1893.] President's Address. 389 



to rapidly evaporate. This last-mentioned result is almost more sur- 

 prising than the phosphorescent glow excited by molecular impacts 

 in bodies not rendered perceptibly phosphorescent by light. Both 

 phenomena will surely be found very telling in respect to the mole- 

 cular 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 something 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 expression 

 I have used. We must not imagine that " presence of molecules is 

 the essential." It is certainly an essential. Ether also is certainly 

 an essential, and certainly has more to do than merely to telegraph to 

 our eyes to tell us of what the molecules and atoms are about. If a 

 first step towards understanding the relations between ether and 

 ponderable matter is to be made, it seems to me that the most hope- 

 ful foundation for it is knowledge derived from experiment on 

 electricity in high vacuum ; and if, as I believe is true, there is good 

 reason for hoping to see this step made, we owe a debt of gratitude 

 to the able and persevering workers of the last forty years who have 

 given us the knowledge we have : and we may hope for more and more 

 from some of themselves and from others encouraged by the fruitful- 

 ness of their labours to persevere in the work. 



The President then presented the Medals awarded by the Society 

 as follows: 



COPLEY MEDAL. 

 Sir G. Gabriel Stokes, Bart., F.RS. 



In presenting the Copley Medal to Sir George Stokes I feel that 

 no " statement of claim " is needed. Nevertheless, it is interesting 

 to recall to memory something of the great work that he has done in 

 mathematical and physical science. Fifty-two years ago he took up 

 the subject of fluid motion with mathematical power amply capable to 

 advance on the lines of Lagrange, Fourier, Cauchy, Poisson, in the 



