Lactic Acid. This acid is present as a constituent of very many natural products, as in the sap of the vine, in certain parts of the body, and in fermented liquids. It occurs in sour milk (hence name), being formed by the fermentation of the milk-sugar, and in this source it was first discovered in 1780 by the Swedish chemist Scheele. It is best prepared by the fermentation of sugar, induced by adding some putrid cheese, sour milk, and zinc carbonate to the solution of sugar and allowing it to stand in a warm place. The sugar first forms glucose. It may also be prepared by numerous synthetic methods. It is a colourless, thick liquid soluble in water, and of specific gravity 1.2. Besides this ordinary lactic acid, an acid of precisely the same constitution is obtained from flesh, called sarco-laetic acid, which only differs from the previous acid by acting on polarised light. Another isomeric acid is hydracylic acid, which, however, differs in many of its properties from lactic acid and has not the same constitution. Another variety has been also described, but its existence not well proved.