Mr. \Y. Crookes on Repulsion, resulting front Radiation. 233 



with mirror, being suspended from the stopper, the rotation of 

 the bulb can only cause a motion of the pith through the inter- 

 vention of the enclosed air. Were there no viscosity of the air, 

 the pith would not move ; but if there be viscosity, the pith will turn 

 in the same direction as the bulb, though not to the same extent, 

 and, after stopping the vessel, will oscillate backwards and for- 

 wards in decreasing arcs, presently setting in its old position rela- 

 tively to space. 



It was suggested by Prof. Stokes that it would be desirable to 

 register not merely the amplitude of the first swing, but the 

 readings of the first five swings or so. This would afford a good 

 value of the logarithmic decrement (the decrement per swing of 

 the logarithm of the amplitude of the arcs), which is the constant 

 most desirable to know. The logarithmic decrement will involve 

 the viscosity of the glass fibre ; but glass is so nearly perfectly 

 elastic, and the fibre so very thm, that this will be practically in- 

 sensible. 



According to Professor Clerk Maxwell, the viscosity of a gas 

 should be independent of its density ; and the experiments with 

 this apparatus have shown that this is practically correct, as the 

 logarithmic decrement of the arc of the oscillation (a constant 

 which may be taken as defining the viscosity of the gas) only 

 slightly diminishes up to as high an exhaustion as I can con- 

 veniently attain — higher, indeed, than is necessary to produce re- 

 pulsion by radiation. 



I next endeavoured to measure, simultaneously with the loga- 

 rithmic decrement of the arc of oscillation, the repulsive force, 

 produced by a candle at high degrees of exhaustion. The motion 

 produced by the rotation of the bulb alone has the advantage of 

 exhibiting palpably to the eye that there is a viscosity between the' 

 suspended body and the vessel ; but once having ascertained that, 

 and admitting that the logarithmic decrement of the arc of os- 

 cillation (when no candle is shining on the plate) is a measure 

 of the viscosity, there is no further necessity to complicate the 

 apparatus by having the ground and lubricated stopper. A move- 

 ment of the whole vessel bodily through a small arc is equally 

 effective for getting this logarithmic decrement ; and the absence 

 of the stopper enables me to have the whole apparatus sealed up 

 in glass, and I can therefore experiment at higher rarefactions than 

 would be possible when a lubricated stopper is present. 



The apparatus, which is too complicated to describe without a 

 drawing, has attached to it : — a, a Sprengel pump ; b, an arrange- 

 ment for producing a chemical vacuum ; c, a lamp with scale, 

 on which to observe the luminous index reflected from the mirror ; 

 d, a standard candle at a fixed distance ; and e, a small vacuum- 

 tube, with the internal ends of the platinum wires close together. 

 I can therefore take observations of : — 



1. The logarithmic decrement of the arc of oscillation when 

 under no influence of radiation ; 



2. The successive swings and final deflection when a candle 

 shines on one end of the blackened bar; 



