352 
ME. W. CROOKES ON REPULSION RESULTING EROM RADIATION. 
and the other ends ( d ) terminate in loops outside, e is a battery, f a contact-key, and 
g a resistance-coil, which I can vary at will. The bulb was perfectly exhausted, and 
the following experiments were tried: — 
176. The resistance-coil was so adjusted that the battery would keep the platinum 
spiral ( c ) at a bright red heat. The arms of the radiometer, which were before quite 
still, moved rapidly until two of the disks were one on each side of the hot spiral, 
the black disk being further off than the white disk, as shown at i. The resistance 
was then gradually increased, and as the temperature of the spiral diminished, the 
black disk gradually approached the spiral, until, when the temperature was just at 
the point of visible redness in a dark room, the black and white disks were practically 
equidistant from the spiral. On diminishing the resistance, the same phenomena took 
place in inverse order. 
177. The resistance was adjusted to give a bright red spiral, and the contact-key 
kept pressed down. The disks stood as at i. A lighted match was momentarily 
brought near the bulb, so as to start a movement. Rotation of the arms commenced, 
and kept up, with some energy, at the rate of about 1 revolution in five seconds, equal 
to that given by a candle 8 inches off. There was some little hesitation as the white 
side came up to the spiral, but this was scarcely noticed when the speed had become 
steady. 
The resistance was slightly increased. The speed became slower as the temperature 
of the spiral diminished, and the hesitation as the white approached the spiral became 
more apparent. The resistance was further increased, with the effect of making rotation 
still slower. I now brought the temperature of the spiral down to just visible redness 
in the dark. The speed of rotation again slackened ; at each approach of the white 
surface to the spiral it stopped, hesitated, and then got past with a rush. Thus 
it went on for a few revolutions, until one white disk, a little nearer perhaps than 
the others, was not able to pass, and the arms after a few oscillations came to rest, 
the black and white surfaces being, as near as I could judge, equidistant from the hot 
spiral. 
These results fully confirm those obtained in experiments 128 & 144, and I think 
justify the conclusions arrived at in my discussion of them at par. 171 — that at tempe- 
ratures between 250° and 100° the repellent action of radiant heat is about equal on 
black and on white surfaces. 
178. I now wished to ascertain whether the continuation of the reasoning (172) was 
correct — whether at temperatures lower than 100° C. the white would be repelled most. 
The resistance of the coil was increased again, and the position of the arms in respect 
to the spiral noticed. When so much resistance was offered to the passage of the 
current that the spiral would only be just warm, I fancied the white set further from 
it than the black ; but the observation was not satisfactory at higher temperatures ; up 
to visible redness the repulsion was equal for each. 
The breath sent the arms rapidly round the negative way (160). 
