288 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
299. A thick brass ring, a little larger in diameter than the bulbs ( b , fig. 16), was 
heated to 400° C., and then held round the centre of the bulb of each radiometer in 
succession in the position b. All the flies revolved in the positive direction. The 
thick mica fly went at a maximum speed of 60 revolutions a minute, the thin mica 
fly made 36 revolutions a minute, the pith 1 5 revolutions a minute, the bright 
aluminium 11a minute, and the blacked aluminium 1 revolution a minute. 
300. When the equatorial hot ring was removed, and the bulbs were allowed to 
cool, no reversal of movement took place with the thick mica or the two aluminium 
flies. The thin mica and the pith flies showed decided reversal on cooling. 
301. To still further test the question of the change of direction on cooling, the 
experiments with the two brass rings were again tried on the five radiometers. 
The rings, this time, were made red hot, and were held in position till the flies were 
in rapid movement. The rings were removed, and as soon as the hot part of the bulb 
was cool enough to bear handling, it was dipped into cold water, so as to chill the 
glass quickly, and still keep the fly warm. Tried in this way very decided results 
were obtained, which will be better understood in the form of a Table. 
Material composing the fly 
of the radiometer ( favourably 
presented.) 
Hot ring applied 
above. 
Hot ring applied 
equatorially. 
Heating. 
Cooling. 
Heating. 
Cooling. 
Thin mica 
_ 
+ 
_ 
Thick mica 
? 
+ 
+ 
Bright aluminium .... 
— 
+ 
+ 
+ 
Blacked aluminium 
— 
+ 
+ 
+ 
Pith 
— 
— 
+ 
— 
The thin mica and the pith when cooling, after being heated above, kept rotating 
negatively for a long time. The others soon came to rest. The movement of the 
thick mica on cooling, after being heated above, was contradictory, some experiments 
giving it -fl and some — . The other movements were very decided. 
Applying the hot ring below had exactly the same effect as applying it above, and 
when the radiometers were heated above and below, the equator being kept cool, the 
rotation in the negative direction was very strong and rapid. 
302. Having described the various phenomena, I will show how far the theory 
explains them, and as the radiometers differ in construction and action, I will first 
discuss the movements common to all. 
It will be observed that when heat is applied round an equatorial ring of the bulbs 
(294. 296, 299, 301), the rotation is always in the positive direction. The hot ring 
of glass generates molecular disturbance, which presses towards the centre and strikes 
the sloping vanes, driving them round as if a wind were blowing on them (276). 
In fig. 17 I have tried to represent this action. The positive movement is hide- 
