ON THE ELECTRICAL CHARGE OF PROTEIN. 141 



longer moves with the electric current. Even before 

 BILTZ'S work other observations had indicated the impor- 

 tance of electrical conditions for colloidal precipitations 

 and had formed the starting-point of theoretical explana- 

 tions. The colloids seem, in general, to be precipitable 

 only through electrolytes; non- electrolytes such as sugar 

 or urea have no precipitating effect even upon very un- 

 stabile colloids. HARDY and BREDIG have, in accord with 

 the theory of electrocapillary phenomena, developed the 

 idea that there exists an antagonism between the forces 

 of surface tension, which, according to BREDIG, cause 

 the colloidal particles to coalesce, and the electrical charges 

 that the colloidal particles carry, in such a way that only 

 after the electrical charge which causes the particles to 

 repel each other has been removed can the surface tension 

 attain its maximum. If a discharge of the colloidal 

 particles is brought about through the addition of the 

 oppositely charged ions of electrolytes, then the optimum 

 of precipitability is produced at the same time, and a 

 precipitate is formed. 



According to a different theory developed by BILLITZER, 

 surface tension does not play the role attributed to it by 

 HARDY and BREDIG. If oppositely charged ions are 

 added to a colloid, the colloidal particles collect about 

 these ions through electrostatic attraction. In this way 

 aggregates are finally formed of sufficient size to fall to 

 the bottom. According to this view, with which many 

 facts agree that contradict the first-mentioned theory, a 

 colloid carrying no electrical charge should be capable 

 of precipitation only with difficulty, as its particles exert 

 no electrostatic forces. One can regard these theories 

 as one pleases; no doubt the necessity of testing the 



