THE RADIOMETER. 15 



screened one-half of the bulb, so that only the blackened faces were 

 exposed to the light as the wheel turned them into the beam. Again, 

 I several times observed the number of seconds during ten turns, which, 

 although equally constant, was greater than before. Lastly, I screened 

 the blackened surfaces so that, as the wheel turned, only the lustrous 

 surfaces of mica were exposed to the light, when, to my surprise, the 

 wheel continued to turn in the same direction as before, although much 

 more slowly. It appeared as if the lustrous surfaces were attracted by 

 the lio-ht. Again I observed the time of ten revolutions, and here I 

 have collected my results, reducing them, in the last column, so as to 

 show the corresponding number of revolutions in the same time : 



It will be noticed that 88 + 232 equals very nearly 319. Evidently 

 the effect, so far from being differential, is concurrent. Hence, the 

 action which causes the motion must take place between the parts of 

 the instrument, and cannot be a direct effect of impulses imparled by 

 ether-waves ; or else we are driven to the most improbable alterna- 

 tive, that lampblack and mica should have such a remarkable selective 

 power that the impulses imparted by the light should exert a repulsive 

 force at one surface and an attractive force at the other. Were there, 

 however, such an improbable effect, it must be independent of the 

 thickness of the mica vanes ; while on the other hand, if, as seemed to 

 us now most probable, the whole effect depended on the difference of 

 temperature between the lampblack and the mica, and if the light pro- 

 duced an effect on the mica surface only because, the mica plate being 

 diathermous to a very considerable extent, the lampblack became heat- 

 ed through the plate more than the plate itself, then it would follow 

 that, if we used a thicker mica plate, which would absorb more of the 

 heat, we ought to obtain a marked difference of effect. Accordingly 

 we repeated the experiment with an equally sensitive radiometer, which 

 we made for the purpose, with comparatively thick vanes, and with this 

 the effect of a beam of light on the mica surface was absolutely null, 

 the wheel revolving in the same time, whether these faces were pro- 

 tected or not. 



But one thing was now wanting to make the demonstration com- 

 plete. A heat-engine is reversible, and if the motion of the radiometer 

 depended on the circumstance that the temperature of the blackened 

 faces of the vanes was higher than that of the glass, then by reversing 

 the conditions we ought to reverse the motion. Accordingly, I care- 

 fully heated the glass bulb over a lamp, until it was as hot as the hand 



