AND MODERN PHYSICS. 57 



electric resistance, and absorption of good dielectrics, such as 

 glass, shell-lac, gutta-percha, ebonite, sulphur, etc. 



"I have also cleared the electromagnetic theory of light 

 from all unwarrantable assumption, so that we may safely 

 determine the velocity of light by measuring the attraction 

 between bodies kept at a given difference of potential, the 

 value of which is known in electromagnetic measure. 



" I hope there will be resistance coils at the British Associa- 

 tion." 



This work resulted in his greatest electrical paper, 

 " A Dynamical Theory of the Electromagnetic Field," 

 read to the Royal Society December 8th, 1864. 



But the molecular theory of gases was still 

 prominently before his mind. 



In 18G2, writing* to H. R Droop, he says : 



"Some time ago, when investigating Bernoulli's theory 

 of gases, I was surprised to find that the internal friction of 

 a gas (if it depends on the collision of particles) should be 

 independent of the density. 



"Stokes has been examining Graham's experiments on the 

 rate of flow of gases through fine tubes, and he finds that the 

 friction, if independent of density, accounts for Grahams 

 results ; but, if taken proportional to density, differs from 

 those results very much. This seems rather a curious result, 

 and an additional phenomenon, explained by the ' collision of 

 particles ' theory of gases. Still one phenomenon goes against 

 that theory- the relation between specific heat at constant 

 pressure and at constant volume, \vbich U in uir = 1*408, 

 while it ought to be i'3:53." 



And again t in the same year, 21st April, 1862, to 

 Lewis Campbell : 



" Herr Claudius of Zurich, one of the heat philosophers, has 

 been working at the theory of gases being little bodies flying 

 * Life of J. C. Maxwell," p. 332. 

 t " Life of J. C. Maxwell/' p. 336. 



