10 INAUGURAL ADDRESS. 



■difficulty, the fact which, more than any other, still blocks the way 

 in spite of much labour spent upon its investigation. It does 

 not prove the theory wrong, but it shows that there are facts 

 about it not yet fully understood ; and the want of that complete 

 ■understanding is at present a serious difficulty in many researches. 



Apart from this, the ionic theory in its essence is now well 

 established. The independent migration through the solvent of 

 positively and negatively electrified ions containing the divorced 

 constituents of the original salt molecules has actually been proved 

 bv special methods, and the velocities of migration of several 

 kinds of ion under known conditions have been experimentally 

 measured. The results obtained in this way admit, in my 

 opinion, of no other interpretation than that which we 

 owe to van't Hoff and Arrhenius. Moreover, the theory has stimu- 

 lated chemists to investigate solutions in other solvents than 

 water, and has thus opened up a field which is already rich in 

 results. Some of these were anticipated by the theory, others are 

 as yet incompletely explained, so that much new light may be 

 looked for from further work in the same direction. As another 

 illustration of the fruitfulness of ionic ideas, I may point to all 

 that has been done by Nernst and others in the development of 

 what may be called the ionic theory of electro-motive force. Here, 

 as elsewhere, new foundations have been provided, and on them 

 is being built from the stones of the old edifice a greatly enlarged 

 and more beautiful superstructure. 



In one way the ionic theory has been, or is in the course of 

 being, extended and improved. Before it was heard of at all, 

 chemists who interested themselves specially in the process of 

 solution were inclined to assume the existence of specific attrac- 

 tions between the molecules of the dissolved substance and those 

 of the solvent, and the formation, in consequence, of complex 

 molecules containing both of these combined, and migrating as 

 such among the more numerous unaltered solvent molecules. 

 This view, unaided, did not lead to any very definite results, for 

 it was qualitative rather than quantitative. The van't Hoff law 

 turned thoughts in another direction, and, being quantitative, 

 proved much more fertile ; but it dealt with the behaviour of 

 the dissolved molecules when in solution, and tacitly set aside 

 the question of whether or not some of the solvent was first used 

 up by chemical combination with the solute. And the ionisation 

 theory followed suit, for the ions were not originally supposed to 

 contain anything but atoms derived from the salt and electrical 



