CHEMISTRY OF PHOTOGRAPHY. It has been said that light does not fall on any body in the universe, on which it does not leave traces of its passage, and that it cannot be absorbed or reflected, without in some way modifying the structure and properties of the sub stance on the surface of which it struck. All the effects which light thus produces are properly the subjects into which photography inquires. But it is impossible here to do more than make a general inquiry as to how light acts in common and easy cases, and en deavour to apply the information thus acquired to the explanation of the usual photographic processes. It will be easy for the reader to adapt the same principles to the phenomena of photo-chemistry in general.
The most simple case of luminous action will be mentioned first, because, while it is less complex than any other, it is also a striking type of the whole class of decompositions and syntheses effected by the solar radiations. The element chlorine is carefully isolated from all other substances, and exposed to the sun's rays. No apparent change takes place in it : its colour, volume, density remain the same ; but it was observed by Dr. Draper, that in such circum stances it undergoes a remarkable modification. For, whereas before solarization, it had no power to combine directly with hydrogen, it combines with it after that event with extreme readi ness, even in the dark. No other element, when alone, has yet been found to experience this kind of change in the light to the same extent, and this fact, therefore, places chlorine in the highest rank as a photographic re-agent : the increase of affinity of chlorine for hydrogen in the light, and of bodies of the same class, as chlorine, for substances resembling hydrogen in their chemical properties, appears to be the thread which runs through all photographic opera tions, and the basis of the art.
The effect produced on chlorine when alone, naturally leads to the expectation that a very decided action will take place, if hydrogen also be present at the time of exposure. Accordingly, we find a mixture of hydrogen and chlorine gases placed in the sunshine, immediately combine with such force as to produce an explosion, and the weakest daylight will produce a perceptible amount of condensation. This mixture has been proposed as an actinometer. The hydrogen and chlorine united, form hydrochloric acid, and the quantity formed in a given time, is in direct proportion to the intensity of the actinic force existing at the time in the luminous ether. No change has yet been found to be effected in hydrogen by solar action, and it may be that the action is confined entirely to the chlorine.
It is not necessary that the hydrogen should be in the elementary state to be capable of photosynthesis with chlorine. A great number of organic and other compounds of hydrogen, are decom posed by chlorine in the light. A solution of chlorine in water, for instance, when exposed to the sun, is speedily decomposed, for hydro chloric acid is formed, and oxygen given off to the atmosphere. H 0 + Cl.=H Cl. O. The action which thus takes place in hydrogenous bodies, in the presence of chlorine, is one of two. If the
compound body contain water, the hydrogen of this water is removed by the chlorine, as hydrochloric acid, and the oxygen unites to the remaining elements of the compound to form a new substance. But if the compound substance contain no water, hydrogen is removed as before, and forms hydrochloric acid with the chlorine ; but for every atom of hydrogen thus abstracted, an atom of chlorine is substituted to supply its place. Thus, in the case of alcohol, C4 H 02, what may be represented as a binhydrate of quadrihydrocarbon, or C4114, 2 HO, that is, one atom of quadrihydrocarbon, and two of water, when chlorine reacts upon it, the first effect is to take away the hydrogen of the water, and leave the two atoms of oxygen to unite with the C4 H4, and form acetic ether ; but when the water is thus decomposed, hydrogen is no longer removed, except by the subtitu tion of chlorine to fill up the gap. One atom after another is replaced in this manner by chlorine, with the production ,of as many different compounds as there may be atoms of hydrogen exchanged, until finally but one atom of hydrogen keeps its place, and the resulting substance is chloral (C4 Hi C1.2 03). Chloracetic acid is another remarkable instance of this change of substance by substitu tion. This compound was discovered by Dumas. It is formed by introducing a small quantity of concentrated acetic acid into bottles filled with dry chlorine, and exposed to the direct sunshine. White vapours are soon formed, and the chlorine disappears. In fierce sun shine the action is sometimes so intense as to cause explosion. The next day crystals are found in the bottles, of chlora.cetic acid ; and, on opening them, vapours as seen to escape of hydrochloric acid, carbonic acid, and chlorocarbonic or phosgene gases. In the crystals it is found that the whole of the hydrogen of the anhydrous acetic acid has be,en replaced by chlorine : thus C4 II3 03 H 0, has become C4 C1.3 03, H O. Chlonne also, in the presence of chloride of ethyl /C4, H Cl.), placed in the direct light of the sun, will remove two of he five atoms of hydrogen by forming hydrochloric acid, and at the same time put two of its own atoms in their place. An intermediate compound is first formed, when single atoms only are exchanged, and one atom of hydrocholoric acid only produced. So far is the mere fact of the hydrogen being in combination, from preventing its union with chlorine in the light, that in some instances the hydrochloric acid is formed with great violence. Thus, when light carburetted hydrogen or marsh gas (C is mixed with chlorine in the propor tion of one volume of the former to three of the latter, even the diffused light of day is sufficient to cause a violent explosion. The vessels are broken, hydrochloric acid formed, and carbon, and chloride of carbon deposited. Of course the difficulty of removing the hydrogen increases with the strength of the affinity which binds together the compound in which it exists, but no affinity is strong enough to prevent it altogether ; water, which is as stable a hydrogen compound as any, is easily decomposed by chlorine and sunshine.