394 On the Residual Gaseous Matter in Crookes Tubes. 



in the residual gaseous matter ; but it is found that when it 

 does this it invariably reverses its rotation almost immediately, 

 and proceeds to rotate more and more rapidly in the direction 

 that indicates a stream of residual gaseous matter passing 

 from the anode to the cathode. Usually, after one or two 

 oscillations, it starts immediately to rotate in this direction. 



An electrometer connected to the wheel through the pivot 

 and needle-point shows that the vanes are always positively 

 electrified. 



In order further to investigate the matter, I have had con- 

 structed another form of radiometer-tube, as shown in the 

 accompanying illustration. Here the vanes (of mica) are 



inclined, and the axis is parallel to the line joining the cathode 

 and anode. The wheel thus rotates in a plane at right angles 

 to the cathode- and anode-streams ; and it is difficult to see 

 how electrification of the vanes should in any way assist its 

 rotation either in one direction or in another. 



The wheel is arranged so that the vanes are all outside the 

 cathode-stream, and when the tube is excited it is found inva- 



