NITROGENOUS COMPOUNDS 65 



atom of hydrogen united to the sulphur of the protein, and 

 form a soluble compound. 



Proteins are precipitated from aqueous solutions on 

 addition of alcohol. The protein, however, is not coagulated. 

 It is apparently unchanged and may be redissolved in water. 

 The action may be compared to the precipitation of dextrine 

 by alcohol ; in both cases it is attributed merely to the 

 insolubility of the compound in the reagent. On prolonged 

 contact with strong alcohol, proteins undergo a change similar 

 to that produced by warming the solutions, they become 

 permanently insoluble in water, and are in fact coagulated. 

 Tannins, phenols, acetone, and several other reagents also 

 cause coagulation. 



Coagulation may also be induced by certain mechanical 

 means, e.g. violent agitation, and by contact with porous sub- 

 stances such as charcoal and unglazed pottery. 



The change which takes place naturally in fibrinogen and 

 caseinogen sometimes called spontaneous coagulation is 

 due to the action of certain ferments. It is not regarded as 

 true coagulation because the precipitates are not so insoluble 

 as those produced by heating the solutions. Connheim l has 

 used the term gerinung (curdling) to describe this change. 



Flocculation. All proteins, including peptones, are com- 

 pletely precipitated from aqueous solutions when the latter are 

 saturated with ammonium sulphate. Zinc sulphate may be 

 used instead of ammonium sulphate. Many other salts, 'e.g 

 potassium acetate, calcium chloride, and calcium nitrate, pre- 

 cipitate all native proteins, but not peptones. Magnesium 

 sulphate precipitates all the less readily soluble proteins, but 

 not those which are soluble in pure water. Common salt, 

 sodium sulphate, acetate, and nitrate are even less effective, 

 but they cause precipitation of caseinogen, fibrinogen, and 

 some others. 



This action is called flocculent precipitation or salting out. 

 The proteins are not coagulated, or apparently changed in any 

 way. The precipitates can be redissolved in water. 



The process affords an important means of separating 

 1 Chemie der Eiweisskorper. 



F 



