Mr. W. Crookes on the Radiometer. 475 



surface was attracted * instead of repelled. Dark heat still caused 

 the opposite rotation to light, repelling the black surface. 



The plates were now folded across the vertical diagonal, the black 

 surface being still inside and the bright metal outside. The actions 

 with a candle and hot glass shade were similar to the last, but more 

 decided. 



The plates were now flattened, and put on the arms at an angle, 

 still being in the vertical plane. When the bright surface was out- 

 side, scarcely any action was produced by a candle ; but when the 

 lampblacked surface was outside, strong repulsion of the black was 

 produced, both with a candle and with a hot shade. 



The square aluminium plates were mounted in the experimental 

 apparatus, one being attached to the arm by" the centre of one of 

 the sides, and the other by an angle. The opposite corner of the 

 one mounted diamond-wise was turned up at an angle. The outer 

 convex surface of the diamond plate was blacked, and the side of 

 the square plate facing the same way was also blacked, so that 

 either two black or two bright surfaces were always exposed to 

 the light, instead of a black and a white surface, as is usual in ra- 

 diometers. As might have been expected, both these black surfaces 

 were repelled ; but the turned-up corner of the diamond-mounted 

 plate proved so powerful an auxiliary to its black surface, that 

 strong rotation was kept up, the square plate being dragged round 

 against the action of light. 



Folding the plates with the angle horizontal has not so decided an 

 action as when the fold is vertical. 



Sloping the plates and disks of a lampblacked mica radiometer 

 so as to have the black outside, and consequently more facing the 

 side of the bulb, greatly increases its sensitiveness. 



The above experiments show that shape has even a stronger in- 

 fluence than colour. A convex bright surface is strongly repelled, 

 whilst a concave black surface is not only not repelled by radiation 

 but is actually attracted. 



I have also tried carefully shaped cups of gold, aluminium, and 

 other metals, as well as cones of the same materials. I will briefly 

 describe the behaviour of a few typical radiometers made with 

 metal cups, which I have the honour of exhibiting to the Society. 



No. 1035. A two-disk cup-shaped radiometer, facing opposite 

 ways ; both sides bright. The disks are 14*5 millims. in diameter ; 

 and their radius of curvature is 14 millims. 



Exposed to a standard candle 3'5 inches off, the fly rotates 

 continuously at the rate of one revolution in 3*37 seconds. A 

 screen placed in front of the concave side so as to let the light 

 shine only on the convex surface, repels the latter, causing con- 

 tinuous rotation at the rate of one revolution in 7*5 seconds. 

 When the convex side is screened off so as to let the light shine 

 only on the concave side, continuous rotation is produced at the 



* I use the word attraction in these cases for convenience of expression. 

 I have no doubt that what looks like attraction in these and other cases is 

 really due to vis a (ergo. 



