which becomes discoloured when the mixture is exposed to the light, should be rejected. One convenient and easy way of approximately testing the strength of acetic acid is by deter-mining the specific gravity by a method explained in the article Hydrometers and Hydrometry, and then referring to the special table which is given for the strengths of acetic acid ; but an irregu-larity there noted renders this method subject to error in certain cases.
Acetone. Formula, C3H60, or di-methyl-ketone. A volatile liquid having a pleasant ethereal odour, prepared by the dry distillation of acetates - calcium acetate, for instance. Acetone mixes with water or alcohol in all proportions, boils at about 57o C, and can be used as a substitute for alkali in the developer, as shown by Lumiere and Seyewetz. (See Development and Developers.) Acetone is a powerful solvent of most resinous substances, and it also very readily dissolves pyroxyline; hence is of special use in repairing articles made of celluloid, the edges being well softened by acetone and pressed into close contact unite firmly, and when dry the article may be as good as if unbroken. Small baths, dishes or protective casings may be easily made from old celluloid films by taking advantage of the softening action of acetone on celluloid.
Aceto-Nitrate of Silver. A mixture of acetic acid and solution of nitrate of silver, used in calotype and some of the earlier processes.
Acetylene. Formula C2H2. A gaseous hydrocarbon, obtained by E. Davy, in 1836, by treating an impure carbide of potassium with water; and somewhat later by Wohler, who obtained it by the action of water on carbide of calcium. Acetylene is formed in many reactions, one of the most interesting being by the direct combination of carbon and hydrogen at a high temperature ; as, for example, when an electric arc is produced in hydrogen. Acetylene is remarkable among hydrocarbons as containing stored in itself a large excess of energy, usually referred to as heat; hence such a substance is called endothermic. By virtue of its endothermic nature it is very ready to enter into reaction with other substances, and can, under certain conditions, explode