Manchester Memoirs, Vol. Hi. (1908) No. 10- 33 



any two ions ever meeting again to unite, as compared 

 with the unaltered chance of any remaining whole 

 molecules becoming divided into ions. Complete ionis- 

 ation must ultimately arrive ; and there is only the 

 question remaining over as to the degree of dilution at 

 which it is practically attained. 



It will be observed that we are in this argument 

 applying the principles of mobile equilibrium to the 

 ionisation of the dissolved substance. Here an appeal 

 to experiment becomes feasible. It was by Ostwald that 

 this test of Arrhenius' view was first applied. As is well 

 known, he found that for acids and salts that are but 

 slightly ionised, ^.^., acetic acid, the mathematical relation 

 expressive of equilibrium of simple dissociation is 

 satisfied ; but for highly ionised substances it is widely 

 departed from. The verification in the former case 

 appears to be sufficient by itself to confirm the general 

 point of view. For it seems natural to suppose that high 

 ionisation indicates the presence of some type of direct 

 affinity with the solvent, which is too powerful to be 

 altogether omitted from the equation which expresses the 

 ultimate equilibrium. How such an influence should be 

 included is one of the main unsolved problems in this 

 subject. At higher concentrations either the tendency 

 to re-combination of the ions is resisted, or else the 

 tendency to ionisation of the molecules is promoted. 

 Thus if increased ionisation were a result of collision of 

 molecules with the existing ions, in analogy with known 

 effects of collision of (rapidly moving) ions with the 

 molecules of gases in promoting further ionisation, the 

 equation of equilibrium would be altered from n^ = kn 

 to n^ = k7i-\-k'?tn^, in which k'fk would be sensible only for 

 easily ionisable substances (cf. Appendix). The solution 

 of this problem, if there is any simple colligatory 



