Solidifying points of acetic acid of different strength : A curious fact that should be noted is that the liquid containing zoo per cent. of acetic acid has the same specific gravity as one containing between so and 6o per cent.
It often happens that a trace of sulphuric acid used in its manufacture is unintentionally left in it, and the presence of either sulphuric or hydrochloric acid is fatal to photographic processes, for the reason that they quickly precipitate nitrate of silver. This fact enables one to readily detect the presence of either in the following manner : Dissolve a small crystal of nitrate of silver in half a drachm of water, and then add another half a drachm of the glacial acetic acid to be tested, and if this mixture remains quite clear when exposed for a little time to the light, the acid can be used with safety. If, on the other hand, there is either hydrochloric or sulphuric acid present, a white deposit of chloride or sulphide of silver is quickly formed, and if aldehyde or any other volatile matter be present, the mixture will soon discolor if exposed to the action of daylight. If we have the white deposit only, and we are desirous of finding out
whether it be hydrochloric or sulphuric acid that is present, it is easily done by the addition of nitric acid, which rapidly dissolves the sulphite, but leaves the chloride unchanged.
Another form of acetic acid is sold in England under the name of Beaufoy's Acid. It contains 3o per cent, of real acid, so that in making up formulas it is necessary to treble the quantity, in order to get the correct amount of acetic acid. It should also be tested for impurities, as it usually contains many.
Acetic acid is sold in this country as " Acetic Acid No. 8." This is usually glacial acetic acid diluted with eight times its volume of water.
Acetic acid is miscible in all proportions with water and is employed in photography as a solvent for gelatine in aceto-gelatine emulsions, and as a restrainer in acid development forming silver acetate with the depositing silver.