Acids * Acid-Sulphite
an acid liquid giving it a reddish tint. Alkalies or alkaline substances restore the blue colour.
Acids may be defined as compounds of hydrogen whose atom or atoms of hydrogen are replaceable by metals, or by radicals having metallic characteristics, and the compound resulting from such substitution is termed a salt. Acids, as the term is now understood, are not necessarily sour, even though they be soluble. (See Acidity above.)
Acid-Sulphite Fixing Bath. The use of a sulphite and a free acid in the fixing bath for negatives was shown some years ago by Lainer to be a useful expedient for removing the yellow tint resulting from pyrogallic development, and acid fixing baths are now very commonly used in fixing negative plates, it being generally considered that greater clearness and brightness results than when the simple hyposulphite bath is used. Professor Eder's method of preparing the acid fixing bath is perhaps the most convenient for general use, and is as follows: -
Mix the sodium sulphite solution with the tartaric acid solution, and then add to the hyposulphite. The above is almost equiva-lent to adding bisulphite of sodium to the hyposulphite solution, but for occasional use has the advantage of being perhaps more certain, as the simple sulphite of sodium is less liable to change by keeping than is the bisulphite. When bisulphite is used the following formula proves satisfactory: -
Large consumers may advantageously use a concentrated solu-tion of sodium bisulphite saturated with sulphurous acid gas, which is manufactured for use in bleaching works. That made by Messrs. Nashold of Aussig, Bohemia, and sold by them as acid-sulphite lye (saure sulphite lauge), is specially recom-mended by Dr. Eder as being more convenient - better and cheaper than the first-mentioned preparation. One litre (about