294 CHEMISTRY OF THE PROTEIDS CHAP. 



albumins with bases are much less readily precipitable than are the 

 natural albumins ; urea, according to Spiro, 1 acts also as a base, and 

 prevents thereby precipitation. 



According to the unanimous statements of Kuhne, Umber, and 

 Siegfried, 2 it is quite impossible to precipitate one or several of the 

 deutero-albumoses without having previously converted them into 

 sulphates or into ammonia salts. The observation of Hopkins 3 and 

 Krieger, 4 . that albumins crystallise more readily from half-saturated 

 ammonium-sulphate solutions if the solution be acid instead of 

 neutral, seems also to come under this heading, for the crystalline 

 albumins are apparently not free albumin, but the sulphate or some 

 analogous salt (see p. 326). 



5. Precipitation of the Colloid due to the withdrawal of 

 the Hydrogen or Hydroxyl Radical 5 



For descriptive purposes, the hydrogen and hydroxyl compounds 

 of colloids are discussed by themselves, but it must be remembered 

 that they differ from other compounds only in degree and not in kind. 



Pure albumins and globulins Starke 6 regards as acid or alkali 

 albumins or globulins. They are insoluble in water and neutral salt 

 solutions, but are soluble in very dilute acid or alkalies, because they 

 unite with the H or the OH'-ions to form new compounds, as can be 

 demonstrated by using tropaeolin as an indicator. 



Alkali and acid albumins and globulins occur naturally. Thus 

 globulins are found both in the acid extracts of lentils made with 5 

 per cent salt solution and also in the blood. They occur in combina- 

 tion with either the acid H- or the alkaline OH'-ions. The relationship 

 between the colloid and the acid H or the alkaline OH' has been 

 expressed as follows : 



1. The acid, or alkali, and the proteid form a salt which is capable 

 of dissociation, and therefore of remaining in solution (Starke). 



2. The proteid is partly in solution and partly in the colloidal 

 state. The colloid as a whole is kept in suspension, because the acid 

 or alkali which was added establishes a difference of potential 

 between the solid and the fluid phases of the colloid, giving rise 

 to a double electric layer round each solid particle (Hardy). 



1 K. Spiro, Hofmeister's Beitrage, IV. p. 300 (1903). 



2 M. Siegfried, Zeitschr. f. physiol. Chem. 27. 335 (1899). 



3 F. G. Hopkins and S. N. Pinkus, Journ. of Physiol. 23. 130 (1898). 



4 H. Krieger, Dissertation, Strassburg, 1899. 



5 Reprinted from the author's Physiological Histology, p. 55. 



6 Starke, Zeitschr. f. BioL 42. 187 (1901). 



