Thebo-lactic acid was discovered in Turkey opium by Messrs T. & H. Smith, the eminent morphia manufacturers of this city. It was examined by Stenhouse, and found to have the same composition as lactic acid, from which, however, it was supposed by the Messrs Smith to differ in the crystalline form of its copper and morphia salts. At present we are acquainted with three isomerie lactic acids, two of them differing from each other chemically, whilst the third is distinguished by its power of rotating the plane of polarisation of light. The last named acid, having been but recently discovered, it is impossible to say whether it possesses any decidedly distinctive chemical properties or not. The other two, namely, the ordinary or ethyliden—and the ethylen-lactic acids, possess perfectly distinct chemical properties, determined by the different relative position in each of the alcoholic hydroxyl.