66 THE CHEMISTRY OF CATTLE FEEDING 



them from various impurities and from one another. It 

 appears to be analogous to the salting out of soaps by brine, 

 and is supposed to be due to partial dehydration or withdrawal 

 of water from the protein by the salt. Other colloids exhibit 

 the same property, and it is not improbable that many com- 

 pounds of high molecular weight, e.g. carbohydrates, may also 

 be capable of flocculent precipitation. 



Crystallisation. When a certain quantity of acetic acid 

 is added to a solution of white of egg, half saturated with 

 ammonium sulphate, the protein comes down as a bulky 

 amorphous precipitate which on standing from six to twelve 

 hours gradually assumes the crystalline form. It is uncertain 

 whether these crystals consist of the simple proteins or salts 

 (the acetate or sulphate) of the same. 



Chemical Composition. The exact chemical composition 

 of the proteins is still a matter of some doubt owing to the 

 difficulty of separating the compounds from each other, and 

 from ash and other impurities. The composition of each 

 particular compound is, doubtless, constant, but it is almost 

 impossible to determine whether any given product represents 

 a single pure compound. Also, the difference in the com- 

 position of some of the substances of widely different origin 

 and different. properties is often no greater than that obtained 

 by different observers for one and the same substance. The 

 following results, obtained in numerous analyses of serum 

 albumin a typical compound may be taken as a practical 

 average for the group. 



Per cent. 



Carbon 53'8 



Hydrogen . . . ". . . 7*10 



Oxygen . . . . . . 21-99 



Nitrogen . . ... . 15 '93 



Sulphur 1*90 



100*00 



The composition of some of the compounds, however, 

 differs considerably from the average. In general, they may 

 contain from 52 to 54 per cent, of carbon, and from 15 to 19 



