MR, W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
261 
from ordinary clear mica, through the agency of volcanic or other natural heat. The 
results show how well Professor Stokes’s theoretical reasoning is confirmed by experi- 
ment. With a radiometer, the fly of roasted mica lampblacked on one side, the 
power of radiation to cause rotation will be 1 1 1 *7 on the black, and —7 ‘6 on the 
white side, or a total of 119 - 3, as against 100 with lampblacked pith. The 
employment of silver flake mica is still more advantageous, as here the blacked side 
is repelled with a force of 151, whilst the bright side is (apparently) attracted 
with a force of 9 '8, making a total force to turn the fly round of 16 O' 8. The negative 
action, or apparent attraction, seen when radiation falls on the clear face of a mica disk 
lampblacked behind, is caused by the heat passing through the thin plate and warming 
the hinder surface of lampblack. The thinner the mica the greater is this action, up to 
the point when the lampblack begins to show through. With a thick piece of silver 
flake mica no negative repulsion takes place, but it becomes positive, increasing in 
power until it reaches 12 ‘5, which is the amount given by silver flake mica with no 
black behind it. Taking the action on the blacked silver flake mica as 151, and that 
on the same mica unblacked as 12 '5, the action exerted on the black surface is seen 
to be 12 times greater than that on the white surface. In some experiments on 
roasted mica radiometers, published by Professor E. Wartmann,* the action on the 
black face is found to be 11 ‘9 times stronger than that on the white fane. 
On heating 1 clear mica in the blowpipe flame, the side on which the flame plays 
becomes opaque and silvery, whilst the other side retains its glassy reflecting surface. 
The employment of one or the other side as the lampblacked surface exerts a 
considerable difference on the results. 
239. The very low results given by the three kinds of charcoal I explain thus : — 
Radiation falling on the front surface is absorbed and converted into thermometric 
heat (107, 195), which warms the front side of the charcoal. Although charcoal is 
one of the worst known conductors of heat, there is no doubt that the vibration 
caused by heat on the surface of a plate of charcoal is quickly communicated to the 
layer beneath, and thus a very short time suffices for the conduction of heat through 
a disk only half a millimetre thick. Having arrived at the hinder surface, the heat 
which there becomes sensible causes negative repulsion to take place, as in the case 
of the roasted and blacked mica (238), and the numbers 18*4, 15 - 2, and 1P6 are 
simply the differences between these two opposing actions. 
This explanation is proved by an experiment. A reference to Table XII. shows 
that the action of radiation on plain pith is equal to 17‘7, whilst that on cocoa- 
nut shell charcoal is only 11 ’6. Now were this amount of 11*6 the total action 
of radiation instead of the difference only, lampblacking the front surface of the 
charcoal should bring it up to 100, and a radiometer made of cocoa-nut shell charcoal 
lampblacked on one side should work better than one made of lampblacked pith. On 
making such an instrument it was however found to be almost insensitive, the feeble 
* Societe de Physique et d’Histoire Naturelle de Geneve, Seance du 2 Mars, 1876. 
