556 POPULAR LECTURES AND ADDRESSES. 



constitution of matter and the origination of 

 thermal radiation, whether visible as light or not. 

 In the whole train of Crookes' investigations on 

 the radiometer, the viscosity of gases at high 

 exhaustions, and the electric phenomena of high 

 vacuums, ether seems to have nothing to do except 

 the humble function of showing to our eyes some- 

 thing of what the atoms and molecules are doing. 

 The same confession of ignorance must be made 

 with reference to the subject dealt with in the 

 important researches of Schuster and J. J. Thomson 

 on the passage of electricity through gases. Even 

 in Thomson's beautiful experiments showing 

 currents produced by circuital electromagnetic 

 induction in complete poleless circuits, the presence 

 of molecules of residual gas or vapour seems to be 

 the essential. It seems certainly true that without 

 the molecules there could be no current, and that 

 without the molecules electricity has no meaning. 

 But in obedience to logic I must withdraw one ex- 

 pression I have used. We must not imagine that 

 "presence of molecules is the essential." It is 

 certainly an essential. Ether also is certainly an 



