220 POPULAR LECTURES AND ADDRESSES. 



its pressure if not allowed to expand for elevation 

 of temperature, a phenomenon which perhaps he 

 scarcely knew, still less the elevation of tempera- 

 ture produced by compression, and the lowering 

 of temperature by dilatation, and the consequent 

 necessity of waiting for a fraction of a second or 

 a few seconds of time (with apparatus of ordinary 

 experimental magnitude), to see a subsidence from 

 a larger change of pressure, down to the amount 

 of change that verifies Boyle's law. The considera- 

 tion of these phenomena forty years ago by Joule, 

 in connection with Bernoulli's original conception, 

 formed the foundation of the kinetic theory of 

 gases as now we have it. But what a splendid and 

 useful building has been placed on this foundation 

 by Clausius and Maxwell, and what a beautiful 

 ornament we see on the top of it in the radiometer 

 of Crookes, securely attached to it by the happy 

 discovery of Tait and Dewar, 1 that the length of 

 the free path of the residual molecules of air in 

 a good modern vacuum may amount to several 

 inches ! Clausius' and Maxwell's explanations of 



1 Proc. R. S. E. March 2, 1874, and July 5, 1875. 



