Hitherto, for the sake of simplicity, no mention has been made of any photo-sensitive element but chlorine ; yet oxygen, bromine, iodine, cyanogen, and others, might have been, at the same time, adverted to. Neither of these is capable of uniting, as chlorine does, with gaseous hydrogen directly when exposed to light : they are, therefore, weaker reagents in those changes which are deter mined by solar radiations ; but from other indications we conclude that their action on combustible bodies, which contain combined hydrogen and carbon, is still very powerful when light falls upon them. Thus, the drying oils, as they are called, confined in a vessel with pure oxygen, at first scarcely absorb any in the dark, but, ex posed to strong light, they are speedily oxidized and converted into gelatinous substances which do not stain paper. As in the case of chlorine, substituted for hydrogen, which we mentioned aboye, the oxygen here removes carbon in the form of carbonic acid, and at the same time replaces it with oxygen ; no water is given off, though pro bably formed in the oil itself. A similar though weaker action takes place with other fat oils. Fats and butter absorb oxygen in the light, and become acid. The essential oils absorb oxygen, and this absorp tion is much more actively carried on in the light, and is the reason of their being odorous. During the oxidation, some give off car bonic acid and a small quantity of hydrogen, become more and more soluble in alcohol, and more solid, and are partially converted into camphors 'or resins : others give hydrogen to form water with the oxygen, which water unites with the remaining elements, to form solid hydrates. Bitter almond oil, in the air and sunshine, absorbs two atoms of oxygen, and is converted into solid hydrated benzoic acid 116 a Os, HO). In the similar case of chlorine, bitter almond oil and light, 2 Cl. +H Cl., but the hydrochloric acid (H Cl.), instead of remaining in the compound as the water (H 0) does, is liberated. Oil of cin namon is decomposed in precisely the same manner. Gum guaia cum, in the light, absorbs oxygen and changes colour to green. The bleaching and the coloration of oils by light, are attributable to the same species of reaction. In the photographic processes with bitumen, this property is taken advantage of, and in the photo lithographic processes generally. Carbon, in certain of its organic forms, has also the property of uniting with oxygen in the light, and in many of the gradual changes which light operates in complex organic substances, both the carbon and the hydrogen which they contain are oxidized. Bromine and iodine unite directly with car bon and hydrogen, existing in the gaseous state in the form of olefiant gas, when exposed to the sun's rays, though they do not appear to have any such power on isolated hydrogen or carbon. They are also both capable of entering like oxygen and chlorine, by a process of gradual substitution, into numberless organic com pounds of hydrogen, carbon, and oxygen : and an inquiry into the exact interference of light in these cases, constitutes the real field of scientific photography. Whenever chemists speak of the pro tracted or gradual effects of these electronegative bodies upon others which are more electropositive, the influence of light is to be suspected ; and their mutual reaction, when recently prepared, and when they have never seen the light at all, ought to be in such cases examined. The processes of fermentation, putrefaction, spon taneous decomposition, and change of form, and spontaneous com bustion, and other changes often spoken of as if they were effects without causes, should also be studied in their relation to actinic phenonfta.
The other imponderable fluids, as well as light, have the power of increasing the affinities of chlorine and hydrogen, and of their respective congeners. Thus, hydrochloric acid may be formed by passing electric sparks through the mixed gases, or by subjecting them to a certain degree of heat, and both these are capable of effecting combinations which light is too weak to produce. These same agents are even found, where they exert different degrees of force, or in different circumstances, to have both the faculty of uniting elements, and of decomposing the compound which their union forms. Thus, hydrochloric acid may be decomposed into its elements by the electric spark as well as formed by its agency.
And the like effect has been observed in photography, to attend the ac tion of the actinic rays, and has been called " reverse action of light." From a comparison of the effects produced by these fluids upon the two classes of substances which have been spoken of above, in different circumstances, we learn that the order of affinities, as far as relates to the "sensitiveness" of the elements, is as follows : Chlorine Fluorine Phosphorus Oxygen Cyanogen Sulphur Bromine Hydrogen Selenium Iodine Carbon Boron.
This order is the same for electricity, light, and heat alike. The metals have not been classed, because if, as,analogy would intimate, the affmity of chlorine for them is increased by light, and not dimi nished, as some suppose, this has not hitherto been taken advantage of in photography. If we wish to form a table of hydrogen and car bon compound bodies, arranged in the order of their affinity for chlorine, &c. in the light, some modification of it would be ne,cessary for each element, in accordance with the nature of the decomposition which would take place.
With respect to the mode in which light operates in modifying the chenaical relations of bodies, we are almost entirely in the dark. It has been disputed in some cases, whether actual decomposition is produced, or only a change in the arrangement of the constituent atoms ; but it is clear that a molecular change must first be induced by light, and that this must end in decomposition. The precise moment when such decomposition commences is beyond our per ception, and is not, in any case, necessary to be known. What the nature is of this molecular modification, is the interesting question. With respect to oxygen, we know that it is capable of existing in two forms—the active 'or allotropic form found in ozone, and the usual and less active form. Allotropic oxygen is often formed from ordlinary oxygen by electricity, and recent experiments have shovvn that it is also formed by light, in those cases where bitt,er almond oil, and other such bodies are present. It is therefore fair to con clude that light acts on the bodies which resemble oxygen in a like manner, and the chlorine, which has been altered in its degre,e of activity by solarization, will be active or allotropic chlorine. And so of the rest. Bodies like hydrogen, combustible hydro-c,arbons, and phosgene gas, are so remarkable for their high refractive power on light, that Newton was led to suppose that the diamond was combustible merely from its index of refraction. Some essential oils and hydro-carbons can only be distinguished by a peculiar power of rotating a polarized ray of light to the right or to the left. These, and other like phenomena, would lead us to suppose that the action of light is to polarize the elementary atoms, and that their different properties are the effects of their polarity. A change of polarity in one element in a compound would immediately affect the whole, and might lead to its decomposition in the presence of a se cond body, or to the assumption of a different arrangement of its own elements if isolated. In either case the result would be a dis tinct chemical change. But light may perhaps, without producing an actual change of polarity, set up in certain cases a vibratory action among the particles themselves, similar to the regular vibra tions produced by sound in sonorous bodies. This kind of action is sufficient to account for the phenomena observed in the formation of images by the condensation of vapours, as in the experiments of M. Moser. The condensed vapours then arrange themselves in the lines of vibration, just as in the figure.s of Chladni, minute solid par ticles arrange themselves on the sonorous surface.
The facts, therefore, which are established by experiment, and which must explain for us the formation of photo-chemical images, are— First, The increase of affinity in the, light of chlorine, oxygen, bromine, and iodine, for hydrogen and carbon.
Second, The regular decomposition which occurs in certain cases in consequence of this increase, capable of being represented by chemical formula).
Third, The gradual and progressive nature of this decomposition.
It remains to apply these to the explanation of the two processes to which all others used by photographers may be referred