268 CHEMISTRY OF THE PROTEIDS CHAP. 



such as that due to the liberation of alkalies by the decomposition of 

 glass vessels or due to the absorption of gases, the following conception 

 seems to the author worthy of being considered : The transformation 

 of insoluble arsenic sulphide into the colloidal electrolytic variety 

 requires a strong force to overcome the initial inertia of the iso-electric 

 sulphide, which is held together by a high surface tension ; but once 

 having been set in motion a comparatively small force will be sufficient 

 to keep the sulphide in the colloidal state, and therefore it is possible 

 to dialyse away a great deal of the acid which was required in making 

 the colloidal solution without leading to a precipitation of the colloid. 

 If, however, a feeble force in one direction is sufficient to maintain a 

 colloid involution, then also will it require only a feeble force in the 

 opposite direction to precipitate a colloid from its solution, and this 

 accounts for the ease with which colloidal solutions may be thrown 

 down. As in the case of a body once set in motion, its momentum 

 will carry it a certain distance, so also in the case of colloids. A 

 ' solution ' made by mechanical means will remain for a certain time 

 in solution, but gradually as the component particles lose their 

 momentum it will become insoluble. 



As pointed out on pp. 266 and 323, true 'colloidal solutions' 

 depending for their solubility on the presence of electrolytes are 

 permanent as long as all chemical change is prevented. 



FURTHER EVIDENCE THAT COLLOIDS ARE ELECTROLYTES 



To the same conclusion as the author, namely, that colloids are 

 electrolytes, have subsequently come Billitzer, working under Nernst, 

 and Freundlich, working in Ostwald's laboratory. The author's views 

 were expressed in 1902 in his Physiological Histology. See this book, 

 pp. 259, 262, 264. 



Billitzer 1 arrived in 1903 at the conclusion that colloids may be 

 regarded as ions, for he found it impossible to explain the movement 

 of colloidal particles in an electrical field on v. Helmholtz's hypothesis 2 

 that a separation of the positive and the negative charge in electro- 

 lytes is brought about by the formation of a double electrical layer, 

 which was so constituted that on two sides of a plane immeasurably 

 thin 3 there were developed equivalent but opposite amounts of 

 electricity. 



Freundlich 4 explains the behaviour of colloids on the assumption 



1 Jean Billitzer, Ann. d. Physik, 316. 902 and 937 (1903). 



2 H. v. Helmholtz, Pogg. Ann. 165. 228 (1853). 



3 See Lord Kelvin, Nature, 31st March and 19th May 1870. 



4 Herbert Freundlich, Zeit. f. physik. Ghem. 44. 129 (1903). 



