MR. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
307 
348. Pressure 14 millims. — The disk is still stationary. The vanes move very 
slowly in the negative direction Cp> . 
Pressure 1 millim. — The disk has commenced to rotate in the same direction as the 
vanes, at a speed of 3 revolutions a minute. The vanes have been gradually increasing 
in speed as the exhaustion has progressed until they now rotate at a speed of 43 revo- 
lutions a minute in the negative direction . 
349. These results are sufficient to show that the negative movement met at 
moderately low exhaustions (339, 342) is not caused by air currents. As shown in 
par. 342, the effect of a current of hot air rising from the platinum ring should be the 
same as that of molecular pressure coming from the ring. If the molecular wind may 
be supposed to blow the vanes round in a positive direction a molar wind should cer- 
tainly act in the same manner. The present apparatus shows that this supposition is 
correct. In air at normal density the action of air currents is strong, blowing the 
vanes round positively. As the density diminishes, the strength of the air currents 
lessens likewise, until at a pressure of 19 millims. the ascending current of hot air has 
not strength enough to blow the vanes round positively, in opposition to the friction 
of the needle point, and possibly in opposition to the tendency to a negative move- 
ment which at a little less pressure begins to be apparent. 
350. Pressure 706 millionths of an atmosphere* — The disk and the vanes both 
rotate in the same direction ; the disk making 1 0 revolutions and the vanes 40 revo- 
lutions a minute negatively . 
Pressure 400 millionths of an atmosphere. — Movements and direction as at 706 
millionths. The disk making 12 and the vanes 25 revolutions a minute negatively. 
Pressure 294 millionths of an atmosphere. — At this pressure the speed of the disk 
and of the vanes is exactly alike. They rotate together in the same direction as when 
last observed, as if they were fixed to the same axis, at a speed of 12’ 5 revolutions a 
minute . 
351. Pressure 141 millionths of an atmosphere. — Up to this observation the vanes 
have been gradually diminishing whilst the disk has been increasing in speed. At 
this pressure, under the influence of the ignited wire, the disk rotates at a speed of 
26 revolutions a minute. The vanes, however, do not rotate at all, but oscillate a 
little as if under the influence of two opposing forces. 
352. Pressure 129 millionths of an atmosphere. — Between this experiment and the 
last a sudden change has occurred. The vanes which then were still now rotate 
rapidly in the positive direction with a speed of 100 revolutions a minute. The disk 
continues to rotate in the same direction as before, but with slightly diminished speed 
(18‘5 revolutions a minute) It is probable that some of the speed of the disk is 
* At low exhaustions I speak of millimetres of pressure, hut at high exhaustions I prefer to count in 
millionths of an atmosphere. The inconvenience of using two units of measure is less than that of 
employing one system for both ends of the scale. 
2 R 2 
