Nor is it necessary, it may be further remarked, that the chlorine, any more than the hydrogen, should be in the elementary state in order to produce its usual reaction with hydrogen in the light. Both the hydrogen and the chlorine may exist in combination in compounds more or less stable, in both of which the affinities may be far from weak, and yet the light may have the power of causing their mutual decomposition. Thus, the sub-chloride of sulphur will be gradually decomposed by water in ordinary daylight, and tetra hedral crystals of sulphur may be thus obtained. The compounds of phosphorus, silenium, carbon, silicon, and perhaps sulphur with chlorine, decompose water, and hydrogenous liquids in the same manner. The terchloride of nitrogen is a substance very dangerously explosive in the presence of hydrogenous and other combustible bodies. A small globule of this under water causes a tremendous explosion by mere contact with a drop of fixed oil, which disperses the water with great violence, and emitting a momentary flash of light breaks the containing vessel into atoms. But these compounds of chlorine are not the most stable of the class. As the affinities increase in strength, the difficulty of overcoming them by hydrogen increases. The alkaline and earthy chlorides are not decomposable in this manner, nor are the majority of metallic chlorides. Those only appear to be thus affected which are very easily reduced to the metallic state, or which pass without great difficulty from a higher to a lower state of chloridation, as from bichloride to chloride, or from chloride to sub-chloride, as the chloride of silver.
These are the principal cases in which chlorine is combined with hydrogen by the action of light. But hydrogen, though by far the most important, is not the only body with which chlorine has greater power of combining in the light than the dark. The element ap proaching nearest to it in this property is carbon. It requires, pre vious to its combining with chlorine, to be like hydrogen in the gaseous state, or else in the state in which it exists in organic com pounds. These two bodies are thus associate,d together universally in all animal and vegetable bodies, and are also acted upon by chlorine in the same manner, but in very different degree. When mixtures of olefiant gas and chlorine (C2 H2 and Cl.), or marsh gas and chlorine (C H2 and Cl.), are exposed to light, both the hydrogen and the carbon enter into union with chlorine, and in the first mixture a triple compound is formed, unless great excess of chlorine is present. The results in both cases are hydrochloric acid and chloride of carbon. The chlorided compounds of carbon are either solid or liquid, but of hydrogen gaseous. It might be supposed that it was the hydrogen which in these cases det,ermined the com bination of carbon and chlorine, but the fact that the hydrogen does not take the whole of the chlorine when there is not enough to saturate it, shows that this is not the case; and instances occur where no hydrogen is present, and where still the carbon and chlorine are induced to unite by solar action. Equal volumes of chlorine and carbonic oxide gases (Cl. and C 0) exposed to ordinary daylight, unite very gradually, but in the sunshine very quickly, and in the dark not at all. It was on account of its mode of formation, called by Dr. Davy, phosgene (ow, light, and yevvato, to produce). Carbon also, existing in alcohol, and in many organic compounds, has a great tendency to unite with chlorine, and this tendency is favoure,d by the action of light. Phosphorus, sulphur, boron, selenium, silicon, would seem, from many reactions, to be similarly attracted by solarized chlorine, though in a degree not equal to hydrogen and carbon ; and this property, as affecting them, is scarcely resorted to in photography.
But is chlorine separated from any of its compounds by the influence of light ? Its union with hydrogen and carbon, as pro duced by that agency, is sufficiently demonstrated by the preceding experiments. Are its compounds ever decomposed by the same
agency ? In the presence of hydrogen or its compounds, yes ; otherwise it seems probable that they are not. This is opposed to the popular opinion on this subject. Chloride of silver, bichromate of potass, citrate of silver, ammonio-tartrate of iron, and other such preparations, are constantly spoken of by photographers as if they suffered decomposition merely as the effect of solar light. The light is said to loosen the affinity of chlorine and silver for each other, to have a deoxidizing power, to be a destructive agent. Of course, it would not be more strange that it should lessen affinities than that it should exalt them, and the question is one entirely of experiment. It does not appear, however, that any experiment has yet been recorded which proves that the change produced by light is of this kind ; while the experiments made by Seebeck and Scheele, with a direct view to determine the point, are strongly conclusive against it. Chloride of silver, covered, in a stoppe,d phial, with sulphuric acid, was found by Seebeck to remain uncoloured in-the sun's rays ; taking the stopper oult was sufficient to cause its discoloration, as was also the addition of water to the acid. Scheele's idea was, from his experiments, that chloride of silver was not decomposed by light, except in the presence of hydrogen or its compounds. Chloride of silver in the dark is not discoloured, even when pure dry hydrogen is present, but is instantly in the light. Compounds of chlorine, much less stable than the silver salt, are not decomposed by light in this manner, though they are very quickly changed when hydrogen in any form is present. Such bodies appear to be very stable when perfectly protected from all combustible substances which can take the chlorine. Even the much stronger current of electricity, pro duced by a galvanic circle, appears to have no power of separating elements in such a direct and abrupt manner. We are not without indications in our ordinary photographic operations, which show that the elements which are concerned in the decompositions must be balanced with tolerable nicety to allow the luminous ether to inter fere with them, and such a compound as chloride of silver would be too stable to allow its affinities to be overcome by so weak a force. The affinity of chkaine for silver is not much weaker than the attrac tion of hydrogen for chlorine, or for oxygen, if we may judge by their mutual reactions, and would therefore be as likely to resist the action of light as wat,er itself. The only example produced by chemists, of decomposition taking place directly by light, so far as we are aware, is that of chlorous acid. Chlorine is here (Cl. 0) as sociated with an element so nearly resembling itself in its relations, that it is difficult to say which is the negative and which the positive. They replace each other in many compounds : their affinities for other bodies are of equal strength, tmd directed to the same elements, and their attraction for each other in this compound so slight, that the warmth of the hand is often sufficient to cause their dissolution. It is said that chlorous acid is not changed by several hours' exposure to daylight, but that a few minutes' sunshine decompose it without explosion. But it is more probably the heat of the sun's rays which effect this change than the actinism, since a slight elevation of temperature will decompose this gas with explo sion. As hydrogen in many views resembles the metals, analogy would suggest that the effect of light would be to increase the affinity of chlorine for silver, rather than diminish it. For these reasons it seems necessary to regard the presence of hydrogen, or some such substance, as requisite in photal reactions. . The nature of the hydrogenous compounds must therefore be calculated, in all photographic preparations, and in all reasonings on the chemistry of the processes.