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CHEMICAL ACTIONS OF LIGHT.

[127.

transparent solids and liquids exert upon light with various. degrees of intensity, when subjected to magnetic power of very exalted degree. Some of these singular results will be more particularly described at a future point (322).

§ III. INFLUENCE OF LIGHT ON CHEMICAL ATTRACTION

PHOTOGRAPHY.

(127 a) Supposed Influence of Light on Crystallization.—It is a familiar observation that bodies which crystallize as they are condensed after spontaneous sublimation,—such as camphor, naphthalin, and Faraday's chloride of carbon,—if placed in glass vessels often collect upon the side of the glass which is exposed to the light, whilst no crystals are deposited upon the other side of the vessels. This effect, however, is not confined to crystallizable substances. If a few drops of water be placed at the bottom of a bottle, the sides of which are kept dry, and the mouth of the bottle be closed, a deposit of globules of moisture will generally be observed upon a particular portion of the side, and often this deposit occurs upon the illuminated side of the bottle. A similar effect is frequently seen in the vacuum of a barometer, globules of mercury being condensed upon the side of the tube. It was generally supposed that these effects were due to some subtle influence exerted by light, but Tomlinson has shown conclusively that they are simply owing to differences in temperature; the crystals, or the liquid, becoming condensed upon that part of the vessel which, from accidental circumstances, is the coldest (Phil. Mag. 1862 [4], xxiv. 358).

(127 b) Chemical Actions of Light.-The rays of the sun are not only the great source both of light and heat to the globe which we inhabit, but they are constantly exerting upon the various substances upon its surface, a chemical influence of the utmost importance to the existence of animal and vegetable life, and to the permanence of the present order of creation. The occurrence of this remarkable chemical activity in the solar rays may be shown in various ways:-When perfectly dry chlo rine is mixed in the dark with hydrogen, no chemical change takes place; if the two gases have been exposed separately to the beams of the sun, and have subsequently been mixed in the dark, they may be preserved in this condition also without change, so long as they are screened from the light; but if the mixture be exposed to diffused daylight, it will be observed that the two gases begin gradually to combine, and if they be free from admixture with uncombined oxygen or excess of hydrogen, sudden combination with explosion occurs when they are exposed to the direct rays of the sun. The rapidity with which this combination occurs is proportional to the intensity of the light; and an instrument for measuring the amount of the action which is produced by diffused daylight was described by

127 d.]

ACTION OF LIGHT ON MIXED GASES.

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Draper, under the somewhat fanciful name of the tithonometer (Phil. Mag. 1843 [3], xxiii. 402).

(127 c) Photo-chemical Induction.-An elaborate investigation of the circumstances which influence the action of light upon a mixture of chlorine and hydrogen, by Bunsen and Roscoe, will be found in the Phil. Trans. 1857, 355.

From these investigations it appears to be probable that a species of induction precedes the chemical action. It was ascertained by Draper that on exposing the explosive mixture of chlorine and hydrogen to diffused daylight, the amount of condensation gradually increases for a few minutes, until it attains a maximum, at which point the rate of combination between the two gases continues to be constant for equal amounts of incident light. Draper attributed this slow attainment of the maximum rate, to an effect of light upon chlorine alone, in consequence of which it was gradually converted into a new and more active modification. Bunsen and Roscoe, however, did not find that either chlorine or hydrogen, when separately exposed to light, exhibited, after they had been mixed and again exposed, any action different from that observed when the gases were prepared and mixed in the dark and then exposed to light; they consider that the light acts by overcoming certain resistances which oppose the combination of the two gases; and this peculiar action they term photo-chemical induction. The time which elapses before the maximum action, due to the light, is attained, varies considerably in different experiments,-ranging from 3 or 4 minutes up to 10 or 15. The more intense the light, the more rapidly is the maximum attained, but the increase is in a greater ratio than the mere increase of light. This inductive influence upon the gases is not permanent. If they are placed in the dark for a short time, and are afterwards again exposed to the light, an interval of exposure similar to the first is required before the maximum rate of combination is attained. Dr. E. Budde (Phil. Mag. 1871 [4], xlii. 290) finds that when a differential thermometer containing chlorine gas, instead of air, is exposed to sunlight or to the ultra-violet rays of the spectrum, the gas expands: this he explains by supposing that some of the chlorine molecules are decomposed, free atoms of the element being thus produced, and which would account for the very energetic character of chlorine under the influence of actinic rays.

(127 d) Action of Light on Mixed Gases.-The presence of foreign gases in the mixture of chlorine aud hydrogen greatly

256

ACTION OF LIGHT ON MIXED GASES.

[127 d. diminishes its sensitiveness to the action of light: the addition, for example, of 3 parts of hydrogen, to 1000 of the mixture reduced the rate of combination for equal amounts of exposure from 100 to 378. The effect of oxygen is still greater; 5 parts of oxygen in 1000 of the mixture reduced its sensitiveness under similar exposure, from 100 to 9'7, and 13 parts of oxygen to 27. The following table shows the results obtained with these and some other gases,-taking the amount of condensation observed in equal times with the pure mixture of equal measures of chlorine and hydrogen as in all cases equal to 100:

Rate of Combination for Intervals of Equal Exposure to Light.

Bunsen and Roscoe found that a gas consisting of equal volumes of chlorine and hydrogen could be obtained with certainty, by the electrolysis of a solution of hydrochloric acid of density 1148, if a sufficient interval were allowed for the liquid to become saturated with the two gases. This gaseous mixture gave perfectly constant results for equal exposure to a light of uniform intensity, provided care was taken to insure the complete expulsion of air from the apparatus. The constant source of light which they employed was that of a jet of coal-gas, burned from a platinum nozzle, and connected with a special apparatus for regulating the efflux of the gas. The colouration of the flame by a trace of foreign matter materially affected its chemical activity.

Chlorine and carbonic oxide gas also enter slowly into combination under the influence of sunshine. Two volumes of chlorine and two of carbonic oxide in this manner become condensed into two volumes; the result is the formation of the irritating pungent gas known as phosgene gas, in allusion to the mode of its production. It is remarkable that the direct union of these gases cannot be effected by any other means.

Organic chemistry abounds with instances in which combinations and decompositions are effected by chlorine, under the in

127 e.]

DEOXIDIZING INFLUENCE OF LIGHT.

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fluence of the solar ray: some remarkable cases of this kind occur in the transformations of Dutch liquid, or ethylenic chloride, produced by chlorine. The operation of bleaching linen, by exposure to moisture and light for several weeks during summer, is another process which illustrates the influence of solar light in the production of chemical changes. But the chemical actions produced by the sun's rays, which are taking place unperceived around us, are of infinitely greater importance than those limited transformations which can be effected in the laboratory or the bleach-field; for it is upon these unobserved, yet daily renewed alterations, that the growth and renovation of the entire vegetable kingdom are dependent. The great chemical effect of light appears to be that of a powerful reducing or deoxidizing agent. Under the influence of solar light, the green parts of plants perform their allotted function in the purification of the atmosphere, by absorbing and removing carbonic anhydride, in virtue of which they fix the carbon in their tissues, and thus supply themselves with food; by a similar decomposition of water they obtain the hydrogen needed for their growth, while they return into the aërial ocean a portion of the oxygen with which the carbon and the hydrogen were previously in combination, and thus assist in maintaining that uniformity in the composition of the atmosphere which is indispensable to the healthful existence of animal life.

If solar light be excluded from plants, none of these decompositions are effected; the carbonic anhydride escapes unchanged into the air, and no fixation of carbon ensues; the plant becomes pale and succulent, whilst its functions languish. Gardeners take advantage of this knowledge in order to procure vegetables of more delicate flavour; by earthing up the plant, as is practised with celery, or by covering it with a tile, as in the case of endive, or by enclosing it in a bell-jar, as is usual with seakale, the light is more or less excluded, and the bleaching which is desired in vegetables for the table is produced.

(127 e) Reducing Influence of Light on Metallic Salts.-Much attention has been given to the study of the chemical actions produced by light, in consequence of the beautiful inventions of the Talbotype and the Daguerreotype. These remarkable processes, as well as others of a somewhat similar character, appear to depend upon the power which the more refrangible rays of the solar spectrum possess of causing the decomposition of the oxide, chloride, or bromide of silver, and of certain compounds of

258

PHOTOGRAPHIC PRINTING.

[127 e. some other of the metals. This decomposition by means of light, usually takes place under the concurrent influence of hydrogen, or of some metallic body, or other reducing agent, which acts either by setting free the silver or other metal, or by producing a lower oxide, chloride, or other compound of the metal. In other instances, as with argentic iodide, a molecular, and not a chemical change, appears to be produced by exposure to light. In this case there is no immediate change of colour, but it may be rendered visible by the reactions produced by the application of suitable chemical reagents to the compound after it has been so exposed to the solar ray.

The following instances of the effects of light have long been observed by chemists-If a piece of white silk be dipped into a solution of auric chloride, and exposed whilst in a moist state to the sun's light, the silk becomes first green, then purple, and in less than an hour a film of metallic gold is produced upon its surface. Argentic nitrate in solution in pure water undergoes no change when exposed to the light, but if any organic matter be added to the liquid, a black deposit is gradually formed; and if the salt be placed upon the surface of the skin, upon paper, or upon linen, the well-known blackening effect for which it is valued as a marking ink for linen is produced. Moist argentic chloride retains its dazzling whiteness if preserved in total darkness, but it assumes a violet tint, which gradually deepens in intensity, if exposed even to the diffused light of day, a portion of chlorine being liberated in the process.

(127) Photographic Printing.-The earliest experiments that have been published upon the production of pictures by the action of light, appear to have been made by Thos. Wedgwood and Davy in the year 1802. Wedgwood moistened white paper, or white leather, with a solution of argentic nitrate (nitrate of silver), and by its means copied paintings on glass, and took profiles; but neither he nor Davy was able to devise any means for preserving those pictures when exposed to diffused light.

The subject attracted but little attention until the commencement of the year 1839, when Fox Talbot made known (Phil. Mag. 1839 [3], xiv. 196) his process of photogenic drawing, which consisted in soaking ordinary writing-paper in a weak solution of common salt, and when dry, washing it over upon one side with a solution of argentic nitrate, consisting of 1 part of a saturated solution of the nitrate with 6 or 8 parts of water. This operation was performed by candle-light, and the paper was dried at the fire; in this manner a film of argentic chloride, mixed with an excess of argentic nitrate, was formed upon the surface of the paper. Suppose that it were desired to obtain a copy of an engraving, or of the leaf of a tree; one of the sheets so prepared was laid under the engraving or the leaf which was to be copied :