TEE RADIOMETER. 13 



and the only opening is then hermetically sealed. If, now, the sun's 

 light or even the light from a candle shines on the vanes, the black- 

 ened surfaces which are coated with lamp-black are repelled, and, 

 these being symmetrically placed around the wheel, the several forces 

 conspire to produce the rapid motion which results. The effect has 

 all the appearance of a direct mechanical action exerted by the light, 

 and for some time was so regarded by Mr. Crookes and other eminent 

 physicists, although in his published papers it should be added that Mr. 

 Crookes carefully abstained from speculating on the subject aiming, 

 as he has since said, to keep himself unbiased by any theory, while he 

 accumulated the facts upon which a satisfactory explanation might be 

 based. 



Singularly, however, the first aspect of the new phenomena proved 

 to be wholly deceptive; and the motion, so far from being an effect of 

 the direct mechanical action of the waves of light, is now believed to 

 be a new and very striking manifestation of molecular motion. To 

 this opinion Mr. Crookes himself has come, and in a recent article he 

 writes : " Twelve months' research, however, has thrown much light 011 

 these actions ; and the explanation afforded by the dynamical theory of 

 gases makes, what was a year ago obscure and contradictory, now rea- 

 sonable and intelligible." 



As is frequently the case in Nature, the chief effect is here obscured 

 by various subordinate phenomena, and it is not surprising that a great 

 difference of opinion should have arisen in regard to the cause of the 

 motion. This would not be an appropriate place to describe the nu- 

 merous investigations occasioned by the controversy, many of which 

 show in a most striking manner how easily experimental evidence may 

 be honestly misinterpreted in support of a preconceived opinion. I 

 will, however, venture to trespass further on your patience, so far as to 

 describe the few experiments by which very early in the controversy I 

 satisfied my own mind on the subject. 



When two years ago I had for the first time an opportunity of ex- 

 perimenting with a radiometer, the opinion was still prevalent that the 

 motion of the wheel was a direct mechanical effect of the waves of 

 light, and therefore that the impulses came from the outside of the in- 

 strument, the waves passing freely through the glass envelope. At 

 the outset this opinion did not seem to me to be reasonable, or in har- 

 mony with well-known facts ; for, knowing how great must be the mo- 

 lecular disturbance caused by the sun's rays as shown by their heating 

 power, I could not believe that a residual action, such as has been re- 

 ferred to, would first appear in these delicate phenomena observed by 

 Mr. Crookes, and should only be manifested in the vacuum of a mer- 

 cury-pump. 



On examining the instrument, my attention was at once arrested 

 by the lampblack coating on the alternate surfaces of the vanes ; and 

 from the remarkable power of lampblack to absorb radiant heat it was 



