50 PROTEINS. 



formation of an insoluble product ; albumin is coagulated by 

 the action of heat, as also is globulin, the raising of the solution 

 to about 70 C. under suitable conditions of acidity being 

 sufficient to precipitate a great portion. Casein, mucoid, and the 

 alcohol soluble protein are gradually removed by nitration 

 through paper, and completely through coarse porcelain ; nitra- 

 tion through fine porcelain removes all the proteins. Properties 

 common to the first three proteins are solubility in alkalies, in- 

 solubility of their copper, mercury, and other salts, insolubility in 

 alcohol ; all are precipitated by tannin and phospho-tungstic acid. 

 Casein. This protein, when pure, is a white amorphous 

 body without taste or smell ; it is practically insoluble in water, 

 dissolving in this menstruum to the extent of about 0-1 per cent. ; 

 it is quite insoluble in alcohol and ether. Very dilute acids seem 

 to dimmish the solubility ; but it is soluble in stronger acids, 

 becoming, however, changed ; a solution of casein in acetic acid 

 has been used as glue ; it is completely soluble in caustic alkaline 

 solutions even when very dilute ; the solutions of the carbonates, 

 bi-carbonates, and phosphates of the alkalies also dissolve it, and 

 from these solutions, as well as from those of the alkalies, it is 

 precipitated unchanged by the addition of sufficient acid to neut- 

 ralise the alkali. It has the property of forming an opalescent 

 solution when it is dissolved in the least possible excess of sodium 

 phosphate, and the addition of small quantities of calcium 

 chloride is made ; it gives then a solution having the appearance 

 of milk. It is highly probable that milk contains casein in this 

 form. Casein has a peculiar affinity for calcium salts, especially 

 the phosphate. It is extremely difficult to free it from this 

 body, the purest preparations that have been prepared having 

 always been contaminated with small amounts of this compound. 

 Casein yields a comparatively small amount of sulphide if boiled 

 with an alkali, and contains less of this element than either 

 globulin or albumin ; it also differs from these compounds in 

 containing phosphorus ; on analysis, like other proteins, it does 

 not yield very concordant results ; the most probable com- 

 position is as follows : 



Per cent. 



Carbon, - - - 53-13 

 Hydrogen, - - 7 '06 

 Nitrogen, - - 15 '65 



Per cent. 



Sulphur, - - 0'77 

 Phosphorus, - 0'85 4 



Oxygen, - - 22 '54 



The composition of casein is variously stated by different 

 authorities, the chief differences lying in the carbon and oxygen 

 percentages. The following are the most reliable results (Table 

 IX.) :- 



* Van Slyke and Bosworth give 0-70 on their purest preparation, while 

 very recently Van Slyke and Baker find 0-80. 



