116 
MR. W. CROOKES OK REPULSION RESULTING- FROM RADIATION. 
exhausted. A candle was placed 2 inches from the glass, and sufficiently above the 
level of the band to allow the rays to pass over it on to the fly. The exhaustion 
was continued until the maximum sensitiveness was obtained. Air was let in, and 
the speed at the pressure of greatest sensitiveness was taken several times ; it was 
Fig. 22. 
found to be 40 revolutions a minute, becoming less if the exhaustion exceeded or fell 
short of the most sensitive point. 
The aluminium band was now removed and the experiment again repeated. At 
the pressure of maximum sensitiveness the revolutions of the fly were only 8'5 
revolutions a minute, diminishing on each side of this pressure. 
The black aluminium band was replaced, and a fly, with clear mica vanes, not 
blacked, and favourably presented (273), was suspended in the instrument. The 
experiments were now repeated as in the last case, and the result was as before— 
that the fly rotated more quickly with the aluminium band in, than when it was 
absent. 
The increased speed in these experiments must not be entirely attributed to the 
fact that the blacked aluminium band presents a better reacting surface for the 
rebounding molecules than does the polished glass. In the first place, the intro¬ 
duction of the metal band slightly diminished the distance between the fly and 
the reacting surface; and in the second place, the rays from the candle not only 
shone on the fly, but also on the opposite he mi-cylinder of black aluminium, and 
warming it converted it into a driving surface. Each of these actions would 
cause an increase of speed in the fly; but added together, they could only account 
for a part of the jump from 8b to 40 revolutions. The diameter was only to a slight 
extent d im ini shed, and it has been shown that a much greater reduction of diameter 
(437) had only power to add 50 per cent, to the speed. The effect which the light 
of the candle shining on the blackened liemi-cylinder would have on the speed 
was ascertained by placing the candle in such a position that it would illuminate 
the inner half of the aluminium band, while its direct action on the fly was cut off 
by screens. The driving action of the pressure from the cylinder was shown in this 
way to be only equal to about 1 3b revolutions a minute. 
