1879-3 
Molecular Physics in High Vacua. 
435 
very shallow cup. In front of the cup is a mica screen ( c , d), 
wide enough to intercept nearly all the molecular rays 
coming from the negative pole. Behind this screen is a 
mica wheel (e, f) with a series of vanes, making a sort of 
paddle-wheel of it. So arranged, the molecular stream 
from the pole a h will nearly all be cut off from the wheel, 
Fig. 19. 
and what escapes over and under the screen will hit the 
vanes equally, and will not produce any movement. I 
now put a magnet, g, over the tube, so as to defied! the 
stream over or under the obstacle c d, and the result will be 
rapid motion in one or the other direction, according to the 
way the magnet is turned. I now throw the image of 
the apparatus on the screen. The spiral lines painted on 
the wheel show which way it turns. I arrange the 
magnet to draw the molecular stream so as to beat 
against the upper vanes, and the wheel revolves rapidly, as 
if it were an over-shot water-wheel. I now turn the 
magnet so as to drive the molecular stream underneath ; the 
wheel slackens speed, stops, and then begins to rotate the 
other way, as if it were an under-shot water-wheel. This can 
be repeated as often as I like to reverse the position of the 
magnet, the change of rotation of the wheel showing imme- 
diately the way the molecular stream is defledled. 
This experiment illustrates the last of the phenomena 
which time allows me to bring before you, attending 
the passage of the induction spark through a highly ex- 
hausted atmosphere. It will now be naturally asked, What 
have we learned from the phenomena described and 
exhibited, and from the explanations that have been 
