644 SCIENCE PROGRESS 



As long ago as 185 1, it was argued by Williamson in his 

 classical essay on the Theory of Electrification, that — 



The formation of ether from alcohol and sulphuric acid is 

 neither a process of simple separation nor, one of mere synthe- 

 sis ; but it consists in the substitution of one molecule for 

 another and is effected by double decomposition between two 

 compounds. This view of the matter (he said) is therefore 

 consistent with the contact theory, inasmuch as it acknowledges 

 the circumstance of contact as a necessary condition of the 

 reaction of the molecules upon one another. ... It may 

 naturally be asked, how do hydrogen and carburetted hydrogen 

 thus continually change places ? It cannot be from any such 

 circumstance as superior affinity of one molecule over another, 

 for one moment sees reversed with a new molecule the transfer 

 effected during the preceding one. Now, in reflecting upon this 

 remarkable fact, it strikes the mind at once that the facility of 

 interchange must be greater the more close the analogy between 

 the molecules exchanged ; that if hydrogen and amyl can replace 

 one another in a compound, hydrogen and ethyl, which are more 

 nearly allied in composition and properties, must be able to 

 replace one another more easily in the same compound ; and that 

 the facility for interchange of hydrogen and methyl, which are 

 still more similar, will be still greater. But if this be true, must 

 not the exchange of one molecule for another of identical 

 properties be the most easily effected of all ? Surely it must, 

 if there be any difference at all ; and if so, the law of analogy 

 forbids our imagining the fact to be peculiar to hydrogen among 

 substances resembling it in other respects. We are thus forced 

 to admit that, in an aggregate of molecules of any compound, 

 there is an exchange constantly going on between the elements 

 which are contained in it. For instance, a drop of hydrochloric 

 acid being supposed to be made up of a great number of 

 molecules of the composition C1H, the proposition at which we 

 have just arrived would lead us to believe that each atom of 

 hydrogen does not remain quietly in juxtaposition with the 

 atom of chlorine with which it first united but, on the contrary, 

 is constantly changing places with other atoms of hydrogen or, 

 what is the same thing, changing chlorine. 



It is clear that Williamson thought of the decomposition 

 as taking place during the times when the molecules were in 

 contact. A few years later, in 1857, however, in order to 

 account for the fact that the smallest currents gave rise to a 

 sensible amount of electrolytic decomposition, Clausius intro- 

 duced the conception of ionic dissociation — of the separation 

 of the compound in solution into its two ions — which he 



