IN THE LAST HALF-CEXTURY. 59 



tiated form of matter ? Fifty years ago, 

 such a suggestion would have been 

 scouted as a revival of the dreams of the 

 alchemists. At present, it may be said to 

 be the burning question of physico-chemi- 

 cal science. 



In fact, the so-called ' vortex-ring ' hy- 

 pothesis is a very serious and remarkable 

 attempt to deal with material units from a 

 point of view which is consistent with the 

 doctrine of evolution. It supposes the 

 ether to be a uniform substance, and that 

 the ' elementary ' units are, broadly speak- 

 ing, permanent whirlpools, or vortices, of 

 this ether, the properties of which depend 

 on their actual and potential modes of 

 motion. It is curious and highly inter- 

 esting to remark that this hypothesis re- 

 minds us not only of the speculations of 

 Descartes, but of those of Aristotle. The 

 resemblance of the 'vortex-rings' to the 

 ' tourbillons ' of Descartes is little more 

 than nominal ; but the correspondence be- 



