282 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
0‘0005 inch in thickness.* No. 3 of pith, 0’05 inch in thickness.* No. 4 of 
aluminium, 0'002 inch in thickness.* These four radiometers were plain on each 
side, no lampblack being applied. Their appearance is shown in plan in fig. 12. 
No. 5 was made of aluminium, identical with No. 4, but the vanes were lampblacked 
Fig. 12. 
on each side instead of being bright. Had the vanes pointed radially, there could 
have been no tendency for any one of the flies to move either way, but being 
inclined, the normal movement, on exposure to radiation, should be in the direction of 
the arrows — a direction which I will call the positive direction. 
274. The radiometers were tested with a candle. They all moved positively, but 
with very different rates. With the candle 3 inches off, no movement took place 
at first ; after a little time repulsion was observed, but rotation could not be started. 
By following up the retreating vane with the candle, rotation was set up, and on 
then removing the candle 3 inches off, rotation continued in the case of the 
pith and the two aluminium instruments, but not with the mica. The black 
aluminium was the easiest to move, then the pith. The bright aluminium fly was 
more difficult to start than the thick mica fly, but when once set going it moved the 
quicker of the two. The thin mica fly was the most sluggish of all. 
These radiometers are evidently affected much more by the heat from the candle 
than by the light. I explain the above actions in the following manner. 
275. I assume that the glass bulb is transparent to the light of the candle, but 
offers considerable obstruction to the ultra-red heat rays which accompany the light 
(246), and that the dark heat rays which are arrested raise the temperature of the 
glass, which thereupon gives out dark heat. The side of the bulb exposed to the air 
radiates dark heat outwards, and the inner side of the bulb in contact with the highly 
rarefied gas, generates molecular pressure, which reacts in all directions (218 postscript), 
but most powerfully in a direction normal to the surface (312). 
276. To explain the action more easily I will first discuss the action of the mica- 
vaned radiometers exposed to the candle. In fig. 13, the candle is represented 
shining on the bulb. The rays of light pass through the first wall without action. 
They then meet the mica, and that also being very transparent, the rays pass through 
it likewise, and then escape through the opposite side of the bulb, as is shown by 
* Measured by a Whitworth’s measuring machine. 
