Manchester Memoirs, Vol. Hi. (1908), No. 10- 41 



in the energy of expansion. Fortunately for exact 

 knowledge, the principles of thermodynamics give 

 in all such cases the means of estimating the final 

 result, without requiring hypothesis as to the nature 

 of the process that is involved, provided only it be 

 reversible ; in dilute systems the argument can be ex- 

 pressed in terms of the available energies, or thermo- 

 dynamic potentials, of the constituents, interpretable in 

 the simpler cases by partial osmotic pressures — in exten- 

 sion of which the idea of solution pressure of an ion can 

 be employed as a graphic mode of expression, without 

 implying that the processes involved can be placed in 

 effective analogy with those of evaporation or solution. 

 The very remarkable quantitative connexion of the 

 electric potential gradient between solutions with the 

 diffusions of their ions, established by Nernst, must ever 

 remain one of the solid foundations in this subject. 



The modern expression of Helmholtz's provisional 

 generalisation, that chemical affinity arises from intrinsic 

 attraction of the different kinds of matter for electricity, 

 may roughly be that every active atom is an ion, and that 

 saturated inert molecules are welded into unity by each 

 constituent atom keeping hold, through the aethereal 

 agency of electric attraction, of the (perhaps inter- 

 penetrating) electrons belonging to the other. The 

 electrical view provides a reason for the ordinary saturated 

 inactive molecule of elementary bodies being often poly- 

 atomic, a fact otherwise of undiscovered import. The 

 exceptions afforded by monatomic gases and metals in- 

 deed suggest themselves at once : but spectrum analysis 

 shows that these molecules are intrinsically just as com- 

 plex in sub-electric structure as the others, while the 

 physical test of monatomicity perhaps only verifies that 

 the components of the molecule are somehow so closely 



