MR. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
299 
repulsion. The light falls on a polished metallic surface, and about half is reflected 
without absorption (278). That which is absorbed acts as in the last case, but the 
forces on each side being more nearly equal, the balance in favour of the convex side 
is not enough to cause rotation and can only produce slight repulsion. 
329. "When the light is screened from the bright convex side and allowed to fall on 
the black concave side, as in Series II., experiment c, the same action takes place as 
in the two previous cases. The light falls on the black side, and the absorption and 
consequent generation of molecular pressure are greater than in experiment b. The 
balance in favour of the convex side is now enough to drive it round. No light, 
however, falling on the vane presenting the convex side, the slight additional impetus 
which would have been given by this vane is absent, and the rotation is not so rapid 
as when both vanes were illuminated. 
330. In the next series (324) the black is applied to the convex side of the cups. 
The pressure arising from the black surface, conspiring with that due to favour- 
able presentation, drives the fly round with a greater speed than in any of the other 
experiments — viz., at a rate of 46 revolutions a minute. It is easy to understand 
that cutting off the light from the convex cup, as in experiment b, will diminish the 
speed, but it is not immediately apparent why the rotation stops when the light is 
screened from the bright concave cup. A little consideration, however, shows that 
it must be so. The light here shines on a polished metallic surface, and as in 
experiment b of Series II., not more than half is absorbed. The molecular pressure 
generated is therefore insufficient to leave enough balance in favour of the convex side 
to drive it round. 
331. The apparent attraction in Series IV., experiment c, is easy to understand. 
The light falling on the black surface is almost entirely absorbed and converted into 
heat of temperature. The metal being very thin and a good conductor, this heat is 
equally apparent on the convex and concave sides, both being black. The molecular 
pressure on each face is therefore equal, and the more favourable presentation of the 
convex side determines the excess of active pressure in its favour. 
332. In testing the action of dark heat on these metallic radiometers by 
immersing them in hot water, I noticed that the negative rotation which generally 
resulted (304) took much longer to sink to rest when all bright cups were used than 
would appear necessary for the whole instrument to acquire the temperature of the 
water. The communication of heat from the glass bulb to the metallic cups is 
effected not by conduction or convection (in addition to radiation), but by the mole- 
cules travelling to and fro between the glass and the metal ; and each molecule 
depositing or handing on towards the metal the extra force it has received from 
the glass, it is conceivable that the process of equalisation may be slower than under 
ordinary circumstances. This is a point which can be settled by experiment, and I 
intend to return to the subject. 
333. When a radiometer immersed in hot water has come to rest, the approach 
2 q 2 
