MR. W. CROOKES ON REPULSION RESULTING EROM RADIATION. 
351 
black surfaces, the white is repelled more than the black, rotation of the radiometer 
taking place in the negative direction. The same rays falling on the two surfaces do 
more work on the white than on the black ; and this, to my mind, appears sufficient to 
make it almost certain that the white pith absorbs more of these low rays than does, 
the lampblack. 
173. Let us imagine that surfaces of lampblack and pith are carried along the spec- 
trum from the blue to the ultra-red. As long as they are in the visible portion we 
observe an enormous difference between them. In the extreme red we can actually see 
that this difference is becoming less. In my mind’s eye I picture the progress being 
continued along the whole length of the ultra-red spectrum. I can see, by the light of 
the above-quoted experiments, that the absorptive action of the two surfaces gradually 
gets more equal. Soon they become identical in their action on the incident rays, and 
after that they enter a portion of the spectrum whose rays are no longer absorbed by 
lampblack, whilst they are quenched by the pith. Lampblack and pith have now 
changed places ; the latter is black, whilst lampblack has become a white substance. 
174. The normal rotation of the radiometer caused by dropping ether on it (163) is 
perfectly well explained by the above hypothesis. If heat in the act of absorption 
produces motion in one direction, in the act of radiation it produces motion in the 
opposite direction (167). Heat of low refrangibility falling on the radiometer repels 
the white more than it does the black, and produces negative rotation. When the same 
kind of heat is drawn out of the black and white surfaces by the chilling action of the 
ether, movement takes place in an opposite direction, and the arms rotate normally. 
On stopping this efflux of heat by covering the instrument with a hot shade (163), I 
changed the direction of movement by causing the surfaces to absorb instead of emit 
heat. 
An irregular emission or absorption of heat (164) stops the movement altogether, 
for the reasons given in pars. 155 to 158. 
175. I have made an apparatus by means of 
which I hoped to put the above theory to accurate 
test. The results are not so definite as they ought 
to be in order to settle the question ; but they are 
worth giving in detail, as some novel facts have been 
elicited by them. 
Fig. 10 shows the instrument : a is the bulb of 
a radiometer of the usual construction, having pith 
disks blacked on one side, b is a tube sealed into 
one side of the bulb, and having two stout plati- 
num wires passing along it, sealed their whole 
length in glass to prevent leakage of air into the 
interior of the apparatus. At the ends c of the 
wires, a spiral of fine platinum wire is fastened, 
3 c 2 
Fig. 10. 
