Acetic Acid, HC2H302 ( = CH3.CO.OH), the acid principle of vinegar. It is produced in nature by the fermentation of alcoholic liquids, and its formation in this way accompanies the growth of a fungus, Mycoderma aceti, to the activity of which, as a carrier of oxygen, its development is due. In countries where alcohol is cheap, acetic acid is manufactured by this process of fermentation. In England it is mainly obtained by the dry distillation of wood. The crude acid obtained by the latter method is termed Pyroligneous Acid, and requires purification from tar and wood spirit. Pure acetic acid, as obtained from pyroligneous acid and vinegar, by processes of refinement, is a colourless liquid which congeals below 16° C.,and is hence called Glacial Acetic Acid, B.P. 118° C; S.G. 20/4= 1.05. It can be mixed in all proportions with water, alcohol, and ether; and forms salts called Acetates, which, for the most part, crystallise well and are very soluble in water. Acetic acid, as usually sold, is a mixture of pure acid and water; as defined by the British Pharmacopoeia, it contains about 33 per cent. of the glacial acid. It is used in manufactures for calico printing, and the preparation of acetates.