396 THE CHEMICAL COMPOSITION OF THE BODY. [CH. xxv. 



Precipitants of Proteids. Solutions of most proteids are 

 precipitated by : 



1. Strong acids like nitric acid. 



2. Picric acid. 



3. Acetic acid and potassium ferrocyanide. 



4. Acetic acid and excess of a neutral salt like sodium sulphate ; 

 when these are boiled with the proteid solution. 



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 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 a 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, <fec.). 



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

 precipitate formed is readily soluble in suitable reagents like 

 saline solutions, and the proteid continues to show its typical 

 reactions. Such precipitation is not coagulation. Such a preci- 

 pitate is produced by saturation with amnxonium 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 by certain salts, like sodium 

 chloride and magnesium sulphate, which do not precipitate the 

 albumins. 



The precipitation produced by alcohol is peculiar 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 globulins are most 

 readily rendered insoluble by this method ; proteoses and 

 peptones are never rendered insoluble by the action of 

 alcohol. This fact is of value in the separation of these proteids 

 from others. 



