The alchemists, amidst the multiplicity of their processes — in their vain search for the philosopher's stone and the elixir vitae stumbled upon a peculiar combination of silver with chlorine, which they called horn-silver — as, by fusion, the white powder they obtained by precipitation was converted into a horn — like substance. 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 realized. 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 crystallized more readily in the light than they did in darkness.