40 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



5. Salts of the heavy metals, like copper sulphate, mercuric 

 chloride, lead acetate, silver nitrate, &c. 



6. Tannin. 



7. Alcohol. 



8. Saturation with certain neutral salts, such as ammonium 

 sulphate. 



It is necessary that the words coagulation and precipitation should, 

 in connection with the proteins, be carefully distinguished. The 

 term coagulation is used when an insoluble protein (coagulated 

 protein) is formed from a soluble one. This may occur 



1. When the protein is heated heat coagulation. 



2. Under the influence of a ferment ; for instance, when a curd is 

 formed in milk by rennet or a clot in shed blood by the fibrin ferment 

 ferment coagulation. 



3. When an insoluble precipitate is produced by an addition of 

 certain reagents (nitric acid, picric acid, tannin, &c.). 



There are, however, other precipitants of proteins in which the 

 precipitate formed is readily soluble in suitable reagents, like saline 

 solution, and the protein continues to show its typical reactions. 

 This precipitation is not coagulation. Such a precipitate is produced 

 by saturation with ammonium sulphate. Certain proteins, called 

 globulins, are more readily precipitated by such means than others. 

 Thus, serum globulin is precipitated by half-saturation with ammonium 

 sulphate. Full saturation with ammoniun sulphate precipitates all 

 proteins but peptone. The globulins are precipitated by certain salts 

 like sodium chloride and magnesium sulphate, which do not precipi- 

 tate the albumins. The precipitation of proteins by salts in this way 

 is conveniently termed ' salting out.' 



The precipitate produced by alcohol is peculiar in that after 

 a time it becomes a coagulum. Protein freshly precipitated by 

 alcohol is readily soluble in water or saline media ; but after it has 

 been allowed to stand some weeks under alcohol it becomes more 

 and more insoluble. Albumins and globulins are most readily ren- 

 dered insoluble by this method ; proteoses and peptones are appa- 

 rently never rendered insoluble by the action of alcohol. This fact 

 is of value in the separation of these proteins from others. 



