MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
253 
232. Table IX. — Metals. 
„ _ Water screen 
No screen. interposed (5 millims.). 
Lampblack (standard disk ) lOO'O lOO'O 
Precipitated silver (on a pith disk) .... 57'8 58'3 
Iron foil ............. 48'4 5T 
Aluminium leaf (backed -with mica) .... 28' 7 45‘0 
Platinum leaf (backed with mica) .... 28' 7 41 T 
Platinum leaf 20 ; 1 22 ; 3 
Lampblacked platinum leaf (black side exposed) 19’9 
Gold leaf 19-3 51-0 
Lampblacked silver leaf (black side exposed) . 17' 7 
Aluminium leaf 13'8 13'6 
Gold leaf (backed with mica) ...... 10'6 10'3 
Palladium foil (saturated with hydrogen) . . 10’6 
Gold leaf (lampblacked on reverse side) . . 7 '3 
Aluminium leaf (lampblacked on reverse side) 5'1 
As metals move very slowly and do not come back to zero for a considerable time, 
satisfactory observations are not easily obtained. Except when testing precipitated 
silver, which was in fine powder, mica or pith disks were not employed as supports 
in this series. The gold leaf was not of the thinnest, but sufficiently thick to bear 
handling, weighing half a grain per square inch. The silver, platinum, and aluminium 
leaves were about double the thickness of the gold. The iron and palladium were 
thicker still. 
The Table does not show any general relationship between the metals which I 
have tried, but before drawing conclusions, a much larger series of experiments 
should be undertaken. The extra weight of metallic disks prevented much expe- 
riment in this apparatus, but I hope to continue the experiments with metallic surfaces 
in an apparatus specially adapted to them. One or two points are here worth notice. 
The considerable movement of metallic iron with no screen interposed, and the slight 
action behind a water screen, show that the invisible heat rays are those chiefly 
absorbed by this metal ; whilst the greatly increased proportional action on gold 
behind the water screen shows that with this metal, the luminous rays are more 
absorbed than the invisible heat rays. 
The film s of metal employed are so thin that very little time is occupied in the 
conduction of heat from one surface to another. The rise of temperature, therefore, 
on the front surface, caused by the radiation from the candle, cannot be much greater 
than that on the back surface ; and the resulting molecular pressure must exert itself 
on each side of the plate, the movement of repulsion being due to the difference of 
this pressure in favour of the front surface. Backing the metallic leaf with a thin 
plate of mica, by stopping the action of the molecular pressure on the reverse side, 
and throwing it all on the front, should therefore increase the amount of repulsion. 
