ACIDS. The uses of acids for special purposes, not actino-chemi cal, as in the making of pyroxylin, cleaning vessels, removing the size of paper, he., are specified under the proper heads. Their action in photo-chemistry is what concerns us here. Acids are substances sour to the taste, which change vegetable blues to reds, And which unite with alkalis or other bases in such a way as to neutralize each other's properties. They are of two kinds, clienai cally considered : oxy-acids or acid oxides, and hydracids or com pounds of non-metallic substances, and some compound radicals with hydrogen. The hydracids decompose the soluble silver salts, and therefore completely bar the dOvelopment of photogenic images, while the decomposition of the acids also prevents our studying their effects, on the formation itself of the image by light. The oxyacids as a class exert in a greater or less degree a retarding effect on the rapidity of formation of the image, which is very re markable ; this is in proportion to their oxidising power. The gallic and pyrogallic acids exert, on the contrary, a powerful reducing or deoxidizing influence, which has made of them our most usefid developers. But while gallic has very feeble acid properties, pyro gallic is not strictly an acid at all : and both act more like sugar and other neutral organic substances, than like true acids.
Of the oxyacids, those which produce insoluble salts of silver when present in the film or the N. S. bath, retard the production of an invisible image by the liberation of free nitric acid, and if they are added to the developer, they will partially or entirely annul its action : they are the oxalic, phosphoric, sulphurous, and others. Those which give salts of silver more or less soluble, vary in their effects according to their oxidising power. The vegetable acids, as acetic, citric, radio, and tartaric, are the lowest in the scale ; the mineral acids, perchloric, sulphuric, 8tc., are next, and nitric is the
strongest. One twentieth of a minim of nitric acid in the N. S. bath, will perceptibly injure its sensibility, and will have a still more decided effect in weakening the developed image, and keeping the reduced silver pure and free from organic matter. The organic acids, however, affect the sensitiveness but little, while they give colour and intensity in a considerable degree. These effects are most strongly perceived when the acids are added to the exciting bath ; but a larger quantity added to the developing solutions have a corresponding influence. The difference observed in various kinds of collodion, ether, paper and its sizing, and other material, may be always traced to the presence of acid bodies. In all negative pro cesses, it Will. be judged essential, in consequence of these established reactions, to remove every trace of nitric acid, and make use of a vegetable acid to preserve clearness. The acetic, malic, and suc cinic, act much alike, but the acetic is rather the best negative a eid ; and all may be used in tolerable quantities without very rapid difference in the results. Citric acid must be used much more carefully, for while even 5 or 6 minims of acetic may be added to eachl. ounce of the N. S. bath without causing vastly more change in it than 1 minim, citric must be added by proportion at least 20 times as small, and each addition will increase the effect per ceptibly.
The consequences of employing acids in the fixing, toning and other solutions, with the effects of acids in reacting on finished photographs, will be discussed under the proper words.