ALLOXAN or MESOXALYL UREA, crystallizes from water in colourless rhombic prisms containing four molecules of water of crystallization. It possesses a very acid reaction; it colours the skin purple-red, and with ferrous sulphate it develops an in digo-blue coloration. It is an oxidation product of uric acid, being obtained from it by the action of cold nitric acid, drochloric acid in the cold convert it into alloxantin (q.v.), hydrox ylamine to nitroso-barbituric acid, C4H2N203 : NOH, baryta water to alloxanic acid, ; hot dilute nitric acid oxidizes it to parabanic acid (q.v.), hot potassium hydroxide solution hydrolyses it to urea and mesoxalic acid (q.v.), and zinc and hot hydrochloric acid convert it into dialuric acid, Alloxan combines with thiourea in alcoholic solution in the presence of sulphur dioxide to form pseudothiouric acid. Methyl and dimethylal loxans are also known, the former being obtained on oxidation of methyl uric acid, and the latter on oxidation of caffein'. (See PURINES.)