ALKALINE COLLODION. N. S. bath, toning bath, Ste. See "Collodion," &e.
ALILALTs. Certain oxides of metals and compound radicals which are of au acrid taste, which change vegetable blues to greens, and whose properties are masked or neutralized by their union with acids. Their actinic effects are opposed also to those of the acids. As acids, when free in the sensitive surfaces, have a retarding power over the formation of the photogenic imave, so alkalis, by neutralizing the acid, appear to possess a quickening influence; but when an excess of alkali is present, it precipitates oxide of silver by its proper force, without the access of light. Acids preserve the lights of photographs clean, under the action of the developer, by preventing spontaneous reduction of the gallo-nitrate, &c., and often by obliterating the faint impression of light up to a certain point ; alkalis produce universal blackening of the silver sUrface. In the hyposulphite bath, acids liberate sulphur, and produce changes which blacken the prints immersed in it, and then make them turn yellow and fade ; alkalis favour the stability of the bath and of the prints, but redden without weakening it. On the contrary, acids check the toning power of
sel d'or and increase its stability, while alkalis decompose and render it violent.
Potassa and soda are often called alkalis proper and fixed alkalis, because they are bodies which possess alkaline properties in fhe highest degree, and because they are not apt to fly off like ammonia, which has received the name of volatile alkali. But metallic protoxides in general have the alkaline quality of neutralizing acids, and when soluble, of affecting vegetable colours. A N. S. bath may have its free acid neutralized as well by oxide of silver as by potassa.
Alkaline salts, carbonates, chlorides, &c., are the carbonates, Ste., of the alkali. In some of these, as in nitrate of soda, the alkali is completly neutralized by the acid, but in the carbonates and others there still remains some alkaline reaction. The most generally useful alkaline substance for the photographer is carbonate of soda, and will be found to answer in almost all eases where acids are to be got rid of.