simple ones, it does not appear necessary to de- scribe these complex mechanisms in detail. When coal is distilled at moderate temperatures, volatile liquids are obtained. These hydrocarbons, be- ing 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-prod- uct of the coal-gas industry which was then in its early stage of development, were burned to some extent in the HoUiday 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 56 AETIFICIAL LIGHT mentioned by Herodotus and other early writers. In 1847 petroleum was discovered in a coal-mine in Eng- land, 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 comer-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 prog- ress was made until, in 1859, Col. E. L. Drake dis- covered oil in Pennsylvania. By studying the geologi- cal formations and concluding that oil should be ob- tained by boring, Drake gave to the world a means of obtaining petroleum, and in quantities which were des- tined to reduce the price of mineral oil to a level un- dreamed 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. The crude petroleum, of course, is not fit for illum- inating 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. _, V* ^-,> i^ rn'i ■^.--^i^ ^^ B ELABORATE FIXTURES OF THE AGE OF CANDLES OIL-LAMPS 57 stability, etc. It must not be so volatile as to have a dangerously low flashing-point, nor so stable as to hin- der 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 lubricat- ing-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. 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 parafiin hydrocarbons having the general formula of ^"-^2114- 2 ^^^ t^6 individual members of this series vary from CH4 (methane) to CigHgj (pentadecane), although the solid hydrocarbons are still more copi- plex. 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 un- justly in many accidents. A disadvantage of the kero- sene lamp over electric lighting, for example, is the relatively greater possibility of accidents through the 58 ARTIFICIAL LIGHT 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 modem lamp of proper construction a close-fit- ting wick is used and the lamp is extinguished by turn- ing down and blowing across the chimney, there is lit- tle 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 danger- ous because they form very explosive mixtures with air at ordinary temperatures. A mineral oil like kero- sene, 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 res- ervoir which is situated below. It is volatilized by the heat of the flame into a mixture of hydrogen and hydro- carbon 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 modem oil-lamp. That an open flame devours something from the air is easily demon- strated 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 OIL-LAMPS 59 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 breath- ing 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 insuflScient 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 capil- larity, 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 che 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 in- complete combustion are carbon monoxide and oil va- pors, which are a menace to health. To supply the necessary amount of oxygen (air) to 60 AETIFICIAL LIGHT 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 chim- ney 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 vapor- ization 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 grav- ity 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 em- ploying 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 gal- lon of kerosene per hour would yield about 8000 lumens. OIL-LAMPS 61 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 direc- tion 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 con- ducted 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 at- tempted 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 appear- ance of the Welsbach mantle, Kitson 's lamp and others met with success by utilizing it. From this point, at- tention was centered upon the new wonder, which is discussed in a later chapter after certain scientific principles in light-production have been discussed. 62 ARTIFICIAL LIGHT The kerosene or mineral-oil lamp was a boon to light- ing in the nineteenth century and even to-day it is a blessing in majiy 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 ac- cident is ever-present. The consequences of such acci- dents are often serious and are exemplified in the ter- rible 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 light- ing are gradually encroaching upon the territory in which the oil-lamp has reigned supreme for many years. Acetylene plants were introduced to a consider- able 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 Mur- dock 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 Eoyal 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 observations 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 installa- tion 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 63 64 AKTIFICIAL LIGHT 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 car- riage 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 EAELY GAS-LiaHTING 65 gas and hit upon the use of bladders. He was sur- prised 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 blad- ders 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 Eoyal 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 per- taining 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 bum 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 66 ARTIFICIAL LIGHT 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 Eoyal 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. HawTcley's Ground (if I mistake not) about a Mile from the Town, in that Road which leads to Warring- ton and Chester: The People of this Town did confi- dently 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 flammg 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 EAELY GAS-LIGHTING 67 Fire, tho' by putting my Hand into it, I could not per- ceive it so much as warm. This Boiling I conceived to proceed from the Erup- tion of some bituminous or sulphureous Fumes; con- sidering this Place was not above 30 or 40 Yards dis- tant 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 sul- phurous 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 Remem- brance, 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 publi- cation 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 68 AETIFICIAL LIGHT curved tubes, and he particularly noted "its great in- flammability as well as elasticity." He also observed that "it retained the former property after it had passed through a great quantity of water. ' ' His pub- lished account dealt with a variety of facts and compu- tations pertaining to the quantities of coke, tar, etc., produced from different kinds of coal and was a scien- tific 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 exhibi- tion, and to return for more. The inventor passes by and sees applications for some of these scientific trophies which are productive of momentous conse- quences to mankind. Sir Humphrey Davy described his primitive arc-lamp three quarters of a century be- fore Brush developed an arc-lamp for practical pur- poses. 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 illumi- nating property of gas, but practicable gas-lighting was destined soon to be an epochal event in the prog- ress of lighting. The dawn of modern science was coincident with the dawn of a luminous era. EAELY GAS-LIGHTING 69 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 enlight- ened proprietors, Messrs. Boulton and Watt ; and who- ever resided there was surrounded by a circle of scien- tific, 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 dis- play of gas-lighting. The illumination of the Soho works on this occasion is described as ' ' one of extraor- dinary 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 over- come 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 perpendicu- larly in a common furnace, and in each were put about fifteen pounds of coal. In 1804 he constructed them 70 AETIFICIAL LIGHT 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 car- bonization was complete the coke could be removed as a whole by extracting this cage. This retort had a ca- pacity of fifteen hundred pounds of coal. He labored with mechanical details, varied the size and shape of the retorts, and experimented with different tempera- tures, with the result that he laid a solid foundation for coal-gas lighting. For his achievements he is en- titled to an honorable place among the torch-bearers of civilization. The epochal feature of the development of gas-light- ing 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 con- venience, 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 re- spects and the latter was a three-flame burner suggest- ing a fleur-de-lis. In this installation there were 271 Argand burners and 636 cockspurs. Each of the for- mer "gave a light equal to that of four candles; and EARLY GAS-LIGHTING 71 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 naold 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 speci- fications) 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 allow- ing generously for interest on capital invested and de- preciation of the apparatus. The cost of furnishing the same amount of light by means of candles he com- puted to be 2000 pounds sterling. This comparison was on the basis of an average of two hours of artifi- cial 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 achieve- ments 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 snuflSng of candles, is a circum- stance of material importance, as tending to diminish the hazard of fire, to which cotton mills are known to be exposed. 72 ARTIFICIAL LIGHT 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 gradu- ally 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 ex- periments 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 advertise- ments 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. Neverthe- less, he was clever and in 1803 he publicly exhibited his EARLY GAS-LIGHTING 73 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 cer- tain exclusive rights by Act of Parliament failed be- cause of opposition of scientific men toward his claims and of the stand which Murdock justly made in self- protection. These years of controversy yield enter- taining 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 Mur- dock appeared modest and conservative and had the support of reputable scientific men, but Winsor main- tained 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 pro- duction 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." 74 ARTIFICIAL 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 pre- sented in an early treatise on gas-lighting (hy Accum in 1818) were as follows: The power and authorities granted to this corporate body are very restricted and moderate. The individu- als composing it have no exclusive privilege; their charter does not prevent other persons from entering into competition with them. Their operations are con- fined 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 manufactur- ing or conveying the gas into private houses, their capi- tal 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 in- EARLY GAS-LIGHTING 75 troduetd. 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 ex- plosion took place and the members of the Royal Soci- ety set a precedent which has lived and thrived: they appointed a committee to make an inquiry. But ap- parently 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 gase- ous 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 chemica,l analyses showed to a consider- able extent the properties of carbureted hydrogen upon which illuminating value depended. The results of his work were published in various English journals be- tween 1805 and 1825 and they contributed much to the advancement of gas-lighting. 76 AETIFICIAL LIGHT Although Clegg's origmal gas-meter was compli- cated and cumbersome, it proved to be a useful device, lu fact, it appears to have been the most original and beneficial invention occasioned by early gas-lighting. Later Sanluel 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 iUunainate 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 popu- lar 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. EAELY GAS-LIGHTING 77 It was not until the latter half of the nineteenth cen- tury that it was generally used in residences. The gas-burner first employed by Murdock received the name "oockspur" 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 in- fluenced 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, ex- tending 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 cool- ing 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 78 ARTIFICIAL LIGHT ajid, 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 chim- ney 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-con- ducting 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 caudle-power based upon equal rates of consump- tion, namely, one cubic foot of gas per hour : EAELY GAS-LIGHTING 79 Candle-power per cubic foot of gas per hour Fish-tail flames, depending upon size .... 0.6 to 2.5 Argand, depending upon improvements . . 2.9 to 3.5 Eegenerative 7 to 10 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 in- tended merely to serve as examples of the diflSculties 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 im- portance in lighting that another chapter is given its modem developments. VII THE SCIENCE OF LIGHT-PRODUCTION In previous chapters much of the historical develop- ment of artificial lighting has been presented and sev- eral subjects have been traced to the modern period which marks the beginning of an intensive attack by scientists upon the problems pertaining to the produc- tion of efficient and adequate light-sources. Many his- torical events remain to be touched upon in later chap- ters, but it is necessary at this point for the reader to become acquainted with certain general physical prin- ciples 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" ex- tended 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 at- tention 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 prog- ress 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 character- istics of carbon, the chief one being its extremely high 80 THE SCIENCE OF LIGHT-PEODUCTION 81 melting-point. However, niost 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 ex- tent 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 sub- division. 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 incan- descence are responsible for the brightness of a flame. A jet of pure hydrogen gas will bum 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 mix- ing air in proper proportions with the gas, gives a hot 82 AETIFICIAL LIGHT 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 parti- cles, and combustion is not complete. However, it is brighter because the solid particles of carbon in pass- ing upward through the flame become heated to tem- peratures at which they glow and each becomes a minia- ture source of light. A 6lose 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 disinte- grated. 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. Ow- ing to the incoming colder air at this point, the flame is not hot enough for complete combustion. The un- burned 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 tem- perature in its central portion from 300° F. at the bot- THE SCIENCE OF LIGHT-PRODUCTION 83 torn 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 incan- descent solid matter, the way was open for many ex- periments. In the early days of gas-lighting investi- gations were made to determine the relation of illumi- nating 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 heat- ing. 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 produciag 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 84 AETIFICIAL LIGHT 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 em- ployed, 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 de- creases rapidly as the burner is used, if a new surface of Hme 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 magne- sium 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 ob- tained intact. Placing this in the faintly luminous flame, he will be surprised at the brilliance of its in- candescence when it has become heated. The simple THE SCIENCE OF LIGHT-PRODUCTION 85 experiment indicates the possibilities of light-produc- tion in this direction. Naturally, metals of high melt- ing-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 experimenta- tion 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 interest- ing chemical properties some of which have been util- ized to advantage in the development of modern arti- ficial 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 prop- erties 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 radi- ator; 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. 86 ARTIFICIAL 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 ex- cepting 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. Electro- magnetic 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 visi- bility 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 THE SCIENCE OF LIGHT-PRODUCTION 87 near the ends of the visible spectrum will be inefficient in producing light. That energy which excites the sen- sation 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, yeUow, green, orange, blue-green, red, blue, deep red, violet. Newton, an English scientist, first described the dis- covery of the visible spectrum and this is of such funda- mental 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 re- fracted 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, ac- cording 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 deter- mine justly what was the Figure, yet they seemed Semi- circular. Even Newton could not have had the faintest idea of 88 ARTIFICIAL LIGHT 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 &,re "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-produc- tion 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 charac- teristic of sodium and if it is examined by means of a spectroscope, a brilliant yellow line (in fact, a double THE SCIENCE OF LIGHT-PKODUCTION 89 line) will be seen. This forms the basis of spectrum analysis as applied in chemistry. Every element has its characteristic spectrum con- sisting 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 pro- duces 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 possi- bilities 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 develop- ments. 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 arisen if electric illuminants had not entered the field. 90 AETIFICIAL LIGHT 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 pro- duced. 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-produc- tion 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 tem- perature of the sun, about 10,000°F., it would appear white. All this time its luminous efficiency is increas- ing, because it is radiating not only an increasing per- centage 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 THE SCIENCE OF LIGHT-PRODUCTION 91 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 inefl&- cient as a light-producer. In this branch of the science of light-production sub- stances 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 lumi- nous 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 de- partures 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, there- fore, are efficient as radiators of luminous radiant energy. But much remains to be unearthed concerning them before they will be generally applicable to light- ing. 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 uni- form candles are still satisfactory standards where high accuracy is not required. Their luminous in- 92 AETIFICIAL LIGHT tensity 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 lumi- nous intensity of the candle was taken only in the hori- zontal 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 hori- zontal 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 4ir or 12.57 square feet ; therefore, a light-source having a luminous intensity of one candle in all direc- tions 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 THE SCIENCE OF LIGHT-PEODUCTION 93 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 in- terested chiefly in the efficiency of the light-source which may he expressed in lumens per watt, or the amount of light obtained from a given rate of consump- tion 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 effi- cient in producing luminous sensation. Of course, no illuminants are available which approach this theoreti- cal ideal and it is not likely that this would be a prac- tical 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 monochro- matic yellow-green light. The red under present con- ditions is kept alive by contrast with other colors, be- cause the latter live by virtue of the fact that most of our present illuminants contain their hues. It is as- sumed that the reader knows that a red object, for ex- ample, appears red because it reflects (or transmits) 94 AKTIFICIAL LIGHT red rays and absorbs the other rays in the illuminant. In other words, color is due to selective absorption, re- flection, or transmission. Perhaps the ideal illuminant, which is most generally satisfactory for general activities, is a white light cor- responding 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 pro- duce 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 imprac- ticable at present — the luminous efficiency would be about 100 lumens per watt. The normal radiator which emits energy by virtue of its temperature with- out 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 ra- diator. 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 SCIENCE OF LIGHT-PRODUCTION 95 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 dan- gerous 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 lumi- nous efficiencies of several light-sources is given in round numbers. These efficiencies are in terms of the most efficient (yellow-green) light. Efficiency in per cent. Sperm-candle 0.02 Open gas-flame 04 Incandescent gas-mantle .19 Carbon filament lamp .05 Vacuum Mazda lamp 1.3 96 AETIFICIAL LIGHT BflSciency in per cent. Gas-filled Mazda lamp 2 to 3 Arc-lamps 2 to 7 White light radiated by "black-body" 16 Most efficient white light 40 Firefly 95 Most efficient light (yellow-green) 100 The luminous eflSciency of a light-source is distin- guished 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 be- sides the light-source are taken into account. These additional losses are appreciable in the mechanisms of arc-lamps but are almost negligible in vacuum incan- descent 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 MODEKN GAS-LIGHTINa 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 nine- teenth 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 con- siderably. 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? 97 98 AETIFICIAL LIGHT Although gas-lighting was bom 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 conse- quences 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 civ- ilization by increased and adequate lighting but they reduced the cost of lighting over the span of the nine- teenth 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! Wash- ington's birthday anniversary was celebrated in 1817 with a grand ball attended by five hundred of the elite. 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 MODERN GAS-LIGHTING 99 Philadelphia with nearly two million inhabitants to exist tinder 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 bum 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 illu- minating 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 elec- tric lamps had already been invented. These were des- tined to become the formidable light-sources of the approaching century and without the gas-mantle gas- lighting would not have prospered. Auer von Wels- bach was conducting a spectroscopic study of the rare- earths when he was confronted with the problem of heating these substances. He immersed cotton in solu- tions 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 100 AETIFICIAL LIGHT illuminating purposes and in 1885 he placed such a mantle in commercial use. His first mantles were un- satisfactory, 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 im- proved 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 man- tle over the burner. After the mantle has dried in MODERN GAS-LIGHTING 101 proper form, it is burned; the organic matter disap- pears 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 ship- ping and handling. The collodion is a solution of gun- cotton in alcohol and ether to which an oil such as cas- tor-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 sSrtificial silk as the base fabric, which re- sults in a mantle superior to previous mantles in strength, flexibility, permanence of form, and perma- nence of luminous property. This artificial silk man- tle 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 102 AETIFICIAL LIGHT into gas-lighting. The light could be directed down- ward 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 elec- tric lighting, notwithstanding the greater difficulties encountered by the designer of gas-lighting fixtures. Many problems were encountered in designing an in- verted 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 de- velopment has been reached. Several methods of electrical ignition have been evolved which in general employ the electric spark. Electrical ignition and developments of remote con- trol have added great improvements especially to street-lighting by means of gas. Gas-valves for re- mote control are actuated by gas pressure and by elec- tromagnets. 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 bum 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 combus- tion 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-pres- MODERN GAS-LIGHTING 103 sure 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 con- stituents 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 dis- tillation of gas from coal for illuminating purposes be- gan in the latter part of the eighteenth century. From this beginning the manufacture of coal-gas has been developed to a great and complex industiy. 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 104 ARTIFICIAL LIGHT emanate. These are generally paraflSns and olefins. As the temperature increases, these hydrocarbons be- gin to be affected. The chemical combinations which have long existed are broken up and there are rear- rangements 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 prod- ucts from a charge of coal. Many interesting chemi- cal 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 ma- terials, 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 re- moved. One method of accomplishing this is by wash- ing the gas with water and ammonia, which also re- moves some of the carbon dioxide and hydrocyanic acid. Various other undesirable constituents are re- moved by chemical means, depending upon the condi- tions. The purified gas is now delivered to the gas- holder ; but, of course, all this time the pressure is gov- erned, 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 decom- posing the tar obtained during the distillation of coal MODERN GAS-LIGHTING 105 and adding these gases to the coal-gas ; by mixing car- bureted water-gas with coal-gas; by carbureting in- ferior 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 wa- ter-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 de- termined 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 man- ufacture, 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, 10,000 cubic feet of gas 17 per cent. coke 70 " " tar 5 " " ammoniaeal liquid 8 " " 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 106 ARTIFICIAL LIGHT 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. Hydrocarbons 40 per cent. Hydrogen 50 Carbon monoxide 4 Nitrogen 4 Carbon dioxide 1 Various other gases 1 It is seen that illuminating gas is not a definite com- pound but a mixture of a number of gases. The pro- portion 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 con- stituents are seen to consist of light-giving hydrocar- bons, of gases which yield chiefly heat, and of impuri- ties. The chief hydrocarbons found in illuminating gas are. ethylene C.H, crotonylene C.H. propylene C3H, benzene CeH. butylene C,H, toluene C.H« amylene CbHio xylene CgHio acetylene C,H, methane CH, aJlylene C3H, ethane C,H« 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 vol- MODERN GAS-LIGHTING 107 ume, 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 ia making other illumi- nating 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 efl&ciency 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 cal- cium carbide are brought together is a double decom- position and is represented by, CaCa + H2O = C2H2 + CaO It will be seen that the products are acetylene gas and calcium oxide or lime. The lime, being hydroscopic 108 ARTIFICIAL LIGHT and being in the presence of water or water-vapor in the acetylene generator, reaUy becomes calcium hy- droxide Ca(0H)2, commonly called slaked lime. If there are impurities in the calcium carbide, it is some- times 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 va- rious 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 cheap- ness 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 ma- terials of a furnace process. The discovery of a com- mercial scale of manufacture of calcium carbide has MODERN GAS-LIGHTING 109 been a boon to isolated lighting. Electric lighting has usurped its place on the automobile and is making in- roads 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 pas- senger-cars in this country, is an oil-gas produced by the destructive distillation of petroleum or other min- eral oil in retorts heated externally. The product con- sists 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 illum- inating-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 authori- ties 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 re- gardless 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 re- ducer which would deliver the gas to the burner at about one pound per square inch. This regulator 110 AETIFICIAL LIGHT 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 illum- inants 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 to- tally 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 vol- taic pile. This letter was published in the Transac- tions 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 sufiicient, 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 con- structed was to place pairs of strips of copper and zinc in cups containing water or dilute acid. Volta re- ceived many honors for his discovery, which contrib- uted 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 111 112 ARTIFICIAL LIGHT 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 Hum- phrey Davy was the first to use charcoal for the spark- ing points. In a lecture before the Koyal 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 imme- diately 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 re- ports 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 Avas 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 bril- liant 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 THE ELECTEIC AECS 113 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 ac- count 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 dis- charge took place through the heated air, in a space a,t least equal to four inches, producing a most bril- liant ascending arch of light, broad and conical in form in the middle. This is suTely a description of the electric arc. Ap- parently the electrodes were ia 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 114 AETIFICIAL LIGHT 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 in- clined carbons are used. In these arcs a magnet keeps the aye in place, for without the magnet the arc would tend to climb up the carbons and go out. In 1838 Gassiot made the discovery that the tempera- ture of the positive electrode of an electric arc is much greater than that of the negative electrode. This is explained in electronic theory by the bombardment of the positive electrode by negative electrons or cor- puscles of electricity. This temperature-difference was later taken into account in designing direct-current arc-lamps, for inasmuch as most of the light from an ordinary arc is emitted by the end of the positive elec- trode, this was placed above the negative electrode. In this manner most of the light from the arc is di- rected doAvnward where desired. In the few instances in modern times where the ordinary direct-current arc has been used for indirect lighting, in which case the arc is above an inverted shade, the positive carbon is placed below the negative one. Gassiot first proved that the positive electrode is hotter than the negative one by striking an arc between the ends of two hori- zontal wires of the same substance and diameter. After the arc operated for some time, the positive wire THE ELECTRIC ARCS 115 was melted for such a distance that it bent downward, but the negative remained quite straight. Charcoal was used for the electrodes in all the early- experiments, but owing to the intense heat of the arc, it burned away rapidly. A progressive step was made in 1843 when electrodes were first made by Foucault from the carbon deposited in retorts in which coal was distilled in the production of coal-gas. However, charcoal, owing to its soft porous character, gives a longer arc and a larger flame. In 1877 the "cored" carbons were introduced. These consist of hard molded carbon rods in which there is a core of soft carbon. In these are combined the advantages of char- coal and hard carbon and the core in burning away more rapidly has a tendency to hold the arc in the center. Modem carbons for ordinary arc-lamps are generally made of a mixture of retort-carbon, soot, and coal-tar. This paste is forced through dies and the carbons are baked at a fairly high temperature. A variation in the hardness of the carbons may be ob- tained as the requirements demand by varying the proportions of soot and retort-carbon. Cored car- bons are made by inserting a small rod in the center of the die and the carbons are formed with a hollow core. This may be filled with a softer carbon. If two carbons connected to a source of electric cur- rent are brought together, the circuit is completed and a current flows. If the two carbons are now slightly separated, an arc will be formed. As the arc bums the carbons waste away and in the case of direct current, the positive decreases in length more rapidly than the negative one. This is due largely to the extremely 116 ARTIFICIAL LIGHT high temperature of the positive tip, where the carbon fairly boils. A crater is formed at the positive tip and this is always characteristic of the positive car- bon of the ordinary arc, although it becomes more shal- low as the arc-length is increased. The negative tip has a bright spot to which one end of the arc is at- tached. By wasting away, the length of the arc in- creases and likewise its resistance, until finally insuffi- cient current will pass to maintain the arc. It then goes out and to start it the carbons must be brought together and separated. The mechanisms of modern arc-lamps perform these functions automatically by the ingenious use of electromagnets. The interior of the arc is of a violet color and the exterior is a greenish yellow. The white-hot spot on the negative tip is generally surrounded by a fringe of agitated globules which consist of tar and other in- gredients of carbons. Often material is deposited from the positive crater upon the negative tip and these ac- cretions may build up a rounded tip. This deposit sometimes interferes with the proper formation of the arc and also with the light from the arc. It is often responsible for the hissing noise, although this hissing occurs with any length of arc when the current is suffi- ciently increased. The hissing seems to be due to the crater enlarging under excessive current until it passes the confines of the cross-section of the carbon. It thus tends to run up the side, where it comes in contact with oxygen of the air. In this manner the carbon is di- rectly burned instead of being vaporized, as it is when the hot crater is small and is protected from the air by the arc itself. The temperature of the positive crater THE ELECTRIC AECS 117 is in the neighborhood of 6000° to 7000°F. The brightness of the arc under pressure is the greatest produced by artificial means and is very intense. By putting the arc under high pressure, the brightness of the sun may be attained. The temperature of the hot- test spot on the negative tip is about a thousand de- grees below that of the positive. No great demand arose for arc-lamps until the de- velopment of the Gramme dynamo in 1870, which pro- vided a practicable source of electric current. In 1876 Jablochkov invented his famous "electric candle" con- sisting of two rods of carbon placed side by side but separated by insulating material. In this country Brush was the pioneer in the development of open arc- lamps. In 1877 he invented an arc-lamp and an effi- cient form of dynamo to supply the electrical energy. The first arc-lamps were ordinary direct-current open arcs and the carbons were made from high-grade coke, lampblack, and syrup. The upper positive carbon in these lamps is consumed at a rate of one to two inches per hour. Inasmuch as about 85 per cent, of the total light is emitted by the upper (positive) carbon and most of this from the crater, the lower carbon is made as small as possible in order not to obstruct any more light than necessary. The positive carbon of the open arc is often cored and the negative is a smaller one of solid carbon. This combination operates quite satis- factorily, but sometimes solid carbons are used out- doors. The voltage across the arc is about 50 volts. In 1846 Staite discovered that the carbons of an arc enclosed in a glass vessel into which the air was not freely admitted were consumed less rapidly than when 118 AETIFICIAL LIGHT the arc operated in the open air. After the appear- ance of the dynamo, when increased attention was given to the development of arc-lamps, this principle of enclosing the arcs was again considered. The early attempts in about 1880 were unsuccessful because low voltages were used and it was not until the discovery was made that the negative tip builds up considerably for voltages under 65 volts, that higher voltages were employed. In 1893 marked improvements were con- summated and Jandus brought out a successful en- closed arc operating at 80 volts. Marks contributed largely to the success of the enclosed arc by showing that a small current and a high voltage of 80 to 85 volts were the requisites for a satisfactory enclosed arc. The principle of the enclosed arc is simple. A closely fitting glass globe surrounds the arc, the fit be- ing as close as the feeding of the carbons will permit. When the arc is struck the oxygen is rapidly consumed and the heated gases and the enclosure check the sup- ply of fresh air. The result is that the carbons are consumed about one tenth as rapidly as in the open arc. There is no crater formed on the positive tip and the arc wanders considerably. The efficiency of the en- closed arc as a light-producer is lower than that of the open arc, but it found favor because of its slow rate of consumption of the carbons and consequent de- creased attention necessary. This arc operates a hun- dred hours or more without trimming, and will there- fore operate a week or more in street-lighting without attention. "When it is considered that open arcs for all- THE ELECTEIC AECS 119 night burning were supplied with two pairs of car- bons, the second set going into use automatically when the first were consumed, the value of the enclosed arc is apparent. However, the open arc has served well and has given way to greater improvements. It is rapidly disappearing from use. The alternating-current arc-lamp was developed after the appearance of the direct-current open-arc and has been widely used. It has no positive or negative car- bons, for the alternating current is reversing in direc- tion usually at the rate of 120 times per second; that is, it passes through 60 complete cycles during each second. No marked craters form on the tips and the two carbons are consumed at about the same rate. The average temperature of the carbon tips is lower than that of the positive tip of a direct-current arc, with the result that the luminous efficiency is lower. These arcs have been made of both the open and en- closed type. They are characterized by a humming noise due to the effect of alternating current upon the mechanism and also upon the air near the arc. This humming sound is quite different from the occasional hissing of a direct-current arc. When soft carbons are used, the arc is larger and apparently this mass of vapor reduces the humming considerably. The hum- ming is not very apparent for the enclosed alternating- current arc. The alternating arc can easily be de- tected by closely observing moving objects. If a pen- cil or coin be moved rapidly, a number of images ap- pear which are due to the pulsating character of the light. At each reversal of the current, the current 120 ARTIFICIAL LIGHT reaches zero value and the arc is virtually extinguished. Therefore, there is a maximum brightness midway be- tween the reversals. Various types of all these arcs have been developed to meet the different requirements of ordinary light- ing and to adapt this method of light-production to the needs of projection, stage-equipment, lighthouses, search-lights, and other applications. Up to this point the ordinary carbon arc has been considered and it has been seen that most of the light is emitted by the glowing end of the positive carbon. In fact, the light from the arc itself is negligible. A logical step in the development of the arc-lamp was to introduce salts in order to obtain a luminous flame. This possibility as applied to ordinary gas-flames had been known for years and it is surprising that it had not been early applied to carbons. Apparently Bremer in 1898 was the first to introduce fluorides of calcium, barium, and strontium. The salts deflagrate and a luminous flame envelops the ordinary feeble arc-flame. From these arcs most of the light is emitted by the arc itself, hence the name "flame-arcs." By the introduction of metallic salts into the car- bons the possibilities of the arc-lamp were greatly ex- tended. The luminous output of such lamps is much greater than that of an ordinary carbon arc using the same amount of electrical energy. Furthermore, the color or spectral character of the light may be varied through a wide range by the use of various salts. For example, if carbons are impregnated with calcium fluoride, the arc-flame when examined by means of a spectroscope will be seen to contain the characteristic THE ELECTRIC ARCS 121 spectrum of calcium, namely, some green, orange, and red rays. These combine to give to this arc a very yellow color. As explained in a previous chapter, the salts for this purpose may be wisely chosen from a knowledge of their fundamental or characteristic flame- spectra. These lamps have been developed to meet a variety of needs and their luminous efficiencies range from 20 to 40 lumens per watt, being several times that of the ordinary carbon open-arc. The red flame-arc owes its color chiefly to strontium, whose characteristic visible spectrum consists chiefly of red and yellow rays. Bar- ium gives to the arc a fairly white color. The yellow and so-called white flame-arcs have been most com- monly used. Flame-arcs have been produced which are close to daylight in color, and powerful blue-white flame-arcs have satisfied the needs of various chemical industries and photographic processes. These arcs are generally operated in a space where the air-supply is restricted similar to the enclosed-arc principle. In- asmuch as poisonous fumes are emitted in large quan- tities from some flame-arcs, they are not used indoors without rather generous ventilation. In fact, the flame-arcs are such powerful light-sources that they are almost entirely used outdoors or in very large interiors especially of the type of open factory build- ings. They are made for both direct and alternating current and the mechanisms have been of several types. The electrodes are consumed rather rapidly so they are made as long as possible. In one type of arc, the carbons are both fed downward, their lower ends form- ing a narrow V with the arc^flame between their tips. 122 ARTIFICIAL LIGHT Under these conditions the arc tends to travel ver- tically and finally to "stretch" itself to extinction. However, the arc is kept in place by means of a magnet above it which repels the arc and holds it at the ends of the carbons. The chief objection to the early flame-arcs was the necessity for frequent renewal of the carbons. This was overcome to a large extent in the Jandus regener- ative lamp in which the arc operates in a glass enclos- ure surrounded by an opal globe. However, in addi- tion to the inner glass enclosure, two cooling chambers of metal are attached to it. Air enters at the bottom and the fumes from the arc pass upward and into the cooling chambers, where the solid products are de- posited. The air on returning to the bottom is thus relieved of these solids and the inner glass enclosure remains fairly clean. The lower carbon is impregnated with salts for producing the luminous flame and the upper carbon is cored. The life of the electrodes is about seventy-five hours. The next step was the introduction of the so-called "luminous-arc" which is a "flame-arc" with entirely different electrodes. The lower (negative) electrode consists of an iron tube packed chiefly with magnetite (an iron oxide) and titanium oxide in the approximate proportions of three to one respectively. The mag- netite is a conductor of electricity which is easily va- porized. The arc-flame is large and the titanium gives it a high brilliancy. The positive electrode, usually the upper one, is a short, thick, solid cylinder of cop- per, which is consumed very slowly. This lamp, known THE ELECTRIC AECS 123 as the magnetite-arc, has a luminous efficiency of about 20 lumens per watt with a clear glass globe. The mechanisms which strike the arc and feed the carbons are ingenious devices of many designs depend- ing upon the kind of arc and upon the character of the electric circuit to which it is connected. Late de- velopments in electric incandescent filament lamps have usurped some of the fields in which the arc-lamp reigned supreme for years and its future does not ap- pear as bright now as it did ten years ago. High-in- tensity arcs have been devised with small carbons for special purposes and considered as a whole a great amount of ingenuity has been expended in the develop- ment of arc-lamps. There will be a continued demand for arc-lamps, for scientific developments are opening new fields for them. Their value in photo-engraving, in the moving-picture production studios, in moving- picture projection, and in certain aspects of stage- lighting is firmly established, and it appears that they will find application in certain chemical industries be- cause the arc is a powerful source of radiant energy which is very active in its effects upon chemical reac- tions. The luminous efficiencies of arc-lamps depend upon so many conditions that it is difficult to present a con- cise comparison; however, the following may suffice to show the ranges of luminous output per watt under actual conditions of usage. These efficiencies, of course, are less than the efficiencies of the arc alone, because the losses in the mechanism, globes, etc., are included. 124 AETIFICIAL LIGHT Lumens per watt Open carbon arc 4 to 8 Enclosed carbon arc 3 to 7 Enclosed flame-arc (yellow or white) . - 15 to 25 Luminous arc 10 to 25 Another lamp differing widely in appearance from the preceding arcs may be described here because it is known as the mercnry-arc. In this lamp mercury is confined in a transparent tube and an arc is started by making and breaking a mercury connection between the two electrodes. The arc may be maintained of a length of several feet. Perhaps the first mercury-arc was produced in 1860 by Way, who permitted a fine jet of mercury to fall from a reservoir into a vessel, the reservoir and receiver being connected to the poles of a battery. The electric current scattered the jet and between the drops arcs were formed. He ex- hibited this novel light-source on the mast of a yacht and it received great attention. Later, various inves- tigators experimented on the production of a mercury- arc and the first successful ones were made in the form of an inverted U-tube with the ends filled with mer- cury and the remainder of the tube exhausted. Cooper Hewitt was a successful pioneer in the pro- duction of practicable mercury-arcs. He made them chiefly in the form of straight tubes of glass up to several feet in length, with enlarged ends to facilitate cooling. The tubes are inclined so that the mercury vapor which condenses will run back into the enlarged end, where a pool of mercury forms the negative elec- trode. The arc may be started by tilting the tube so that a mercury thread runs down the side and con- THE ELECTRIC ARCS 125 nects with the positive electrode of iron. The heat of the arc volatilizes the mercury so that an arc of con- siderable length is maintained. The tilting is done by electromagnets. Starting has also been accomplished by means of a heating coil and also by an electric spark. The lamps are stabilized by resistance and in- ductance coils. One of the defects of the light emitted by the incan- descent vapor of mercury is its paucity of spectral colors. Its visible spectrum consists chiefly of violet, blue, green, and yellow rays. It emits virtually no red rays, and, therefore, red objects appear devoid of red. The human face appears ghastly under this light and it distorts colors in general. However, it pos- sesses the advantages of high eflficiency, of reasonably low brightness, of high actinic value, and of revealing detail clearly. Various attempts have been made to improve the color of the light by adding red rays. Reflectors of a fluorescent red dye have been used with some success, but such a method reduces the luminous eflGiciency of the lamp considerably. The dye fluoresces red under the illumination of ultra-violet, violet, and blue rays; that is, it has the property of converting radiation of these wave-lengths into radiant energy of longer wave-lengths. By the use of electric incandes- cent filament lamps in conjunction with mercury-arcs, a fairly satisfactory light is obtained. Many experi- ments have been made by adding other substances to the mercury, such as zinc, with the hope that the spec- trum of the other substance would compensate the defects in the mercury spectrum. However no suc- cess has been reached in this direction. 126 AETIFICIAL LIGHT By the use of a quartz tube whieh can withstand a much higher temperature than glass, the current den- sity can be greatly increased. Thus a small quartz tube of incandescent mercury vapor will emit as much light as a long glass tube. The quartz mercury-arc produces a light which is almost white, but the actual spectrum is very different from that of white sunlight. Although some red rays are emitted by the quartz arc, its spectrum is essentially the same as that of the glass- tube arc. Quartz transmits ultra-violet radiation, which is harmful to the eyes, and inasmuch as the mercury vapor emits such rays, a glass globe should be used to enclose the quartz tube when the lamp is used for ordinary lighting purposes. It is fortunate that such radically different kinds of light-sources are available, for in the complex activi- ties of the present time all are in demand. The quartz mercury-arc finds many isolated uses, owing to its wealth of ultra-violet radiation. It is valuable as a source of ultra-violet for exciting phosphorescence, for examining the transmission of glasses for this radia- tion, for sterilizing water, for m'edical purposes, and for photography. THE ELECTEIC INCANDESCENT FILAMENT LAMPS Prior to 1800 electricity was chiefly a plaything for men of scientific tendencies and it was not until Volta invented the electric pile or battery that certain scien- tific men gave their entire attention to the study of electricity. Volta was not merely an inventor, for he was one of the greatest scientists of his period, en- dowed with an imagination which marked him as a genius in creative work. By contributing the electric battery, he added the greatest impetus to research in electrical science that it has ever received. As has al- ready been shown, there began a period of enthusiastic research in the general field of heating effects of elec- tric current. The electric arc was born in the cradle of this enthusiasm, and in the heating of metals by elec- tricity the future incandescent lamp had its beginning. Between the years 1841 and 1848 several inventors attempted to make light-sources by heating metals. These crude lamps were operated by means of Grove and Bunsen electric cells, but no practicable incandes- cent filament lamps were brought out until the devel- opment of the electric dynamo supplied an adequate source of electric current. As electrical science prog- ressed through the continued efforts of scientific men, it finally became evident that an adequate supply of 127 128 ARTIFICIAL LIGHT electric current could be obtained by mechanical means ; that is, by rotating conductors in such a manner that current would be generated within them as they cut through a magnetic field. Even the pioneer inventors of electric lamps made great contributions to electrical practice by developing the dynamo. Brush developed a satisfactory dynamo coincidental with his invention of the arc-lamp, and in a similar manner, Edison made a great contribution to electrical practice in devising means of generating and distributing electricity for the purpose of serving his filament lamp. Edison in 1878 attacked the problem of producing light from a wire or filament heated electrically. He used platinum wire in his first experiments, but its volatility and low melting-point (3200°F.) limited the success of the lamps. Carbon with its extremely high melting-point had long attracted attention and in 1879 Edison produced a carbon filament by carbonizing a strip of paper. He sealed this in a vessel of glass from which the air was exhausted and the electric cur- rent was led to the filament through platinum wires sealed in the glass. Platinum was used because its ex- pansion and contraction is about the same as glass. Incidentally, many improvements were made in incan- descent lamps and thirty years passed before a ma- terial was found to replace the platinum leading-in wires. The cost of platinum steadily increased and finally in the present century a substitute was made by the use of two metals whose combined expansion was the same as that of platinum or glass. In 1879 and 1880 Edison had succeeded in overcoming the many 6 g s H H a Eh o INCANDESCENT FILAMENT LAMPS 129 difficulties sufficiently to give to the world a prac- ticable incandescent filament lamp. About this time Swan and Steam in England had also produced a suc- cessful lamp. In Edison's early experiments with filaments he used platinum wire coated with carbon but without much success. He also made thin rods of a mixture of finely divided metals such as platinum and iridium mixed with such oxides as magnesia, zirconia, and lime. He even coiled platinum wire around a piece of one of these oxides, with the aim of obtaining light from the wire and from the heated oxide. However, these experiments served little purpose besides indi- cating that the filament was best if it consisted solely of carbon and that it should be contained in an evac- uated vessel. One of the chief difficulties was to make the carbon filaments. Some of the pioneers, such as Sawyer and Mann, attempted to cut these from a piece of carbon. However, Edison and also Swan turned their attention to forming them by carbonizing a fiber of organic mat- ter. Filaments cut from paper and threads of cotton and silk were carbonized for this purpose. Edison scoured the earth for better materials. He tried a fibrous grass from South America and various kinds of bamboo from other parts of the world. Thin fila- ments of split bamboo eventually proved the best ma- terial up to that time. He made many lamps contain- ing filaments of this material, and even until 1910 bam- boo was used to some extent in certain lamps. Of these early days, Edison said : 130 ARTIFICIAL LIGHT It occurred to me that perhaps a filament of carbon could be made to stand in sealed glass vessels, or bulbs, which we were using, exhausted to a high vacuum. Separate lamps were made in this way independent of the air-pump, and, in October, 1879, we made lamps of paper carbon, and with carbons of common sewing thread, placed in a receiver or bulb made entirely of glass, with the leading-in wires sealed in by fusion. The whole thing was exhausted by the Sprengel pump to nearly one-millionth of an atmosphere. The fila- ments of carbon, although naturally quite fragile ow- ing to their length and small mass, had a smaller rad- iating surface and higher resistance than we had dared hope. We had virtually reached the position and con- dition where the carbons were stable. In other words, the incandescent lamp as we still know it to-day [1904], in essentially all its particulars unchanged, had been bom. After Edison's later success with bamboo, Swan in- vented a process of squirting filaments of nitrocellulose into a coagulating liquid, after which they are carbon- ized. Very fine uniform filaments can be made by this process and although improvements have been made from time to time, this method has been employed ever since its invention. In these later years cotton is dis- solved in a suitable solvent such as a solution of ziuc chloride and this material is forced through a small diamond die. This thread when hardened appears similar to cat-gut. It is cut into proper lengths and bent upon a form. It is then immersed in plumbago and heated to a high temperature in order to destroy the organic matter. A carbon filament is the result. From this point to the finished lamp many operations are performed, but a discussion of these would lead INCANDESCENT FILAMENT LAMPS 131 far afield. The production of a high vacuum is one of the most important processes and manufacturers of incandescent lamps have mastered the art perhaps more thoroughly than any other manufacturers. At least, their experience in this field made it possible for them to produce quickly and on a large scale such devices as X-ray tubes during the recent war. During the early years of incandescent lamps, im- provements were made from time to time which in- creased the life and the luminous eflS.ciency of the car- bon filaments, but it was not until 1906 that any radi- cal improvement was achieved. In that year in this country a process was devised whereby the carbon fila- ment was made more compact. In fact, from its ap- pearance it received the name "metallized filament." These carbon filaments are prepared in the same man- ner as the earlier ones but are finally "treated" by heating in an atmosphere of hydrocarbons such as coal- gas. The filament is heated by electric current and the heat breaks down the hydrocarbons, with the result that carbon is deposited upon the filament. This "treated" filament has a coating of hard carbon and its electrical resistance is greater than that of the un- treated filament. The luminous efficiency of a carbon filament is a func- tion of its temperature and it increases very rapidly with increasing temperature. For this reason it is a constant aim to reach high filament temperatures. Of all the materials used in filaments up to the present time, carbon possesses the highest melting-point (per- haps as high as 7000° F.), but the carbon filament as operated in practice has a lower efficiency than any 133 AETIFICIAL LIGHT other filament. This is because the highest tempera- ture at "which it can be operated and still have a rea- sonable life is much lower than that of metallic fila- ments. The incandescent carbon in the evacuated bulb sublimes or volatilizes and deposits upon the bulb. This decreases the size of the filament eventually to the breaking-point and the blackening of the bulb de- creases the output of light. The treated filament was found to be a harder form of carbon that did not volatilize as rapidly as the untreated filament. It im- mediately became possible to operate it at a higher temperature with a resulting increase of luminous effi- ciency. This "graphitized" carbon filament lamp be- came known as the gem lamp in this country and many persons have wondered over the word "gem." The first two letters stand for "General Electric" and the last for "metallized." This lamp was welcomed with enthusiasm in its day, but the day for carbon fila- ments has passed. The advent of incandescent lamps of higher efficiency has made it uneconomical to use carbon lamps for general lighting purposes. Al- though the treated carbon filament was a great im- provement, its reign was cut short by the appearance of metal filaments. In 1803 a new element was discovered and named tantalum. It is a dark, lustrous, hard metal. Pure tantalum is harder than steel; it may be drawn into fine wire; and its melting-point is very high (about 5100 °F.). It is seen to possess properties desirable for filaments, but for some reason it did not attract attention for a long time. A century elapsed after its discovery before von Bolton produced the first tan- INCANDESCENT FILAMENT LAMPS 133 talum filament lamp. Owing to the low electrical re- sistance of tantalum, a filament in order to operate sat- isfactorily on a standard voltage must be long and thin. This necessitates storing away a considerable length of wire in the bulb without permitting the loops to come into contact with each other. After the fila- ments have been in operation for a few hundred hours they become brittle and faults develop. When exam- ined under a microscope, parts of the filament operated on alternating current appear to be offset. The ex- planation of this defect goes deeply into crystalline structure. The tantalum filament was quickly fol- lowed by osmium and by tungsten in this country. The osmium filament appeared in 1905 and its in- vention is due to Welsbach, who had produced the mar- velous gas-mantle. Owing to its extreme brittleness, osmium was finely divided and made into a paste of organic material. The filaments were squirted through dies and, after being formed and dried, they were heated to a high temperature. The organic mat- ter disappeared and the fine metallic particles were sintered. This made a very brittle lamp, but its high efficiency served to introduce it. In 1870 when Scheele discovered a new element, known in this country as tungsten, no one realized that it was to revolutionize artificial lighting and to alter the course of some of the byways of civilization. This metal — which is known as "wolfram" in Germany, and to some extent in English-speaking countries — is one of the heaviest of elements, having a specific gravity of 19.1. It is 50 per cent, heavier than mercury and nearly twice as heavy as lead. It was early used in 134 ARTIFICIAL LIGHT German silver to the extent of 1 or 2 per cent, to make platinoid, an alloy possessing a high resistance which varies only slightly as the temperature changes. This made an excellent material for electrical resistors. The melting-point of tungsten is about 5350°F., which makes it desirable for filaments, but it was very brittle as prepared in the early experiments. It unites very readily with oxygen and with carbon at high tempera- tures. The first tungsten lamps appeared on the market in 1906, but these contained fragile filaments made by the squirting process. When the squirted filament of tungsten powder and organic matter was heated in an atmosphere of steam and hydrogen to remove the bind- ing material, a brittle filament of tungsten ^yas ob- tained. The first lamps were costly and fragile. After years of organized research tungsten is now drawn into the finest wires, possessing a tensile strength perhaps greater than any other material. Filaments are now made into many shapes and the greatest strides in artificial lighting have been due to scientific research on a huge scale. The achievements which combined to perfect the tungsten lamp to the point where it has become the mainstay of electric lighting are not attached to names in the Hall of Fame. Organization of scientific re- search in the industrial laboratories is such that often many persons contribute to the development of an im- provement. Furthermore, time is usually required for a full perspective of applications of scientific knowl- edge. In the early days organized research was not practised and the great developments of those days INCANDESCENT FILAMENT LAMPS 135 were the works of individuals. To-day, even in pure science, some of the greatest contributions are made by industrial laboratories ; but sometimes these do not become known to the public for many years. The whole scheme of scientific development has changed materially. For example, the story of the development of ductile tungsten, which has revolutionized lighting, is complex and more or less shrouded in secrecy at the present time. Many men have contributed toward this accomplishment and the public at the present time knows little more than the fact that tungsten filaments, which were brittle yesterday, are now made of ductile tungsten wire drawn into the finest filaments. The earlier tungsten filaments were made by three rival processes. By the first, a deposit of tungsten was "flashed" on a fine carbon filament, the latter being eliminated finally by heating in an atmosphere of hy- drogen and water-vapor. By the second, colloidal tungsten was produced by operating an arc between tungsten electrodes under water. The finely divided tungsten was gathered, partially dried, and squirted through dies to form filaments. These were then sin- tered. The third was the "paste" process already de- scribed. These methods produced fragile filaments, but their luminous efficiency was higher than that of previous ones. However, in this country ductile tung- sten was soon on its way. An ingot of tungsten is sub- jected to vigorous swaging until it takes the form of a rod. This is finally drawn into wire. Much of this development work was done by the laboratories of the General Electric Company and they were destined to contribute another great improve- 136 ARTIFICIAL LIGHT ment. The blackening of the lamp bulbs was due to the evaporation of tungsten from the filament. All filaments up to this time had been confined in evacu- ated bulbs and the low pressure facilitates evaporation, as is well known. It had long been known that an inert gas in the bulb would reduce the evaporation and rem- edy other defects; however, under these conditions, there would be a considerable loss of energy through conduction of heat by the gases. In the vacuum lamp nearly all the electrical energy is converted into ra- diant energy, which is emitted by the filament and any dissipation of heat is an energy loss. A high vacuum was one of the chief aims up to this time, but a radical departure was pending. If an ordinary tungsten-lamp bulb be filled with an inert gas such as nitrogen, the filament may be oper- ated at a very much higher temperature without any more deterioration than takes place in a vacuum at a lower temperature. This gives a more efficient light but a less efficient lamp. The greater output of light is compensated by losses by conduction of heat through the gas. In other words, a great deal more energy is required by the filament in order to remain at a given temperature in a gas than in a vacuum. However, elaborate studies of the dependence of heat-losses upon the size and shape of the filament and of the physics of conduction from a solid to a gas, established the foundation for the gas-filled tungsten lamp. The,-, knowledge gained in these investigations indicated that a thicker filament lost a relatively less percentage of energy by conduction than a thin one for equal amounts of emitted light. However, a practical fila- INCANDESCENT FILAMENT LAMPS 137 ment must have sufficient resistance to be used safely on lighting circuits already established and, therefore, the large diameter and high resistance were obtained by making a helical coil of a fine wire. In fact, the gas-filled tungsten lamp may be thought of as an ordi- nary lamp with its long filament made into a short helical coil and the bulb filled with nitrogen or argon This development was not accidental and from a scientific point of view it is not spectacular. It did not mark a new discovery in the same sense as the dis- covery of X-rays, However, it is an excellent exam- ple of the great rewards which come to systematic, thorough study of rather commonplace physical laws in respect to a given condition. Such achievements are being duplicated in various lines in the laboratories of the industries. Scientific research is no longer mo- nopolized by educational institutions. The most elab- orate and best-equipped laboratories are to be found in the industries sometimes surrounded by the smoke and noise and vigorous activity which indicate that achievements of the laboratory are on their way to mankind. The smoke-laden industrial district, pul- sating with life, is the proud exhibit of the present civilization. It is the creation of those who discover, organize, and apply scientific facts. But how many ap- preciate the debt that mankind owes not only to the individual who dedicates his life to science but to the far-sighted manufacturer who risks his money in or- ganized quest of new benefits for mankind? A glimpse into a vast organization of research, which, for ex- ample, has been mainly responsible for the progress 138 AETIFICIAL LIGHT of the incandescent lamp would alter the attitude of many persons toward science and toward the large in- dustrial companies. The progress in the development of electric incan- descent lamps is shown in the following table, where the dates and values are more or less approximate. It should be understood that from 1880 to the present time there has been a steady progress, which occa- sionally has been greatly augmented by sudden steps. Approximate Values Lumens per Date Filament Temperatura watt 1880 Carbon 3300°F. 3.0 1906 Carbon (graphitized) 3400 4.5 1905 Tantalum 3550 6.5 1905 Osmium 3600 7.5 1906 Tungsten (vacuum) 3700 8.0 1914 Tungsten (gas-filled) up to 5300°F. 10 to 25 Throughout the development of incandescent fila- ment lamps many ingenious experiments were made which resulted usually in light-sources of scientific in- terest but not of practical value. One of the latest is the tungsten arc in an inert gas. By means of a heating coil, a small arc is started between two elec- trodes consisting of tungsten, but this as yet has not been shown to be practicable. Another type of filament lamp was developed by Nernst in 1897. It was an ingenious application of the peculiar properties of rare-earth oxides. His first lamp consisted essentially of a slender rod of mag- nesia. This substance does not conduct electricity at INCANDESCENT FILAMENT LAMPS 139 ordinary temperatures, but when heated to incandes- cence it becomes conducting. Upon sufficient heating of this filament by external means while a proper volt- age is impressed upon it, the electric current passes through it and thereafter this current will maintain its temperature. Thus such a filament becomes a con- ductor and will continue to glow brilliantly by virtue of the electrical energy which it converts into heat. Later lamps consisted of "glowers" about one inch long made from a mixture of zirconia and yttria, and finally a mixture of ceria, thoria, and zirconia was used. The glower is heated initially by a coil of plat- inum wire located near it but not in contact with it. Owing to the fact that this glower decreases rapidly in resistance as its temperature is increased, it is necessary to place in series with it a substance which increases in resistance with increasing current. This is called a "ballasting resistance" and is usually an iron wire in a glass bulb containing hydrogen. The heater is cut out by an electromagnet when the glower goes into operation. This lamp is a marvel of in- genuity and when at its zenith it was installed to a considerable extent. Its light is considerably whiter than that of the carbon filament lamps. However, its doom was sounded when metallic filament lamps ap- peared. An interesting filament was developed by Parker and Clark by using as a core a small filament of car- bon. This flashed in an atmosphere containing a vapor of a compound of silicon, became coated with silicon. This filament was of high specific resistance and ap- 140 ARTIFICIAL LIGHT peared to have promise. It has not been introduced commercially and doubtless it cannot compete with the latest tungsten lamps. Electric incandescent lamps are the present main- stay of electric illumination and, it might be stated, of progress in lighting. Wonderful achievements have been accomplished in other modes of lighting and the foregoing statement is not meant to depreciate those achievements. However, the incandescent filament lamp has many inherent advantages. The light-source is enclosed in an air-tight bulb which makes for a safe, convenient lamp. The filament is capable of subdivision, with the result that such lamps vary from the minutest spark of the smallest miniature lamp to the enormous output of the largest gas-filled tungsten lamp. The outputs of these are respectively a frac- tion of a lumen and twenty-five thousand lumens ; that is, the luminous intensity varies from an equivalent of a small fraction of a standard candle to a single light-source emitting light equivalent to two thousand standard candles. Statistics are cold facts and are usually uninterest- ing in a volume of this character, but they tell a story in a concise manner. The development of the modern incandescent lamp has increased the intensity of light available with a great decrease in cost, and this pro- gressive development is shown easily by tables. For example, since the advent of the tungsten lamp the average candle-power and luminous efficiency of all the lamps sold in this country has steadily increased, while the average wattages of the lamps have remained vir- tually stationary. INCANDESCENT FILAMENT LAMPS 141 AVEEAGE CanDLE-PoWEK, WaTTS, AND EFFICIENCY OP AlL THE Lamps Sold in This Country Lumens Year Candle-power Watts per watt 1907 18.0 53 3.33 1908 19.0 53 3.52 1909 21.0 52 3.96 1910 23.0 51 4.42 1911 25.0 51 4.82 1912 26.0 49 5.20 1913 29.4 47 6.13 1914 38.2 48 7.80 1915 42.2 47 8.74 1916 45.8 49 9.60 1917 48.7 51 10.56 It will be noted that the luminous intensity of in- candescent filament lamps has steadily increased since the carbon lamp was superseded, and that in a period of ten years of organized research behind the tung- sten lamp the luminous efficiency (lumens per watt) has trebled. In other words, everything else remain- ing unchanged, the cost of light in ten years was re- duced to one third. But the reduction in cost has been more than this, as will be shown later. During the same span of years the percentage of carbon fila- ment lamps of the total filament lamps sold decreased from 100 per cent, in 1907 to 13 per cent, in 1917. At the same time the percentage of tungsten (Mazda) lamps increased from virtually zero in 1907 to about 87 per cent, in 1917. The tantalum lamp had no oppor- tunity to become established, because the tungsten lamp followed its appearance very closely. In 1910 the 142 AETIFICIAL LIGHT sales of the former reached their highest mark, which was only 3.5 per cent, of all the lamps sold in the United States. From a lowly beginning the number of in- candescent filament lamps sold for use in this country has grown rapidly, reaching nearly two hundred mil- lion in 1919. XI THE LIGHT OF THE FUTURE In viewing the development of artificial light and its manifold effects upon the activities of mankind, it is natural to look into the future. Jules Verne pos- sessed the advantage of being able to write into fiction what his riotous imagination dictated, and so much of what he pictured has come true that his success tempts one to do likewise in prophesying the future of light- ing. Surely a forecast based alone upon the past achievements and the present indications will fall short of the actual realizations of the future! If the imagination is permitted to view the future without restrictions, many apparently far-fetched schemes may be devised. It may be possible to turn to nature 's sup- ply of daylight and to place some of it in storage for night use. One millionth part of daylight released as desired at night would illuminate sufficiently all of man's nocturnal activities. The fictionist need not heed the scientist's inquiry as to how this daylight would be bottled. Instead of giving time to such in- quiries he would pass on to another scheme, whereby the earth would be belted with optical devices so that day could never leave. When the sun was shining in China its light would be gathered on a large scale and sent eastward and westward in these great optical "pipe-lines" to the regions of darkness, thus banish- 143 144 ARTIFICIAL LIGHT ing night forever. The writer of fiction need not bother with a consideration of the economic situation which would demand such efforts. This line of con- jecture is interesting, for it may suggest possibilities toward which the present trend of artificial lighting does not point; however, the author is constrained to treat the future of light-production on a somewhat more conservative basis. At the present time the light-source of chief inter- est in electric lighting is the incandescent filament lamp ; but its luminous efficiency is limited, as has been shown in a previous chapter. When light is emitted by virtue of its temperature much invisible radiant energy accompanies the visible energy. The highest luminous efficiency attainable by pure temperature radiation will be reached when the temperature of a normal radiator reaches the vicinity of 10,000 °F. to 11,000°F. The melting-points of metals are much lower than this. The tungsten filament in the most efficient lamps employing it is operating near its melt- ing-point at the present time. Carbon is a most at- tractive element in respect to melting-point, for it melts at a temperature between 6000°F. and 7000°F. Even this is far below the most efficient temperature for the production of light by means of pure tempera- ture radiation. There are possibilities of higher effi- ciency being obtained by operating arcs or filaments under pressure; however, it appears that highly effi- cient light of the future will result from a radical departure. Scientists are becoming more and more intimate with the structure of matter. They are learning secrets THE LIGHT OF THE FUTUEE 145 every year which, apparently are leading to a funda- mental knowledge of the subject. When these mys- teries are solved, who can say that man will not be able to create elements to suit his needs, or at least to alter the properties of the elements already avail- able? If he could so alter the mechanism of radia- tion that a hot metal would radiate no invisible en- ergy, he would have made a tremendous stride even in the production of light by virtue of high tempera- ture. This property of selective radiation is pos- sessed by some elements to a slight degree, but if treat- ment could enhance this property, luminous eflSiciency would be greatly increased. Certainly the principle of selectivity is a byway of possibilities. A careful study of commonplace factors may result in a great step in light-production without the crea- tion of new elements or compounds, just as such a pro- cedure doubled the luminous efficiency