A Manual Of Photography

4 HISTORY OF PHOTOGRAPHY. It has been stated—but on doubtful authority—that the jug- glers of India were for many ages in possession of a secret process, by which they were enabled in a brief space to copy the profile of any individual by the action of light. However this may have been, it does not appear that they know anything of such a process in the present day. The alchemists, amidst the multiplicity of their manipulatory processes, in their vain search for the philosopher’s stone and the elixir vitæ, stumbled upon a peculiar combination of silver with chlorine, whieli they called horn silver—as, by fusion, the white powder they obtained by precipitation was converted into a horn-like substance, chlorine as a chemical element being un- known to them. They observed that this horn silver was blackened by light, and as they taught that “ silver only differed from gold in being mercury interpenetrated by the sulphureous principle of the sun’s rays,” they concluded that this change was the commencement of the process by which their dreams were to be realised. Failing, however, to produce gold from horn silver, the fact of its blackening was simply recorded, and no further investigations were made into this remarkable phenomenon. Petit, in 1722, noticed that solutions of nitrate of potash and muriate of ammonia crystallised more readily in the light than they did in darkness. The illustrious Scheele (1777), in his admirable Traité de V Air et du Feu, gave us the first philosophical examination of the peculiar change in the salts of silver, and showed the dissimilar powers of the different rays of light in effecting this change. He writes, “ It is well known that the solution of silver in acid of nitre poured on a piece of chalk, and exposed to the beams of the sun, grows black. The light of the sun reflected from a white wall has the same effect, but more slowly. Heat without light has no effect on this mixture.” Again, “ Fix a glass prism at the window, and let the refracted sun-beams fall on the floor; in this coloured light put a paper strewed witli luna cornua, and you will observe that this horn-silver grows sooner black in the violet ray than in any of the other rays.” Senebier repeated these experiments, and he states that he found chloride of silver darkened in the violet ray in fifteen seconds to a shade which required the action of the red ray for twenty minutes. He also experimented on the influence of light in bleaching wax. In the Philosophical Transactions for 1798 will be found a memoir by Count Rumford, entitled, " An Inquiry concerning the Chemical Properties that have been attributed to Light.” In tins paper a number of experiments are brought forward to prove