Chapter III: Preface (3)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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
Regenerative 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 intended merely to serve as examples of the difficulties encountered. Doubtless, similar controversies arose in other countries in which pioneers were also nursing gas-lighting to maturity. However, it is certain that much of the early progress of lighting of this character was fathered in England. Gas-lighting was destined to become a thriving industry, and is of such importance in lighting that another chapter is given its modern developments.

VII

THE SCIENCE OF LIGHT-PRODUCTION

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

VIII

MODERN GAS-LIGHTING

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

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

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

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

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

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

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

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

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

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

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

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

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