A.-PREPARATION OP PAPER WITH SALTS OF SILVER.

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after adding alum, with alkaline solutions. Both alumina and oxide oflead are well known to have an affinity to many of these fugitive organiccompounds which cannot be concentrated by evaporation without injury,—an affinity sufficient to carry them down in combination, when pre-cipitated, either as hydrates or as insoluble salts. Such precipitates,when collected, were applied in the state of cream on paper, and whendry were washed with the nitrate. It was here that the first promin-ently successful result was obtained. The precipitate thrown downfrom a liquid of this description by lead, was found to give a far higherdegree of sensitiveness than any I had before obtained, receiving anequal depth of impression, when exposed, in comparison with meronitrated paper, in less than a fifth of tlio time; and, moreover, acquiringa beautiful ruddy brown tint, almost amounting to crimson, with apeculiarly rich and velvety effect.* Alumina, similarly precipitatedfrom the same liquid, gave no such result. Struck by this difference,which manifestly referred itself to the precipitate, it now occurred tome to omit the organic matter, (whoso necessity I had never beforethought-of questioning,) and to operate with an alkaline precipitant ona mere aqueous solution of nitrate of lead, so as to produce simply ahydrate of that metal. The result was instructive. A cream of thishydrate being applied and dried, acquired, when washed with nitrate ofsilver, a considerable increase of sensitiveness over what the nitratealone would have given, though less than in the experiment whereorganized matter was present. The rich crimson hue also acquired inthat case under the influence of light, was not now produced. Twopeculiarities of action were thus brought into view; the one that of theoxide of lead as a mordant, (if we may use a tenn borrowed from theart of dyeing,) the other, that of organic matter as a colorific agent.—

“ Paper washed with acetate of lead was impregnated with variousinsoluble salts of that metal, such as the sulphate, phosphate, muriate,liydriodate, borate, oxalato, and others, by washing with their appro-priate neutral salts, and when dry, applying the nitrate of silver asusual. The results, however, were in no way striking, as regards sen-sitiveness, in any case but in that of the muriatic applications. In allcases where such applications were used, a paper was produced infinitelymore sensitive than any I had at that timo made. And I may heroobserve, that in this respect the muriate of strontia appeared to havedecided advantage.—

“ The paper with a basis of lead turns yellow by keeping in the dark,and tho tint goes on gradually deepening to a dark brown. But, whatis very singular, this change is not equally rapid on all kinds of paper,a difference depending no doubt on the size employed, which, it may beobserved hero once for all, is of tho utmost influence on all photographicprocesses. In one sort of paper, known by tho name of blue wove post,

* It lias been found that this rich tint may be communicated by soaking the drawingformed on many of the nitrated papers in a saturated solution of sugar of lead.

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