MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
89 
of silver-flake mica blacked on different sides. I quote the following from par. 237 of 
Part V. of this series :— 
Material of which the experimental disk in the torsion apparatus 
is composed. 
Amount of repulsion 
under the influence 
of a standard light. 
Lampblack (standard disk, pitb) . . 
100- 
Mica, silver-flake, lampblacked on one side, black side exposed 
151- 
Mica, silver-flake 
12-5 
Mica, silver-flake, blacked on one side, black turned away from light.. 
- 9-8 
391. It would throw much light on these two hypotheses, and probably enable the 
true law governing the movements to be disentangled from the individual results, if 
the screen were easily movable. An instrument was accordingly constructed as 
shown in fig. 3. a a are disks of silver-flake mica, lampblacked on one side. They 
Fig. .3. 
are mounted on aluminium arms, in the centre of which is a glass cap, c, balanced on 
the needle point, e, of the radiometer. The cap, c, has a small needle point fused into 
the top, and this serves as the support of a second glass cap, d, to which arms are 
attached, carrying the clear mica disks, b b. The upper cap, with the clear disks, 
is therefore pivoted on c, and will revolve freely round until arrested in one direction 
by striking against the blacked disks, and in the other direction by meeting the stop, f 
which prevents the two pairs of arms from separating beyond a right angle, as shown 
by the dotted line, V b '. The two pairs of disks will rotate together on the cap, c, 
and will not separate if no repulsive force is exerted between them; but if pressure is 
generated at the black surface, the disks will be pushed asunder. 
Experiment shows that, in whatever way the light is allowed to shine on the disks, 
there is an actual repulsive force produced between them, which causes them to separate 
to the furthest possible extent. By gentle tapping in a faint light, the clear disks can 
be brought within 2 millims. of the lampblacked surfaces; a candle is now brought 
near, shaded by a screen ; on removing the screen, so that the rays pass through one 
MDCCCLXXIX. N 
