Sir George Gabriel Stoke*. 213 



a surrounding gas. In most cases a solid body vibrates without much- 

 change of volume, so that the effect is represented by a distribution of 

 sources over the surface, of which the components are as much negative- 

 as positive. The resultant is thus largely a question of interference, and 

 it would vanish altogether were it not for the different situations and 

 distances of the positive and negative elements. In any case it 

 depends greatly upon the wave-length (in the gas) of the vibration 

 in progress. Stokes calculates in detail the theory for vibrating 

 spheres and cylinders, showing that when the wave-length is large 

 relatively to the dimensions of the vibrating segments, the resultant 

 effect is enormously diminished by interference. Thus the vibrations of 

 a piano-string are communicated to the air scarcely at all directly, but 

 only through the intervention of the sounding board.* 



On the foundation of these principles he easily explains a curious, 

 observation by Leslie, which had much mystified earlier writers, 

 When a bell is sounded in hydrogen, the intensity is greatly reduced. 

 Not only so, but reduction accompanies the actual addition of hydrogen 

 to rarefied air. The fact is that the hydrogen increases the wave- 

 length, and so renders more complete the interference between the 

 sounds originating in the positively and negatively vibrating segments. 



The determination of the laws of viscosity in gases was much 

 advanced by him. Largely through his assistance and advice, the first 

 decisive determinations at ordinary temperatures and pressures were 

 effected by Tomlinson. At a later period he brilliantly took advantage 

 of Crookes' observations on the decrement of oscillation of a vibrator 

 in a partially exhausted space to prove that Maxwell's law holds up to 

 very high exhaustion and to trace the mode of subsequent departure 

 from it. Throughout the course of Crookes' investigations on the 

 electric discharge in vacuum tubes, in which he was keenly interested 

 and closely concerned, he upheld the British view that the cathode 

 stream consists of projected particles which excite phosphorescence in 

 obstacles by impact : and accordingly, after the discovery of the- 

 Rontgen rays, he came forward with the view that they consisted of 

 very concentrated spherical pulses travelling through the aether, but 

 distributed quite fortuitously because excited by the random collisions 

 of the cathode particles. 



A complete estimate of Stokes' position in scientific history would 

 need a consideration of his more purely mathematical writings, 

 especially of those on Fourier series and the discontinuity of arbitrary 

 constants in semi-convergent expansions over a plane, but this would 

 demand much space and another pen. The present inadequate survey 

 may close with an allusion to another of those " notes," suggested by 



* It may be worth notice that similar conclusion? are more simply reached b.\ 

 considering the particular case of a plane vibrating surface. 



