Some one in an early century was the first to notice that the sun's rays tanned the skin, and this unknown individual made the initial discovery in what is now an extensive branch of science known as photo-chemistry. The fading of dyes, the bleaching of textiles, the darkening of silver salts, the synthesis and decomposition of compounds are common examples of chemical reactions induced by light. There are thousands of other examples of the chemical effects of light some of which have been utilized by mankind. Others await the development of more efficient light-sources emitting greater quantities of active rays, and many still remain interesting scientific facts without any apparent practical applications at the present time. Visible and ultra-violet rays are the radiations almost entirely responsible for photochemical reactions, but the most active of these are the blue, violet, and ultra-violet rays. These are often designated chemical or actinic rays in order to distinguish the group as a whole from other groups such as ultra-violet, visible, and infra-red. Light is a unique agent in chemical reactions because it is not a material substance. It neither contaminates nor leaves a residue. Although much information pertaining to photochemistry has been available for years, the absence of powerful light-sources emitting so-called chemical rays in large quantities inhibited the practical development of the science of photochemistry. Even to-day, with vast applications of light in this manner, mankind is only beginning to utilize its chemical powers.

STERILIZING WATER WITH RADIANT ENERGY FROM QUARTZ MERCURY-ARCS

Although it appears that the chemical action of light was known to the ancients, the earliest photochemical investigations which could be considered scientific and systematic were those of K.W. Scheele in 1777 on silver salts. An extract from his own account is as follows:

I precipitated a solution of silver by sal-ammoniac; then I edulcorated (washed) it and dried the precipitate and exposed it to the beams of the sun for two weeks; after which I stirred the powder and repeated the same several times. Hereupon I poured some caustic spirit of sal-ammoniac (strong ammonia) on this, in all appearance, black powder, and set it by for digestion. This menstruum (solvent) dissolved a quantity of luna cornua (horn silver), though some black powder remained undissolved. The powder having been washed was, for the greater part, dissolved by a pure acid of nitre (nitric acid), which, by the operation, acquired volatility. This solution I precipitated again by means of sal-ammoniac into horn silver. Hence it follows that the blackness which the luna cornua acquires from the sun's light, and likewise the solution of silver poured on chalk, is silver by reduction. I mixed so much of distilled water with the well-washed horn silver as would just cover this powder. The half of this mixture I poured into a white crystal phial, exposed it to the beams of the sun, and shook it several times each day; the other half I set in a dark place. After having exposed the one mixture during the space of two weeks, I filtrated the water standing over the horn silver, grown already black; I let some of this water fall by drops in a solution of silver, which was immediately precipitated into horn silver.

This extract shows that Scheele dealt with the reducing action of light. He found that silver chloride was decomposed by light and that there was a liberation of chlorine. However, it was learned later that dried silver chloride sealed in a tube from which the air was exhausted is not discolored by light and that substances must be present to absorb the chlorine. Scheele's work aroused much interest in photochemical effects and many investigations followed. In many of these the superiority of blue, violet, and ultra-violet rays was demonstrated. In 1802 the first photograph was made by Wedgwood, who copied paintings upon glass and made profiles by casting shadows upon a sensitive chemical compound. However, he was not able to fix the image. Much study and experimentation were expended upon photochemical effects, especially with silver compounds, before Niepce developed a method of producing pictures which were subsequently unaffected by light. Later Daguerre became associated with Niepce and the famous daguerreotype was the result. Apparently the latter was chiefly responsible for the development of this first commercial process, the products of which are still to be found in the family album. A century has elapsed since this earliest period of commercial photography, and during each year progress has been made, until at the present time photography is thoroughly woven into the activities of civilized mankind.In those earliest years a person was obliged to sit motionless in the sun for minutes in order to have his picture taken. The development of a century is exemplified in the "snapshot" of the present time. Photographic exposures outdoors at present are commonly one thousandth of a second, and indoors under modern artificial light miles of "moving-picture" film are made daily in which the individual exposures are very small fractions of a second. Artificial light is playing a great part in this branch of photochemistry, and the development of artificial light for the various photographic needs is best emphasized by reminding the reader that the sources must be generally comparable with the sun in actinic or chemical power. The intensity of illumination due to sunlight on a clear day when the sun is near the zenith is commonly 10,000 foot-candles on a surface perpendicular to the direct rays. This is equivalent to the illumination due to a source 90,000 candle-power at a distance of three feet. The sun delivers about 200,000,000,000 horse-power to the earth continuously, which is estimated to be about one million times the amount of power generated artificially on the earth. Of this inconceivable quantity of energy a small part is absorbed by vegetation, some is reflected and radiated back into space, and the balance heats the earth. To store some of this energy so that it may be utilized at will in any desired form is one of the dreams of science. However, artificial light-sources are depended upon at present in many photographic and other chemical processes.

Although two illuminants may be of the same luminous intensity, they may differ widely in actinic value. It is impossible to rate the different illuminants in a general manner as to actinic value because the various photochemical reactions are not affected to the same extent by rays of a given wave-length. Nearly all human eyes see visible rays in approximately the same manner, but the multitude of chemical reactions show a wide variation in sensitivity to the various rays. For example, one photographic emulsion may be sensitive only to ultra-violet, violet, and blue rays and another to all these rays and also to the green, yellow, and red. Therefore, one illuminant may be superior to another for one photochemical reaction, while the reverse may be true in the case of another reaction. In general, it may be said that the arc-lamps including the mercury-arcs provide the most active illuminants for photochemical processes; however, a large number of electric incandescent filament lamps are used in photographic work.

The photo-engraver has been independent of sunlight since the practical development of his art. In fact, the printer could not depend upon sunlight for making the engravings which are used to illustrate the magazines and newspapers. The newspaper photographer may make a "flashlight" exposure, develop his negative, and make a print from it under artificial light. He may turn this over to the photo-engraver who carries out his work by means of powerful arc-lamps and in an hour or two after the original exposure was made the newspaper containing the illustration is being sold on the streets.

The moving-picture studio is independent of daylight in indoor settings and there is a tendency toward the exclusive use of artificial light. In this field mercury-vapor lamps, arc-lamps, and tungsten photographic lamps are used. Similarly, in the portrait studio there is a tendency for the photographer to leave the skylighted upper floors and to utilize artificial light. In this field the tungsten photographic lamp is gaining in popularity, owing to its simplicity and to other advantages. Artificial light in general is more satisfactory than natural light for many kinds of photographic work because through the ease of controlling it a greater variety of more artistic effects may be obtained. In ordinary photographic printing tungsten lamps are widely used, but in blue-printing the white flame-arc and the mercury-vapor lamp are generally employed. Not many years ago the blue-printer waited for the sun to appear in order to make his prints, but to-day large machines operate continuously under the light of powerful artificial sources. How many realize that the blue-print is almost universally at the foundation of everything at the present time? Not only are products made from blue-prints but the machinery which makes the products is built from blue-prints. Even the building which houses the machinery is first constructed from blue-prints. They form an endless chain in the activities of present civilization.

Artificial light has been a great factor in the practical development of photography and it is looked upon for aid in many other directions. Although there is a multitude of reactions in photographic processes which are brought about by exposure to light, these represent relatively few of the photochemical reactions. In general, it may be stated that light is capable of causing nearly every type of reaction. The chemical compounds which are photo-sensitive are very numerous. Many of the compounds of silver, gold, platinum, mercury, iron, copper, manganese, lead, nickel, and tin are photo-sensitive and these have been widely investigated. Light and oxygen cause many oxidation reactions and, on the other hand, light reduces many compounds such as silver salts, even to the extent of liberating the metal. Oxygen is converted partially into ozone under the influence of certain rays and there are many examples of polymerization caused by light.

Various allotropic changes of the elements are due to the influence of light; for example, a sulphur soluble in carbon disulphide is converted into sulphur which is insoluble, and the rate of change of yellow phosphorus into the red variety is greatly accelerated by light. Hydrogen and chlorine combine under the action of light with explosive rapidity to form hydrochloric acid and there are many other examples of the synthesizing action of light. Carbon monoxide and chlorine combine to form phosgene and the combination of chlorine, bromine, and iodine, with organic compounds, is much hastened by exposing the mixture to light. In a similar manner many decompositions are due to light; for example, hydrogen peroxide is decomposed into water and oxygen. This suggests the reason for the use of brown bottles as containers for many chemical compounds. Such glass does not transmit appreciably the so-called actinic or chemical rays.

There is a large number of reactions due to light in organic chemistry and one of fundamental importance to mankind is the effect of light on the chlorophyll, the green coloring matter in vegetation. No permanent change takes place in the chlorophyll, but by the action of light it enables the plant to absorb oxygen, carbon dioxide, and water and to use these to build up the complex organic substances which are found in plants. Radiant energy or light is absorbed and converted into chemical energy. This use of radiant energy occurs only in those parts of the plant in which chlorophyll is present, that is, in the leaves and stems. These parts absorb the radiant energy and take carbon dioxide from the air through breathing openings. They convert the radiant energy into chemical energy and use this energy in decomposing the carbon dioxide. The oxygen is exhausted and the carbon enters into the structure of the plant. The energy of plant life thus comes from radiant energy and with this aid the simple compounds, such as the carbon dioxide of the air and the phosphates and nitrates of the soil, are built into complex structures. Thus plants are constructive and synthetic in operation. It is interesting to note that the animal organism converts complex compounds into mechanical and heat energy. The animal organism depends upon the synthetic work of plants, consuming as food the complex structures built by them under the action of light. For example, plants inhale carbon dioxide, liberate the oxygen, and store the carbon in complex compounds, while the animal uses oxygen to burn up the complex compounds derived from plants and exhales carbon dioxide. It is a beautiful cycle, which shows that ultimately all life on earth depends upon light and other radiant energy associated with it. Contrary to most photochemical reactions, it appears that plant life utilize yellow, red, and infra-red energy more than the blue, violet, and ultra-violet.

In general, great intensities of blue light and of the closely associated rays are necessary for most photochemical reactions with which man is industrially interested. It has been found that the white flame-arc excels other artificial light-sources in hastening the chlorination of natural gas in the production of chloroform. One advantage of the radiation from this light-source is that it does not extend far into the ultra-violet, for the ultra-violet rays of short wave-lengths decompose some compounds. In other words, it is necessary to choose radiation which is effective but which does not have rays associated with it that destroy the desired products of the reaction. By the use of a shunt across the arc the light can be gradually varied over a considerable range of intensity. Another advantage of the flame-arc in photochemistry is the ease with which the quality or spectral character of the radiant energy may be altered by varying the chemical salts used in the carbons. For example, strontium fluoride is used in the red flame-arc whose radiant energy is rich in red and yellow. Calcium fluoride is used in the carbons of the yellow flame-arc which emits excessive red and green rays causing by visual synthesis the yellow color. The radiant energy emitted by the snow-white flame-arc is a close approximation to average daylight both as to visible and to ultra-violet rays. Its carbons contain rare-earths. The uses of the flame-arcs are continually being extended because they are of high intensity and efficiency and they afford a variety of color or spectral quality. A million white flame-carbons are being used annually in this country for various photochemical processes.

Of the hundreds of dyes and pigments available many are not permanent and until recent years sunlight was depended upon for testing the permanency of coloring materials. As a consequence such tests could not be carried out very systematically until a powerful artificial source of light resembling daylight was available. It appears that the white flame-arc is quite satisfactory in this field, for tests indicate that the chemical effect of this arc in causing dye-fading is four or five times as great as that of the best June sunlight if the materials are placed within ten inches of a 28-ampere arc. It has been computed that in several days of continuous operation of this arc the same fading results can be obtained as in a year's exposure to daylight in the northern part of this country. Inasmuch as the fastness of colors in daylight is usually of interest, the artificial illuminant used for color-fading should be spectrally similar to daylight. Apparently the white flame-arc fulfils this requirement as well as being a powerful source.

Lithopone, a white pigment consisting of zinc sulphide and barium sulphate, sometimes exhibits the peculiar property of darkening on exposure to sunlight. This property is due to an impurity and apparently cannot be predicted by chemical analysis. During the cloudy days and winter months when powerful sunlight is unavailable, the manufacturer is in doubt as to the quality of his product and he needs an artificial light-source for testing it. In such a case the white flame-arc is serving satisfactorily, but it is not difficult to obtain effects with other light-sources in a short time if an image of the light-source is focused upon the material by means of a lens. In fact, a darkening of lithopone may be obtained in a minute by focusing upon it the image of a quartz mercury-arc by means of a quartz lens. In special cases of this sort the use of a focused image is far superior to the ordinary illumination from the light-source, but, of course, this is impracticable when testing a large number of samples simultaneously. Incidentally, lithopone which turns gray or nearly black in the sunlight regains its whiteness during the night.

An amusing incident is told of a young man who painted his boat one night with a white paint in which lithopone was the pigment. On returning home the next afternoon after the boat had been exposed to sunlight all day, he was astonished to see that it was black. Being very much perturbed, he telephoned to the paint store, but the proprietor escaped a scathing lecture by having closed his shop at the usual hour. The young man telephoned in the morning and told the proprietor what had happened, but on being asked to make certain of the facts he went to the window and looked at his boat and behold! it was white. It had regained whiteness during the night but would turn black again during the day. Although pigments and dyes are not generally as peculiar as lithopone, much uncertainty is eliminated by systematic tests under constant, continuous, and controllable artificial light.The sources of so-called chemical rays are numerous for laboratory work, but there is a need for highly efficient powerful producers of this kind of energy. In general the flame-arcs perhaps are foremost sources at the present time, with other kinds of carbon arcs and the quartz mercury-arc ranking next. One advantage of the mercury-arc is its constancy. Furthermore, for work with a single wave-length it is easy to isolate one of the spectral lines. The regular glass-tube mercury-arc is an efficient producer of the actinic rays and as a consequence has been extensively used in photographic work and in other photochemical processes. An excellent source for experimental work can be made easily by producing an arc between two small iron rods. The electric spark has served in much experimental work, but the total radiant energy from it is small. By varying the metals used for electrodes a considerable variety in the radiant energy is possible. This is also true of the electric arcs, and the flame-arcs may be varied widely by using different chemical compounds in the carbons.

There are other effects of light which have found applications but not in chemical reactions. For example, selenium changes its electrical resistance under the influence of light and many applications of this phenomenon have been made. Another group of light-effects forms a branch of science known as photo-electricity. If a spark-gap is illuminated by ultra-violet rays, the resistance of the gap is diminished. If an insulated zinc plate is illuminated by ultra-violet or violet rays, it will gradually become positively charged. These effects are due to the emission of electrons from the metal. Violet and ultra-violet rays will cause a colorless glass containing manganese to assume a pinkish color. The latter is the color which manganese imparts to glass and under the influence of these rays the color is augmented. Certain ultra-violet rays also ionize the air and cause the formation of ozone. This can be detected near a quartz mercury-arc, for example, by the characteristic odor.

The foregoing are only a few of the multitude of photochemical reactions and other effects of radiant energy. The development of this field awaits to some extent the production of so-called actinic rays more efficiently and in greater quantities, but there are now many practical applications of artificial light for these purposes. In the extensive fields of photography various artificial light-sources have served for many years and they are constantly finding more applications. Artificial light is now used to a considerable extent in the industries in connection with chemical processes, but little information is available, owing to the secrecy attending these new developments in industrial processes. However, this brief chapter has been introduced in order to indicate another field of activity in which artificial light is serving. It is agreed by scientists that photochemistry has a promising future. Mankind harnesses nature's forces and produces light and this light is put to work to exert its influence for the further benefit of mankind. Science has been at work systematically for only a century, but the accomplishments have been so wonderful that the imagination dares not attempt to prophesy the achievements of the next century.