S INAUGURAL ADDRESS. 



as those long ago known to apply to the molecules of a gas. Hence 

 I then spoke of the whole resulting theory as " the gaseous theory 

 of solution," and ventured to liken a gas or vapour to a solution 

 of matter in space and a liquid to a solution of space in matter ; 

 and I pointed out certain deductions from the theory which are 

 now familiar. It was known then, however, that solutions of 

 acids, alkalis, and salts, in water — or, in other words, that electro- 

 lytic solutions — are apparent exceptions to the van't Hoff law ; 

 and it was Arrhenius's explanation of this, adopted by van't Hoff, 

 that rendered the whole subject so vitally interesting to chemists. 

 His hypothesis was that the molecule of any such compound, when 

 dissolved in water, tends automatically to break into two parts, 

 each of which is an atom or group of atoms carrying one or more 

 unit charges of electricity, positive in the one case, negative in 

 the other, so that the solution as a whole remains electrically 

 neutral. Further, these two " ions," when separated, move inde- 

 pendently through the solvent, can recombine when they meet 

 to form the original kind of molecule, and can separate again, so- 

 that each ion leads a definite fraction of its life in the free state ; 

 they may take part, when free, in other chemical changes by 

 collision with molecules of the solvent or with other molecules or 

 ions present in the solution ; and in an electric iield they tend to 

 move towards opposite poles and are, in fact, the actual carriers 

 of the electricity, and thus constitute the current. Thus arose the 

 ionic theory of electro-chemistry and of chemical change. Hardly 

 any other modern development has been so productive of research, 

 during the last quarter of a century. 



To make its significance clearer, let me point out that the 

 theory threw a quite fresh light on the question of the relationship 

 between electricity and chemical phenomena. The work of 

 Faraday had actually led to the view, put forth later by Helmholtz, 

 that electricity, like matter, is atomic ; and there was an accumu- 

 lation of evidence that material atoms and electrical atoms dO' 

 in fact combine ; but it was Arrhenius and van't Hoff who first 

 opened the minds of chemists to the view that such combinations 

 form spontaneously and in quantity under normal conditions, and 

 that many of our most familiar chemical changes are due to 

 collisions of these material-electrical systems, and not to those of 

 molecules built entirely of material atoms. The first and simplest 

 conception of the ion — and at least a useful one to start with — is 

 that it is a combination of one or more material atoms with one- 

 or more electrical atoms, either of the kind called negative of 



