I1ST THE LAST HALF-CENTURY. 55 



pear to be used as mere names for physico- 

 chemical units which have not yet been 

 subdivided, and 'molecules' for physico- 

 chemical units which are aggregates of 

 the former. And these individualised 

 particles are supposed to move in an end- 

 less ocean of a vastly more subtle matter 

 —the ether. If this ether is a continuous 

 substance, therefore, we have got back 

 from the hypothesis of Dalton to that of 

 Descartes. But there is much reason to 

 believe that science is going to make a 

 still further journey, and, in form, if not 

 altogether in substance, to return to the 

 point of view of Aristotle. 



The greater number of the so-called Eie- 



, , ,. , t ^ mentary 



'elementary' bodies, now known, had bodies 

 been discovered before the commencement 

 of our epoch ; and it had become appar- 

 ent that they were by no means equally 

 similar or dissimilar, but that some of 

 them, at any rate, constituted groups, the 

 several members of which were as much 



