232 Royal Society : — 



viscous fluid (194) ; aud this at once led me to think that the re- 

 pulsion caused by radiation was indirectly due to a difference of 

 therinometric heat between the black and white surfaces of the 

 moving body (195), and that it might be due to a secondary action 

 on the residual gas. 



On April 5, 1876, I exhibited at the Soiree of the Koyal Society 

 an instrument which proved the presence of residual gas in a 

 radiometer which had been exhausted to a very high point of sen- 

 sitiveness. A small piece of pith was suspended to one end of a 

 cocoon fibre, the other end being attached to a fragment of steel. 

 An external magnet held the steel to the inner side of the glass 

 globe, the pith then hanging down like a pendulum, about a mil- 

 limetre from the rotating vanes of the radiometer. By placiug 

 a candle at different distances off, any desired velocity, up to several 

 hundreds per minute, could be imparted to the fly of the radiometer. 

 Scarcely any movement of the pendulum was produced when the 

 rotation was very rapid ; but on removing the candle, and letting 

 the rotation die out, at one particular velocity the pendulum set 

 up a considerable movement. Professor Stokes suggested (and, 

 in fact, tried the experiment at the time) that the distance of the 

 candle should be so adjusted that the permanent rate of rotation 

 should be the critical one for synchronism corresponding to the 

 rate at which one arm of the fly passed for each complete oscil- 

 lation. In this way the pendulum was kept for some time swinging 

 with regularity through a large arc. 



This instrument proved that, at a rarefaction so high that the 

 residual gas was a non-conductor of an induction-current, there was 

 enough matter present to produce motion, and therefore to offer 

 resistance to motion. That this residual gas was something more 

 than an accidental accompaniment of the phenomena was rendered 

 probable by the observations of Dr. Schuster, as well as by my own 

 experiments on the movement of the floating glass case of a radio- 

 meter when the arms are fixed by a magnet*. 



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

 criminating between the viscosity of the minute quantity of re- 

 sidual gas and the other retarding 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 blown on the end of a glass tube, to the upper 

 part of which 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 moA^ement 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, 

 * Proc. Boy. Soc. vol. xxiv. p. 409. 



