TARTARIC ACID. 947 



purified, as cream of tartar. To procure tartaric acid, cream of 

 tartar is dissolved in boiling water and neutralised with car- 

 bonate of lime, which throws down one half of the tartaric acid 

 as an insoluble tartrate of lime; the other half of the acid 

 contained in the neutral tartrate of potash, is precipitated by a 

 solution of chloride of calcium. The tartrate of lime is decom- 

 posed by an equivalent quantity of sulphuric acid, and the 

 tartaric acid separated from the insoluble sulphate of lime by 

 nitration ; 3 parts of oil of vitriol are usually taken for 5 parts 

 of cream of tartar. The acid solution is evaporated in leaden 

 vessels by a gentle heat, during which process some sulphate of 

 lime is deposited. A syrupy solution of tartaric acid left in a 

 warm place yields crystals which are oblique prisms of a rhombic 

 base, terminated by dihedral summits and truncated on the 

 longitudinal edges, or hexagonal prisms terminated by three 

 faces of truncation ; but the two parallel faces are generally 

 more developed than the others, so as to give the crystals the 

 appearance of tables. The crystals are persistent in air, of a 

 strongly acid and agreeable taste, dissolve in 1 J parts of cold 

 water and in less hot water, and are equally soluble in alcohol. 

 The aqueous solution of tartaric acid and its salts undergoes 

 decomposition, and becomes covered with mouldiness. When 

 the crystallized acid is heated, it loses water and produces a series 

 of new compounds. Treated at a high temperature with a 

 strong solution of hydrate of potash, tartaric acid is entirely 

 converted into acetate and oxalate of potash ; one atom of crys- 

 tallized tartaric acid actually containing the elements of 1 atom 

 of hydrated acetic acid and two atoms of hydrated oxalic acid : 



2HO + C 8 H 4 10 =HO,C 4 H 3 3 and 2 (HO,C 2 O 3 ). 



A solution of tartaric acid does not disturb solutions of 

 chloride of barium and chloride of calcium, but produces a white 

 precipitate in barytes and lime-water, and in acetate of lead. 

 A solution of tartaric acid is also used in precipitating potash 

 from its salts, when not very dilute, the bitartrate of potash 

 falling down, upon agitation, as a granular precipitate, which is 

 sparingly soluble in water, but dissolves readily in hydrochloric 

 acid. The addition of tartaric acid to many metallic solutions, 

 prevents their precipitation by alkalies. 



Tartrates. According as one or both atoms of water in tar- 



2 Q Q 2 



