determine the Stability of Irreversible Hydrosols. 125 



the action may be said to be on the particles, or rather on the electric 

 layers immediately around them, and the active ions are those whose 

 electric sign is the opposite of that of the charge on the surface of the 

 particles. In this case coagulative power does not vary directly with 

 variations in chemical activity. It rises exceedingly rapidly with a 

 rise in the valency of the active ion. so that the relation 



r : I" : I'" = n:n^ '.n^ 



is approximately satisfied. 



Picton and Linder have shown that when the concentration of the 

 salt is insufficient to completely destroy the system, it is not wholly 

 without action. A fresh point of equilibrium between solid and fluid 

 is reached by an increase in the size of the particles and therefore a 

 diminution in the extent and curvature of the surface of contact. The 

 fact is of importance, because it introduces us to the possibility that 

 the reagent may aifect the size of the pai'ticles by altering the equi- 

 librium between the part of the solid in solution and the part in 

 suspension. Double electric layers round each particle are, according 

 to Thomson, separated by a region of " uncompleted chemical combina- 

 tion " between the components."^ The density of the field round the 

 particles in hydrosols will therefore be a measure of the velocity of the 

 solution and condensation between the particle and the liquid, and 

 therefore the factor which determines whether the particles will on the 

 whole grow, diminish, or remain stationary in size. 



When acids or alkalis are added to hydrosols holding particles of 

 the opposite electric sign to themselves, the simplest relation seems to 

 be that univalent acids or alkalis increase the stability ; bivalent acids 

 or alkalis decrease it. 



The view advanced in this paper implies that each particle in a 

 hydrosol is surrounded by a zone in which the components are in a 

 condition of chemical instability. According to Rayleigh,t such a zone 

 is of finite thickness, and deep enough to contain several molecules. 

 We therefore have in these hydrosols two phases, separated by a layer 

 of extraordinarily large extent, which possesses considerable chemical 

 energy. This, it seems to me, suggests an explanation of the catalytic 

 powers so markedly manifested by hydrosols. 



* 'Discharge of Electricity through. G-ases.' Scribner, 1898, p. 24. 



t Thomson, loc. cit., p. 26; Eayleigh, * Phil. Mag.,' vol. 33, 1892, p. 468. 



VOL. LXVI. 



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