346 
ME. W. CEOOKES ON EEPULSION EESULTINGr EEOM EADIATION. 
this difference of action is not apparent (128). The following experiments were tried 
with various radiometers : — 
A candle was placed at such a distance from a radiometer that the fly would make 
one revolution a minute. A small glass flask of boiling water was then placed half an 
inch from the bulb. The revolutions instantly stopped, two of the arms setting equi- 
distant from the hot-water flask. The candle was kept in the same position, and the 
flask of water was removed. As the portion of the bulb which had been heated by the 
hot water cooled, the white surface gradually crept nearer and nearer to it, the superior 
repulsion of the candle on the black disks urging the arms round, and acting in opposi- 
tion to the repulsion of the hot glass to the white disk. At last the force of the light 
drove the white disk with difficulty past the hot spot of glass. Rotation then commenced, 
but for some revolutions there appeared to be a difficulty in the white disks passing the 
spot of glass which had been warmed by the hot water. 
156. The flask of boiling water was then replaced in its position half an inch from 
the bulb of the radiometer. The rotation immediately stopped. The candle was then 
brought gradually nearer and nearer to the instrument, but with no particular effect. As 
it came very near the arms vibrated to and fro, and appeared to make violent efforts to 
get round, but no force of the light seemed sufficient to overcome the repugnance of the 
white disk to pass the heated portion of the glass. 
157. The radiometer was allowed to cool, and the candle was again placed in the 
first position, where it produced one revolution in a minute. The finger was pressed 
against the side of the bulb. As the spot of glass got warm the white surface experi- 
enced more and more difficulty in getting past it, until at last one disk refused to pass, 
and the arms came to rest. 
The instrument was again allowed to cool, and the revolutions recommenced at the 
usual speed (the laboratory in which this was tried was somewhat cold). I then came 
from a warm room, and stood a foot from the radiometer, watching it. In about a 
minute the radiant heat from my body had warmed the side of the bulb nearest to me 
sufficiently to cause an appreciable difficulty in the movement, and soon the revolutions 
stopped. The same effect has been observed if the radiometer is brought into a very 
warm room, and placed near a cold window. If the daylight is feeble, the instru- 
ment not very sensitive, or an observer stands near the instrument, an appreciable 
sticking is observed as the white disks come near that part of the bulb which is the 
warmest. 
These experiments show that dark heat has quite a different action from that of the 
luminous rays. They also show that many precautions are necessary to guard against 
the interfering action of unequal heating of the radiometer when it is being used for 
accurate measurements. 
158. Having found such an antagonistic action of dark heat, I tried the action of ice. 
This, I have already shown (33, 88), is equivalent to warming the opposite side of the 
instrument. A piece of ice brought near the radiometer on one side cuts off the influx 
