560 JAMES CLERK MAXWELL. 



plenum was on the whole the more prevalent in the ancient 

 world. 



The controversy has been handed on to modern science, 

 or rather it has inevitably reappeared ; some, with Descartes 

 and Spinoza, resolving matter into continuous extension ; 

 while others, as Bacon would say, cut deeper into Nature, and 

 would divide her if possible into her ultimate constituent parts. 

 Experimental investigations, especially those through which 

 chemistry became an exact science, have greatly favoured 

 atomistic views, yet the tendency to maintain the continuity 

 and plasticity of matter is still apparent, and the bold hypo- 

 thesis of the vortex atom, due to Sir William Thomson, 

 affords us perhaps the nearest approach to a reconciliation 

 between these two contradictory theories. According to 

 this hypothesis the whole of space is filled with a uniform 

 and perfect fluid, and material atoms are vortices of one 

 form or another which have been created within this fluid. 



In order to reach the starting-point of Maxwell's in- 

 vestigations in molecular physics we must go back to Daniel 

 Bernoulli who, in his hydrodynamics, published in 1738, 

 explained the pressure of the air on the hypothesis that air 

 consists of a number of particles moving about in all direc- 

 tions and impinging on any surface exposed to its action. 

 Le Sage of Geneva in 1818 explained the pressure of a gas 

 in strict accordance with the modern Dynamical Theory. 

 The theory of Le Sage was based upon his doctrine of " ultra- 

 mundane corpuscles," a doctrine borrowed from the older 

 atomists, and employed by Le Sage to explain gravitation. 

 For a concise account of this and of the older atomic 

 theories the reader is referred to Maxwell's article " ATOM " 

 in the ninth edition of the Encyclopedia Britannica. In 

 1847 John Herapath published his Mathematical Physics, 

 in which he supposed gases to consist of perfectly hard 

 molecules impinging against one another, and against any 

 surface exposed to their action ; and he points out the rela- 

 tion of the motion to the temperature and pressure of a 

 gas and explains gaseous diffusion. 



But we are indebted for all the modern developments 



