THE PEOTEIDS 25 



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 a,nd precipitation should,. 

 in connection with the proteids, be carefully distinguished. The term 

 coagulation is used when an insoluble proteid (coagulated proteid) is 

 formed from a soluble one. This may occur — 



1. When the proteid 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 the addition of 

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



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

 precipitate formed is readily soluble in suitable reagents, like saline 

 solution, and the proteid continues to show its tj^ical reactions. This 

 precipitation is not coagulation. Such a precipitate is produced by 

 saturation with ammonium sulphate. Certain proteids, 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 ammonium sulphate 

 precipitates all proteids but peptone. The globulins are precipitated 

 h\ certain salts hke sodium chloride and magnesium sulphate, which 

 do not precipitate the albumins. The precipitation of proteids by 

 salts in this way is conveniently termed ' salting out.' 



The precipitation produced by alcohol is pecuHar in that after a 

 time it becomes a coagulation. Proteid 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 globuUns are most readily 

 rendered insoluble by this method; albumoses and peptones are 

 apparently never rendered insoluble by the action of alcohol. This 

 fact is of value in the separation of these proteids from others. 



CLASSIFICATION OF PROTEIDS 



Proteids may be of animal or vegetable origin, and both animal 

 and vegetable proteids may be subdivided in the same way. We 

 shall, however, be chiefly concerned with the animal proteids. 



