268 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
equally. Now, I have already shown (128, 144, 155, 156, 157, 161) that dark heat, 
radiated from glass, has a very different action on a lampblack and pith radiometer 
from that of the luminous rays; and at pars. 170, 1 71, 172, reasons are adduced for 
believing that, to rays of low refrangibility radiated from hot glass, white pith and 
lampblack change places, the pith becoming black and the lampblack acting as a light 
substance. 
It is therefore legitimate to carry out the same line of reasoning in the case of 
chromic oxide and copper tungstate. To the total radiation from the candle, unfil- 
tered except through the thin glass wall of the bulb, the chromium compound is the 
more sensitive ; the chromic oxide is black, and the copper tungstate white. But 
when exposed to the heat rays of low refrangibility emitted by warm glass, the two 
powders change places, and chromic oxide becomes white, whilst copper tungstate 
absorbs them powerfully, and behaves as a black powder. The difference between the 
action of dark heat and that due to the other rays is sufficient not only to overcome 
th e plus action of 20 '3, but to give a slight negative action. 4 ' 
When covered with a hot glass shade, negative rotation takes place, the copper 
tungstate being repelled ; on removing the shade and letting the instrument cool, 
reverse rotation takes place, the fly now moving positively. The theory just discussed 
is therefore corroborated. 
247. The next powders experimented with were persulpho-cyanogen and copper 
oxalate ; in a tabular form these stand thus : — 
No screen. 
Water screen. 
Persulpho-cyanogen . . . 
43-9 
10 
Copper oxalate .... 
30T 
33 
+ 13-8 
-2-3 
Showing a positive rotation of 13 ’8 to the naked flame, and a negative rotation of 
2 '3 behind a water screen. When properly exhausted and exposed to the candle 
behind water, negative rotation was produced, and continued steadily as long as the 
experiment was tried. Without moving the candle the water screen was now taken 
away. The direction of rotation instantly changed, and gave place to a rapid posi- 
tive rotation. The speed, however, soon diminished, and in a little time, when the 
bulb had become warm, there was great difficulty in the copper oxalate passing the 
candle, and sometimes a disk would stop on the dead centre. A gentle tap on the 
bulb always sent it on, and then rotation would continue for a short time longer, the 
persulpho-cyanogen always being the powder most repelled by the naked flame. 
When the fly was cold a hot glass shade was inverted over the bulb ; positive 
rotation immediately commenced, which, when the hot shade was removed and the 
apparatus was cooling, changed to negative rotation. 
* I may here remark that this anomalous action is strongly marked in the case of most copper salts, 
which act as if they were nearly white to light, but black to heat of low refrangibility. 
