A Manual Of Photography

SELECTION OE PAPER POP PHOTOGRAPHIC PURPOSES. 191 IV.—Papers prepared with Iodide or Bromide Nitrate of Silver. Nit of Potassium and a. Superfine satin post. b. Thick wove post ... c. Superfine demy.... d. Bath drawing card . e. Thick post........ f. Common hank post. g. Thin post......... h. Tissue paper...... . Darkens slowly. . Results uncertain. . Strong exalting influence. . Very slowly changes. . Depressing influence. . Slight exalting effect. . Ditto. . Results uncertain. Unsized paper has been recommended by some, but in no instance have I found it to answer so well as paper which has been sized. The principal thing to be attended to in preparing sensitive sheets, is to prevent, as far as it is possible, the ab- sorption of the solutions into the body of the paper,—the materials should be retained as much as possible upon the very surface. Therefore the superficial roughness of unsized sheets, and the depth of the imbibitions, are serious objections to their use It must not, however, be forgotten, that these objections apply in their force only to the silver preparations ; in some modifications of the processes, with the bichromate of potash, the common bibulous paper, used for filtering liquids, has been found to answer remarkably well, on account of the facility with which it absorbs any size or varnish. Great annoyance often arises from the rapid discolouration of the more sensitive kinds of photographic drawing paper, inde- pendent of the action of light, which appears to arise from the action of the nitrate of silver on the organic matters of the size. Unsized paper is less liable to this change. If we spread a pure chloride of silver over the paper, it may be kept for any length of time without any change of its whiteness taking place in the dark. Wash it over with a very weak solution of nitrate of silver, and, particularly if the paper is much sized, a very rapid change of colour will take place, however carefully we may screen it from the light. From tins it is evident that the organic matter of the size is the nrincipal cause of the spontaneous darkening of photo- graphic, papers prepared with the salts of silver. 6jc ^st curious part of the whole matter is, that in many eases this change is carried on to such an extent that a revival of metallic silver takes place, to all appearance in opposition to “ce of Xity. This is very difficult to deal with. Che- mistry has not yet made us acquainted with any organic body which would separate either chlorine or nitric acid from their metallic combinations. I can only view it in this light ■.—the