14 CONDITIONS OF STABILITY 



cally charged, but precipitate in the electrically neutral state, 

 as the particles lose their water of hydra tion when discharged. 



The importance of solvation and electric charge of the 

 particles in determining the stability of colloids is shown by 

 numerous researches. The questions of the origin of the 

 electric charge and of the mechanism of discharge and precipita- 

 tion of the colloid particles lead us, on the other hand, into a 

 field of various conflicting theoretical speculations. 



It would appear impossible to suggest a single source of the 

 electric charge which would be applicable to all colloid systems. 

 But in the case of the proteins, as we shall show, all experience 

 points to the conclusion that those which behave as electrolytes 

 owe their charge to typical ionisation processes. Similarly, 

 silicic acid, stannic acid, tungstic acid, etc., which give positive 

 H-ions, form particles which are charged negatively, as also do 

 colloidal solutions of the noble metals, when positive metal 

 ions are produced. Colloidal metallic hydroxides, on the other 

 hand, can be regarded as complex salts of the metallic oxide 

 with the salt from which they are prepared by hydrolysis and 

 dialysis (see the recent work of Wo. Pauli and J. Matula*). 

 These complex salts consist of a positive colloidal ion and a 

 simple anion. For example, a ferric hydroxide sol prepared 

 from ferric chloride contains the ions #Fe(OH) 3 . yFe*" and 

 3 yCl'. 



Difficulties crop up when this purely ionic conception is 

 applied to colloids in which at present no notable ionisation 

 has been detected : for example, emulsion of resins, and such 

 like bodies. But even here the ionisation resulting from 

 slight saponification at the surface of the particles is not 

 excluded. The theory of the charging of colloid particles 

 as an ionic process in which one ion is very large has been 

 developed by J. Billiter. The particles have the charac- 

 teristics of an ion with the charge of the electrode which 

 repels them. 



This view of the origin of the charge on the particles of 

 colloids leads to a theory of the discharging processes which 

 is most simply regarded as an ionic reaction at the surface of 



* Kolloid Zeitsch., 1917, 21, 49 (literature). 



