1877.] 



Movements of Radiometers. 



553 



P.S. — It might be supposed at first sight that the change of rota- 

 tion from negative to positive (in § 7) was due, not to a change in the 

 conditions of absorption, but to the circumstance that the inner surface 

 of the bulb had become warm by conduction, so as to be warmer than the 

 surfaces of the fly instead of colder. For we now know that the " repul- 

 sion resulting from radiation," as in some way or other it undoubtedly 

 does result, is an indirect effect, in which radiation acts only through the 

 alterations it occasions in the superficial temperatures of the solids in 

 contact with the rarefied gas ; and it might be supposed that when the 

 inner surface of the bulb passed from colder than the fly to warmer, the 

 direction of rotation would, on that account alone, be reversed. This, 

 however, is not so. If bulb and fly are at a common temperature, and 

 the instrument is protected from radiation, the fly remains at rest whether 

 the common temperature be high or low. If a small portion of the total 

 surface in contact with the rarefied gas be warmed by any means, repul- 

 sion takes place, through the intervention of the rarefied gas, between 

 the warmed surface and the opposed surfaces, if not too distant ; if it be 

 cooled, the result is attraction. It does not matter whether the surface at 

 the exceptional temperature belong to the fly or the bulb. The former 

 takes place in the ordinary case of a radiomter exposed to radiation, the 

 latter in that of a radiometer at a uniform temperature and protected 

 from radiation when a small portion of the bulb is warmed or cooled, in 

 which case the part at the exceptional temperature repels or attracts the 

 disk irrespectively of its colour or the nature of its coating*. Suppose 

 now that the fly is being warmed by radiation from without, the bulb 

 being cool, at least at its inner surface. Let A, B be the two kinds of 

 faces of the disks, and suppose A to be the better absorber of the total 

 radiation. Then A will be the warmer, and therefore will be more 

 strongly repelled than B. Suppose now that the bulb is heated till its 

 inner surface becomes warmer than the fly. Then the fly will still be 

 receiving heat by radiation, to some extent also by communication from 

 the gas ; but this will be the same for both faces. Hence if A be still the 

 better absorber of the two (A, B), A will be the warmer, and being less 

 below the temperature of the interior surface of the fly will be less 

 attracted, or, which is the same, more repelled. Hence, whether the 

 inner surface of the bulb be cooler or hotter than the fly, a reversal in 

 the direction of rotation, while the fly is being heated, indicates a reversal 

 in the order of absorbing power of the two faces, and that, again, shows 



* Theoretically there would be a minute difference of temperature, produced, other 

 circumstances being alike, by the difference in the absorbing or emitting power of the 

 two faces of a disk, as regards the radiation which is the difference between the radia- 

 tions from or towards the affected portion of the bulb and the same portion at the 

 normal temperature. But this, and the repulsion or attraction corresponding to it, 

 would be only a small quantity of the second order, the main effect being deemed one 

 of the first order. 



