' 

 12 CONDITIONS OF STABILITY 



systems is brought out most clearly when "the effect of the 

 electric charge of the particles on the equilibrium of the system 

 is investigated at the same time. When the behaviour of 

 colloids in an electric field is explored, a movement of the 

 particles towards one of the poles can ordinarily be demon- 

 strated. If electro-negative, they wander to the anode (e.g., 

 metals, metallic sulphides, mastic suspensions, silicic acid, etc.), 

 or, if positive, to the cathode (e.g., metallic hydroxides). The 

 work of H. Schultze, H. Picton and S. E. Linder, W. B. Hardy, 

 W. Biltz, J. Billiter, H. Freundlich, and others has shown that 

 the electric charge on the particles of these colloids is essential 

 for their stability in solution. The removal of the charge on 

 the particles leads to a cessation of movement in an electric 

 field, and also to precipitation. This fact was first noted and 

 precisely stated by W. B. Hardy. Later researches show that 

 even an incomplete discharge of the particles (for oil-emulsions 

 0-03 volt) leads to coagulation (R. Ellis * and F. Powis ). 

 This discharge and precipitation of the colloidal solution 

 can be brought about in various ways : 



I. By addition of electrolytes, in which case relatively 

 low concentrations of the ions are sufficient. Positive 

 ions are effective in precipitating negative sols and 

 negative ions for positive sols (Hardy's Rule}. The 

 potency of the ion increases disproportionately with 

 its valency, polyvalent being more effective than 

 monovalent, and of the latter H- and OH-ions are 

 the most powerful (H. Schultze, W. B. Hardy, H. 

 Freundlich). 



II. Oppositely charged colloids neutralise each other, 

 and are mutually precipitated. Excess of one of 

 them makes precipitation incomplete or may prevent 

 it altogether (W. Biltz, J. Billiter). 



III. If secondary reactions are neglected, precipitation 



occurs on the oppositely charged pole when a colloid 

 is subjected to an electric field. 



IV. The charge on the particles is decreased, even to the 



* Zeitsch. physikal. Chem., 1914, 89, 145. 

 t Zeitsch. physikal. Chem., 1914, 89, 186. 



