542 Proceedings of Royal Society of Edinburgh. 
C TT 
mixture of the two ethers but an intermediate ether V^rr 5 0. In 
Ori 3 
a similar way he prepared the intermediate amyl-methyl and 
amyl-ethyl ethers. He then goes on to use these principles to ex- 
plain the ordinary process for preparing ether, giving the sequence 
of actions now familiar to every student of chemistry, and show- 
ing experimentally that three ethers are formed when a mixture of 
two alcohols is distilled with sulphuric acid. 
Williamson’s work on etherification was published in a paper 
read before the Chemical Section of the British Association at its 
meeting in Edinburgh in 1850 and printed in the Philosophical 
Magazine . It is in this paper that the following striking passage 
occurs : — “ Before quitting the subject of setherification I would wish 
to add a few words on an application which naturally enough 
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 remark- 
able 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 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 of interchange of hydro- 
gen 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 1 
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 
