1876.] 



Mr. W. Crookes on the Radiometer. 



311 



and 107. • A large series of observations have been taken with this 

 apparatus, with the result not only of supplying important data for future 

 consideration, but of clearing up many anomalies which were noticed, and 

 of correcting many errors into which I was led at earlier stages of this 

 research. Among the latter may be mentioned the speculations in which 

 I indulged as to the pressure of sunlight on the earth. 



Hitherto most of my experiments had been carried on with bad con- 

 ductors of heat. To get the maximum action of a radiometer it appeared 

 necessary that no heat should pass through to the back surface, but that 

 all should be kept as much as possible on the surface on which the light 

 fell*. At first I used pith, but since learning the advantage of raising 

 the whole apparatus to a high temperature during exhaustion, I have used 

 roasted mica lampblacked on one side for the vanes ; for this purpose it is 

 almost perfect, 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 ajjparatus 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 

 alumiaium 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 



* I have already shown that when a raj' of light from any part of the spectrum falls 

 on a black surface the ray is absorbed and degraded in refrangibility, warming the black 

 surface and being emitted as radiant heat. In this sense only can the repulsion 

 resulting from radiation be called an effect of heat. 



