Tartaric Acid. The ordinary variety of tartaric acid is a compound which occurs very widely in the vegetable kingdom, being found in many fruits. It is most abundant, however, in grapes, where it exists as an acid potassium salt, which thus forms a deposit in wine-casks, which is known as argol. From this argol most of the commercial tartaric acid is obtained. The acid is a dibasic acid, possessing the formula C4H6O6 or [CH(OH)C02H]2. It forms crystals of the monoclinic system, which dissolve easlly in water. Its solution acts on polarised light, being what is known as dextrorotatory. It melts at 180° and is converted into an amorphous form known as meta-tartaric acid. It forms a series of salts known as tartrates; of these the acid potassium salt is almost insoluble in water and is formed as a white precipitate when the acid is added to a solution of a potassium salt. It is well known under the name of cream of tartar. A double salt of potassium and sodium is known under the name of Rochelle salt or Seignette's salt; while a compound with oxide of antimony is emp1oyed medicinally as tartar emetic (q.v.). Besides this ordinary variety, a form which melts at 167°, and is laevorotatory, and also a form which does not act upton polarised light, are known. A mixture of the two active forms usually results from the artificial preparation of the acid and is known as racemic acld. It very closely resembles the ordinary tartaric in all its properties, and by certain chemical methods may be separated into its two active constituents. Other such compounds, also consisting of a mixture of two optically active compognds, so as to form an apparently inactive substance, are known by the general term of racemates.