A Manual Of Photography

CHLORIDE 0F SILVER. 127 It is necessary now to direct attention to the effects of organic matter in accelerating the blackening process. Sir John Herschel, whose researches in this branch of science are marked with his usual care, has given particular attention to this matter. As it is impossible to convey the valuable information that Sir John has published, more concisely than in his own language, I shall take the liberty of extracting rather freely from his memoir, published in the Philosophical Transactions. " A great many experiments were made by precipitating organic liquids, both vegetable and animal, with solutions of lead; as also, after adding alum, with alkaline solutions. Both alumina and oxide of lead are well known to have an affinity to many of these fugitive organic compounds which cannot be con- centrated by evaporation without injury,—an affinity sufficient to carry them down in combination, when precipitated, either as hydrates or as insoluble salts. Such precipitates, when col- lected, were applied, in the state of cream, on paper, and, when dry, were washed with the nitrate. It was here that the first prominently successful result was obtained. The precipitate thrown down from a liquid of this description by lead, was found to give a far higher degree of sensitiveness than any I had before obtained, receiving an equal depth of impression, when exposed, in comparison with mere nitrated paper, in less than a fifth of the time ; and, moreover, acquiring a beautiful ruddy brown tint, almost amounting to crimson, with a peculiarly rich and velvety effect. Alumina, similarly precipitated from the same liquid, gave no such result. Struck by this difference, which manifestly referred itself to the precipitate, it now occurred to me to omit the organic matter (whose necessity I had never before thought of questioning), and to operate with an alkaline precipitant on a mere aqueous solution of nitrate of lead, so as to produce simply a hydrate of that metal. The result was in- structive. A cream of this hydrate being applied and dried, acquired, when washed with nitrate of silver, a considerable in- crease of sensitiveness over what the nitrate alone would have given, though less than in the experiment where organised matter was present. The rich crimson hue also acquired in that case under the influence of light, was not now produced. Two peculiarities of action were thus brought into view ; the one, that of the oxide of lead as a mordant (if we may use a term borrowed from the art of dyeing), the other, that of organic matter as a colorific agent. " Paper washed with acetate of lead was impregnated with various insoluble salts of that metal—such as the sulphate, phos- phate, muriate, hydriodate, borate, oxalate—and others, by wash-