138 



Mr. W. Crookes on 



[June 15^ 



paniment of tlie phenomena was rendered probable by the observations of 

 Dr. Schuster, as well as by my own experioients on the movement of the 

 floating glass case of a radiometer when the arms are fixed by a magnet*. 



My first endeavour was to get some experimental means of discrimi- 

 nating between the viscosity of the minute quantity of residual gas and 

 the other retardiug forces, such as the friction of the needle-point on the 

 glass cup when working with a radiometer, or the torsion of the glass fibre 

 when a torsion-apparatus was used. A glass bulb is blowTi on the end of 

 a glass tube, to the upper part of w^hich a glass stopper is accurately 

 fitted by grinding. To the lower part of the stopper a fine glass fibre is 

 cemented, and to the end of this is attached a thin oblong plate of pith, 

 which hangs suspended in the centre of the globe ; a mirror is attached 

 to the pith bar, which enables its movement to be observed on a 

 graduated scale. The stopper is well lubricated with the burnt india- 

 rubber which I have already found so useful in similar cases (207). 

 The instrument is held upright by clamps, and is connected to the pump 

 by a long spiral tube. The stopper is fixed rigidly in respect to space, 

 and an arrangement is made by which the bulb can be rotated through a 

 small angle. The pith plate, with mirror, being suspended from the 

 stopper, the rotation of the bulb can only cause a motion of the pith 

 through the intervention 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 forwards in 

 decreasing arcs, presently setting in its old position relatively 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 v^ue 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 loga- 

 rithmic decrement will involve the viscosity of the glass fibre ; but glass 

 is so nearly perfect]}'" elastic, and the fibre so very thin, that this will be 

 practically insensible. 



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 conveiiiently attain — higher, indeed, than is necessary 

 to produce repulsion by radiation. 



I next endeavoured to measure, simultaneously with the logarithmic 

 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 

 * Proc. Roy. Soc. vol. xxiy. p. 409. 



