Casein is the form in which protein appears in the milk, where it constitutes the chief source of nourishment to the young animal, for which purpose it is admirably adapted, not only from the protein it contains, which is readily converted into fibrin and albumen, but also on account of the inorganic salts, espe cially phosphate of lime, with which it is always associated. The proportion of casein contained in the milk of different animals varies consi derably; and a still more striking variation is caused by the food of the animal, as rnay be seen in the following table.
Casein is scarcely known in a state of abso lute purity, as it is extremely difficult to sepa rate it entirely from inorganic impurities : these consist chiefly of lime, potash, soda, and iron, combined with phosphoric, sulphuric, and hy drochloric acids. The purest specimens pre pared by Rochleder left, when burnt, only 0.3 per cent. of incombustible ash ; but as it is generally prepared it contains considerably more, sometimes as much as 10 per cent. It appears to be insoluble in water, and owes its solubility in milk to the small quantity of potash which is always present. The best pro cess for obtaining casein is the following. A quantity of milk is first evaporated to dryness on a.water-bath, and the dry residue, reduced to powder, is boiled in successive portions of ether until the whole of the fatty matter is re moved ; the impure curd should then be eva porated to dryness, and the soluble part sepa rated by digesting in water. To this solution, after filtration, alcohol is added to throw down the casein, which, however, is often still con taminated with a little sugar of milk : this may be removed by again dissolving in water, and once more precipitating the casein by alcohol. When dry it resembles albumen very much in appearance, and its behaviour with reagents is in most cases very similar ; it differs from it chiefly in not coagulating when heated, and it is precipitated by all the acids, but redissolves in an excess of most of them. Sulphuric acid throws down a compound which has been called sulphate of casein; this precipitate al ways contains a certain quantity of phosphate of lime, and it is only by repeatedly dissolving it in an alkaline solution, reprecipitating with dilute sulphuric acid, and well washing with boiling water, that it can be obtained in a state of purity. When milk or a solution of casein
is heated under ordinary circumstances, a thin skin is formed on the surface, which, if re moved, is quickly replaced by another ; this substance has never been properly examined; but as it is not formed unless oxygen is pre sent, it is probably the result of oxidation. Casein is precipitated from its solutions by ferrocyanide and ferridcyanide of potassium, provided the solution is not alkaline, and still more perfectly if a little acetic acid is present. Lactic acid also readily coagulates casein; but the coagulation appears to be most completely effected by the lining membrane of the stomach of the young animal, an action due either to lactic acid, or, what is perhaps more probable, to the presence of pepsine.
Casein has been found in some of the animal fluids besides milk : the saliva, the bile, pan creatic juice, and perhaps the blood, all con tain it in more or less notable quantity. It affords another instance of the admirable adap tability of this interesting class of compounds very similar to that already mentioned when speaking of albumen : in the milk, which is the sole food on which the young of most ani mals subsist, no other protein compound has been detected ; but no sooner has it become the food of the young animal which it is in tended to nourish, than it is for the most part converted into fibrin and albumen, thus fur nishing blood and muscle, together with most of the other tissues of the body, which, though less directly, are scarcely less certainly products of the decomposition of this substance. The composition of casein is represented by the for mula 112,0 N5,, 0,2,, S, or ten equi valents of protein united to one equivalent of sulphur, thus differing from fibrin and albu men in not containing any phosphorus.
There is another modification of protein, very similar to casein in its properties and composition, which has been called both glo bulin and crystalline, from the circumstance that it is found surrounding the blood globules and also in the crystalline lens of the eye. It appears to contain no phosphorus and less sul phur than casein, and is composed, according to Mulder, of fifteen equivalents of protein united to one of sulphur.