286 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
284. The thin mica radiometer behaved differently. When first immersed in hot 
water, it gave one turn in the negative direction, and then changed to positive, 
rotating positively for some time at a good speed. It finally came to rest. 
When removed from the hot water and allowed to cool, the thin mica radiometer 
rotated negatively for a considerable time. 
285. The bright aluminium radiometer, on immersion in hot water, rotated rapidly 
in the negative direction, and soon came to rest. 
When cooling in the air, this radiometer rotated 'positively at a considerable speed. 
286. The aluminium radiometer, blacked on both sides, behaved like the plain 
aluminium one, but in a stronger degree, rotating negatively on heating and positively 
on cooling. 
287. The pith radiometer, when dipped into hot water, moved in an undecided 
manner, first a little one way and then a little the other — the tendency, however, 
being in the positive direction. 
On cooling, the pith radiometer moved continuously, but slowly, in the negative 
direction, and kept up the movement for a considerable time. 
288. The thick mica radiometer was heated over a Bunsen burner till the glass 
approached its softening point. It was then turned upside down, and allowed to cool 
till the temperature of the bulb was about 150° C. The cooling having gone on 
for some time, and taking place entirely by radiation from the outside of the bulb, it 
was certain that the fly inside was somewhat hotter than the bulb. On now inverting 
it, so that the fly rested on the needle point, the vanes immediately commenced to 
rotate in the positive direction. When the temperature of the outer bulb was appre- 
ciably the same as the room, the vanes stopped, then gave two or three turns the 
negative way, and finally came to rest. 
289. Thinking it probable that the kind of dark heat might vary in refrangibility 
according to its source, and that the rays from hot water, hot glass, and hot metal 
might affect the materials composing the vanes in a different manner, and being 
absorbed by one body and transmitted by another might cause the positive or negative 
rotation, I repeated the experiments, varying the source of heat. 
290. The five radiometers were immersed in boiling water ; after cooling, they were 
immersed in water only a few degrees above the temperature of the room. In each 
case the results were of the same kind as already described with water of 7 0° C. 
291. The five radiometers were covered successively with hot shades of English, 
French, and German glass, of different thicknesses, and at different degrees of tem- 
perature. They were sometimes dry, and at others wet and filled with steam. The 
results were even more contradictory than with the hot water, and no law could yet 
be disentangled from the individual results. 
292. Heating the bulbs with a gas or spirit flame gave less uniform results than 
those obtained with the hot shades. 
293. A funnel was heated in boiling water, and then allowed to rest on the five 
