Mr. A. Williamson's Theory of JEtherification. 355 



suggests itself of the fact to which the process is here 

 ascribed. I refer to the transfer of homologous molecules in 

 alternately opposite directions, which, as I have endeavoured 

 to show, is the cause of the continuous action of sulphuric acid 

 in this remarkable process. It may naturally be asked, why 

 do hydrogen and carburetted hydrogen thus continuously 

 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 mo- 

 lecules exchanged ; that if hydrogen and amyle can replace 

 one another in a compound, hydrogen and sethyle, 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 of interchange of hydrogen and methyle, 

 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 hy- 

 drogen among substances resembling it in other respects. 

 We are thus forced to admit, that, in an aggregate of mole- 

 cules of any compound, there is an exchange constantly going 

 on between the elements which are contained in it. For in- 

 stance, a drop of hydrochloric acid being supposed to be made 

 up of a great number of molecules of the composition CI H, 

 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. Of course this change is not directly sen- 

 sible to us, because one atom of hydrochloric acid is like 

 another ; but suppose we mix with the hydrochloric acid some 

 sulphate of copper (of which the component atoms are under- 

 going a similar change of place), the basilous elements hy- 

 drogen and copper do not limit their change of place to the 

 circle of the atoms with which they were at first combined, the 

 hydrogen does not merely move from one atom of chlorine to 

 another, but in its turn also replaces an atom of copper, form- 

 ing chloride of copper and sulphuric acid. Thus it is, that at 

 any moment of time in which we examine the mixture, the 

 bases are divided between the acids ; and in certain cases, 

 where the difference of properties of the analogous molecules 



2 A 2 



