350 
ME. W. CROOKES ON REPULSION RESULTING EROM RADIATION. 
powdered rock-salt) ; and it is therefore reasonable to suppose that other substances may 
exist which, whilst they are very different to the eye, may have the same action on dark 
radiant heat. We may also fairly assume that a substance may exert a considerable 
selection on the rays which it absorbs and reflects — that, in fact, there may be, in the 
ultra-red region of the spectrum, thermic colours, as in the visible spectrum we have 
optical colours ; so that whilst two substances may absorb to the same extent heat-rays 
of one refrangibility, they may be quite different in their actions on heat-rays of another 
refrangibility. These suppositions are not only resonable but very probable : let us see 
how they account for the facts. Light falls on the black and white surfaces of a radio- 
meter, or other similar instrument. That which falls on the white surface is nearly all 
reflected back again. Were the surface perfectly white all the force which went into 
the bulb would be reflected out again ; the incident ray would contain in itself a certain 
amount of potential work ; but as the emergent beam would come out with no loss of 
intensity, no work could have been done on the reflecting surface. In practice this 
does not quite hold good. Pith is not a perfect reflector, some light stops behind, 
that which comes from it is not quite equal to that which it receives, and the balance 
makes itself evident by causing the pith to move to a slight extent. 
171. But in the case of light falling on the lampblacked surface the result is very 
different. Here, practically, the whole of the light is quenched by the lampblack. 
Force is poured into the bulb, but none comes out. What, then, becomes of it 1 ? It is 
changed into motion, and becomes evident in the strong repulsion which is exerted on 
the black surface. 
This I think is clear in the case of light. We can see that there is an enormous 
difference in the absorbing powers of white and black pith for light, and we can also see 
that there is an equally marked difference between the motive power which light exerts 
on them. But with the heat from boiling water or from a hot copper ball our eyes 
cannot tell us whether the same difference obtains or not, and we must use other and 
less direct means of finding out what takes place. 
Let me direct attention to the experiments described in paragraphs 128 & 144. 
Here red-hot copper was seen to repel the black surface with violence, and the white 
surface only moderately. As the copper ball cooled, the repulsion on each surface 
became more nearly equal. At 400° C. the differential action was decided, though 
faint. At 300° C. the black surface was still repelled slightly more than the white 
surface, but at 250° C. down to 100° C. the repellent action of the radiant heat was 
the same on the white as on the black surface. The two surfaces were then thermi- 
cally of the same colour. 
The fact that the work done on each surface was equal, is, I think, proof that the 
absorption of the incident rays was equal. 
172. Let me now carry the reasoning a step further. Experiments described in 
pars. 159 to 168 show that when heat of low refrangibility — from the body (159), the 
breath (160), hot air, or a warm glass shade (161, 165, 168) — falls on the white and 
