ETHER AND PONDERABLE MATTER 



the new theories of energy had made their way, there 

 came a revival of practically the same ideas of the par- 

 ticles of matter (molecules they were now called) 

 which Davy had advocated. Then it was that Clausius 

 in Germany and Clerk-Maxwell in England took up 

 the investigation of what came to be known as the 

 kinetic theory of gases the now familiar conception 

 that all the phenomena of gases are due to the helter- 

 skelter flight of the showers of widely separated mole- 

 cules of which they are composed. The specific idea 

 that the pressure or "spring" of gases is due to such 

 molecular impacts was due to Daniel Bournelli, who 

 advanced it early in the eighteenth century. The idea, 

 then little noticed, had been revived about a century 

 later by William Herapath, and again with some suc- 

 cess by J. J. Waterston, of Bombay, about 1846 ; but it 

 gained no distinct footing until taken in hand by 

 Clausius in 1857 and by Clerk-Maxwell in 1859. 



The considerations that led Clerk-Maxwell to take 

 up the computations may be stated in his own words, 

 as formulated in a paper " On the Motions and Colli- 

 sions of Perfectly Elastic Spheres." 



"So many of the properties of matter, especially 

 when in the gaseous form," he says, "can be deduced 

 from the hypothesis that their minute parts are in 

 rapid motion, the velocity increasing with the tem- 

 perature, that the precise nature of this motion be- 

 comes a subject of rational curiosity. Daniel Bour- 

 nelli, Herapath, Joule, Kronig, Clausius, etc., have 

 shown that the relations between pressure, tempera- 

 ture, and density in a perfect gas can be explained by 

 supposing the particles to move with uniform velocities 



YOU. m. o 295 



