Two peculiar principles have been discovered by Proust, in the caseous matter of milk, which he has named Caseic acid, and Caseous oxide. The former is obtained by keeping the curd of milk for several days in water, when acetic, phosphoric and caseic acids are produced, all saturated with ammonia, which is generated at the same time. The washings of the curd on evaporation yield a saline mass, slightly transparent, and tasting strongly of cheese; this is dissolved in alcohol and boiled with carbonate of lead, which removes the phosphoric acid: the caseate and acetate of lead remain in solution and are to be decomposed by sulphuretted hydrogen; lastly, by distillation, the acetic acid being volatile, is separated from the caseic acid. The latter is of the colour and consistence of syrup, red-lens litmus paper, and has a sour and bitter taste, mixed with that of cheese; it soon becomes solid, forming a mass like honey. It precipitates the oxides of silver, gold, and mercury, but scarcely any of the other metallic oxides. With infusion of galls it forms a thick white precipitate; nitric acid converts it into oxalic acid. It exists in cheese in the state of caseate of ammonia, and gives the peculiar flavour. The caseous oxide remains after the action of alcohol on the saline mass before described. It is a bulky white powder; when purified by frequent washing it is tasteless, soft and friable; it is dissolved by hot water and by liquid potash, but not by alcohol or ether. It is often present in cheese in distinct grains, which are hard and gritty. Caseate of ammonia and caseous oxide form from 30 to 50 per cent. of good cheese.