474 Royal Society:— 



learning the advantage of raising the whole apparatus to a high 

 temperature during exhaustion, I have used roasted mica lamp- 

 blacked on one side for the vanes ; for this purpose it is almost per- 

 fect, being a good absorber on one face, a good reflector on the 

 other, a bad conductor for heat, extremely light, and able to stand 

 high temperatures. Many experiments have been tried with metal 

 radiometers, some of the results being recorded in previous papers 

 which I have read before the Society; but being less sensitive than 

 pith or mica instruments, I had not hitherto worked much with 

 them. I now tried similar experiments to the above, using the best 

 conductors of heat instead of the worst ; and for this purpose thick 

 gold-leaf was selected for the surface on which to try the action of 

 radiation. An apparatus was constructed resembling a radiometer 

 with an opening at the top, capable of being closed with a plate of 

 glass. Through this I could introduce disks of any substance I 

 liked, mounted in pairs on an aluminium arm rotating on a needle- 

 point. The first disks were of gold-leaf, blacked on alternate sides. 

 After exhaustion, a candle repelled the black surface of one of the 

 disks, but, to my surprise, it strongly attracted the black surface of 

 the other disk. I noticed that the disk which moved the negative 

 way was somewhat crumpled, and had the outer edge curved so as 

 to present a slightly concave black surface to the candle. I soon 

 found that the curvature of the disk was the cause of the anomaly 

 observed ; and experiments were then tried with disks of gold and 

 aluminium — the latter being chiefly used as being lighter and stiffer, 

 whilst it acted in other respects as gold. 



A radiometer the fly of which is made of perfectly flat aluminium 

 plates, lampblacked on one side, is much less sensitive to light 

 than one of mica or pith ; but, as I proved in my earlier papers, it 

 is more sensitive to dark heat. Exposed to light, the black face of 

 a metal radiometer moves away as if it were black pith. When, 

 however, it is exposed to dark heat, either by grasping the bulb 

 with the warm hand, dipping it into hot water, or covering it with 

 a hot glass shade, it rapidly rotates in a negative direction, the black 

 advancing, and continuing to do so until the temperature has become 

 uniform throughout. On now removing the source of heat, the fly 

 commences to revolve with rapidity the positive way, the black 

 this time retreating as it would if light shone on it. Pith or mica 

 radiometers act differently from this, dark heat causing them to re- 

 volve in the same direction as light does. 



The outer corners of the aluminium plates, which were mounted 

 diamond-wise, were now turned up at an angle of 45°, the lamp- 

 blacked surface being concave and the bright convex. On being 

 exposed to a candle, scarcely any movement was produced ; when 

 one vane was shaded off the other was repelled slightly, but the 

 turned-up corner seemed to have almost entirely neutralized the 

 action of the black surface. A greater amount of the same corner 

 was now turned up, the fold going through the centres of adjacent 

 sides. Decided rotation was now produced by a candle, but the black 



