548 



Prof. G. G. Stokes on certain [Dec. 20, 



this has ceased, which takes place in a couple of minutes or so, the heated 

 vessel be removed, a negative, though not lively, rotation is produced as 

 the apparatus cools. 



12. These results do not seem difficult to coordinate so far as to reduce 

 them, to their proximate cause. 



As regards the small quantity, if any, of heat radiated directly across 

 the glass of the bulb, the action of which was experimentally distin- 

 guishable by its promptitude, both radiometers behaved in the ordinary 

 way. 



13. As regards the mica radiometer, when the bulb gets heated, and 

 radiates towards the fly, the fly is impelled in the negative direction, as 

 if the white, pearly mica were black and the lampblack were white. And 

 there is nothing opposed to what we know in supposing that such is really 

 their relative order of darkness as regards the heat of low refrangibility 

 absorbed and radiated by the glass ; for the researches of Melloni and 

 others have shown that lampblack is, if not absolutely white, at any rate 

 very far from black as regards heat of low refrangibility. On the other 

 hand, glass and mica are both silicates, not so very dissimilar in chemical 

 composition ; and it would not therefore be very wonderful, but rather the 

 reverse, if there were a general similarity in their mode of absorption of 

 radiant heat, so that the heat most freely radiated by glass, and accord- 

 ingly abounding in the radiation from thin glass such as that of the bulb, 

 were greedily absorbed by mica. The explanation of the reversal of the 

 action when heat and cold were interchanged is too well known to require 

 mention. 



14. With the pith radiometer, when the bulb as a whole is heated, and 

 radiates towards the fly, the impulse is positive, though less strong than 

 in the case of the mica (§ 4) ; and when the bulb as a whole is cooler than 

 the fly the impulse is negative (§ 11). 



But to explain all the phenomena we must dissect the total radiation 

 from or towards the bulb. "When I first noticed the negative rotation 

 produced by a heated wet tumbler, I was disposed to attribute it to radia- 

 tion from the water, which possibly the glass of the bulb might be thin 

 enough to let pass ; but when I found that hot water in a glass vessel 

 outside, even though the glass of it were thin, produced no sensible effect, 

 and that blowing on the heated bulb when it was dry produced a similar 

 effect to blowing on it when dewed, though of much less amount, I per- 

 ceived that the moisture acted, not by direct radiation from it, and in conse- 

 quence of a difference of quality between the radiations from glass and 

 water, but by causing a rapid superficial heating of the bulb; and, 

 similarly, the blowing on the dewed surface acted by causing a rapid 

 superficial cooling. When the dry tumbler radiates to the bulb, the 

 radiation is absorbed at various depths ; the absorption is most copious, 

 it is true, at the outer strata ; but still the change of temperature is not 

 by any means so much confined to the immediate surface as when we 



