68 THE CHEMISTRY OF CATTLE FEEDING 



soluble in water than the original compounds. The potassium 

 and sodium salts are also very soluble. They are of peculiar 

 interest because such compounds occur in nature. Nucleo- 

 albumin is an ajkali-albuminate, and the caseinogen of milk is 

 a calcium caseinate. 



The acid and alkali salts of the proteins are easily saponi- 

 fied, i.e. the proteins are easily separated from the acid or 

 base, by the action of water. 



The albuminates of the heavy metals are all insoluble in 

 water, and are, therefore, precipitated from aqueous solutions 

 on addition of the salts of these elements. These reactions, 

 it will be seen, are not at all analogous to the precipitation of 

 the proteins by coagulating reagents. 



Qualitative Tests. The more important reactions which 

 may be used as qualitative tests for the proteins fall naturally 

 into two divisions, viz. (a) precipitation from solutions, and (b) 

 colour reactions. 



The former includes all those in which the proteins are 

 precipitated 



(1) By reagents in which they are insoluble, e.g. alcohol, 

 chloroform, etc. Flocculent precipitation by ammonium 

 sulphate and other salts may be included under this head. 



(2) In the form of a coagulum produced by heat or by the 

 action of various reagents, e.g. phenol, acetone, etc. 



(3) By salts of the heavy metals, e.g. mercuric chloride, 

 basic acetate of lead, cupric sulphate, etc. 



(4) By mineral acids, e.g. sulphuric, phosphoric, and nitric 

 acids. The last is the most effective, but the precipitates are 

 slightly soluble in excess. 



(5) By alkaloid reagents, e.g. phosphotungstic acid, phos- 

 phomolybdic acid, tannic acid, ferrocyanic acid (potassium 

 ferrocyanide and acetic acid), picric acid, potasso-mercuric 

 iodide. The proteins resemble the alkaloids in many respects, 

 and, like them, they form insoluble compounds with the re- 

 agents mentioned. 



The colour reactions are, for the most part, not produced 

 by the protein molecules as a whole, but by constituent 

 radicles. Certain proteins do not give the colorations 



