122 Mr. W. B. Hardy. On the Conditions which 



The important point, however, which calls for notice is that the 

 function a(u + is a numerical measure of the chemical activity of the 

 substance at a given concentration, so that we reach the important 

 conclusion that the concentration of acids necessary to coagulate electro- 

 negative colloid particles, and of alkalis necessary to coagulate electro-positive 

 particles, is determined by the laws which govern ordinary chemical equilibrium. 



In the case of the action of salts on these hydrosols, the relation is 

 not so simple. K does not vary directly with oi{u + v), but contains a 

 factor which is approximately squared or cubed by a change from a 

 mono-valent to di- or tri-valent ions. The relation can therefore be 

 best expressed as 



K = noL {u + v) A* 



where x is positive and increases rapidly with an increase in the valency 

 of the ion whose electric charge is of the opposite sign to that on the 

 particles. 



I should interpret these relations by the suggestion that in the 

 former the acid or alkali alters the difference of potential at the surface 

 of the particles by altering the character of the fluid, and in that way 

 modifies the stability of the hydrosol ; in the latter the active ions of 

 the salt act directly upon the solid particles, or, perhaps, on the charge 

 which these carry, and thus play a part which is, perhaps, generally 

 similar to the action of ions when they furnish nuclei for the condensa- 

 tion of vapour. Picton and Linder have shown that the active ions 

 are actually entangled in, and form part of, the coagulum.* 



The former relation may profitably be placed beside Bruhl's con- 

 clusions that the action exerted by a fluid upon the substance dis- 

 solved in it is determined by the chemical characters of the former, as 

 well as of the latter. He has shown that the molecular refraction, the 

 dielectric coefficient, and the power possessed by the fluid of disso- 

 ciating or chemically changing the molecules of the substance dissolved 

 in it are measured by the unsatisfied valency, or, to use another phrase, 

 the residual energy of its molecules. 



The action of acids or alkalis on a hydrosol, the particles of which 

 are of the opposite electrical sign, seems to be compounded of these 

 two actions. The acid or alkali may act as a salt, and exhibit the 

 characteristic relation between K and the valency of the ion of the 

 opposite electrical sign. An instance is furnished by the action of 

 various acids on ferric hydrate, or the acid or alkali by increasing the 

 diflference of potential between the fluid and the solid particles may 

 increase the stability of the hydrosol. This is markedly manifested 

 by the increased stability given to the hydrosol of gum mastic by the 

 addition of univalent alkalis. In the action of barium hydrate on 

 this hydrosol, the segregating action of the metal ion overcomes the 

 * ' Journ. of the Chem. Soc.,' vol. 67, 1895, p. 63. 



