254 
ME. W. CEOOKES ON EEPULSION RESULTING PROM RADIATION. 
This reasoning is confirmed by the behaviour of the aluminium and platinum, but 
not by the gold, which is diminished ill fiction by the mica backing. Gold, 
however, is seen to be acted on but little by the heat rays, and it is probable that 
these are the rays which most readily raise the temperature of the plate and cause 
molecular pressure to be exerted from the hinder surface^ According to the same 
theory, if the transference of heat from the back surface be assisted by Coating it with 
lampblack, the repulsion by rays falling on the unblacked surface should diminish. 
This is seen to be the case with aluminium and gold. The following Table will explain 
these actions 
Metallic Plate. 
Backing. 
None. 
Mica. 
Lampblack. 
Aluminium 
13-8 
28-7 
5T 
Platinum ; 
20T 
28-7 
Gold . 
19 ; 3 
10‘6 
7 : 3 
The following experiment proves that when radiation from a candle falls on the 
surface of a metal plate, heat rapidly passes through and causes molecular pressure 
to be exerted from the back of the plate. A torsion apparatus was made similar 
to those used so frequently in these experiments but having a double suspension, and 
P^. 3. 
two independent beams, as shown in fig. 3, each supported by a glass fibre, and having 
at each end a plate of platinum. In the centre of the tube, at a, is a plate glass 
window, through which radiation falls on the plate at the end of the beam b, which 
plate, being larger than the one at the end of beam c, and overlapping it entirely, cuts 
off direct radiation from it. Stops d and <?, at the outer extremities of the beams, prevent 
