i 4 THE POPULAR SCIENCE MONTHLY. 



evident at once that, whatever other effects the rays from the sun or 

 from a flame might cause, they must necessarily determine a constant 

 difference of temperature between the two surfaces of the vanes ; and 

 the thought at once occurred that, after all, the motion might be a direct 

 result of this difference of temperature in other words, that the radi- 

 ometer might be a small heat-engine, whose motions, like those of every 

 other heat-engine, depend on the difference of temperature between its 

 parts. 



But, if this were true, the effect ought to be proportional solely to 

 the heating power of the ra3'S, and a very easy means of roughly testing 

 this question was at hand. It is well known that an aqueous solution 

 of alum, although transmitting light as freely as the purest water, pow- 

 erfully absorbs those rays, of any source, which have the chief heating 

 power. Accordingly, I interposed what we call an alum-cell in the 

 path of the rays shining on the radiometer, when, although the light on 

 the vanes was as bright as before, the motion was almost completely 

 arrested. 



This experiment, however, was not conclusive, as it might still be said 

 that the heat-giving rays acted mechanically ; and it must be admitted 

 that the chief part of the energy in the rays, even from the most brill- 

 iant luminous sources, always takes the form of heat. But, if the action 

 is mechanical, the reaction must be against the medium through which 

 the rays are transmitted, while, if the radiometer is simply a heat- 

 engine, the action and reaction must be, ultimately at least, between 

 the heater and the cooler, which in this case are respectively the black- 

 ened surfaces of the vanes and the glass walls of the inclosing bulb ; 

 and here, again, a very easy method of testing the actual condition at 

 once suggested itself. 



If the motion of the radiometer-wheel is an effect of mechanical 

 impulses transmitted in the direction of the beam of light, it was cer- 

 tainly to be expected that the beam would act on the lustrous as well 

 as on the blackened mica surfaces, however large might be the differ- 

 ence in the resultants producing mechanical motion in consequence of 

 the great absorbing power of the lampblack. Moreover, since the 

 instrument is so constructed that of two vanes, on opposite sides of the 

 wheel, one always presents a blackened and the other a lustrous surface 

 to an incident beam, we should further expect to find in the motion of 

 the wheel a differential phenomenon, due to the unequal action of the 

 light on these surfaces. On the other hand, if the radiometer is a heat- 

 engine, and the reaction takes place between the heated blackened sur- 

 faces of the vanes and the colder glass, it is evident that the total effect 

 will be simply the sum of the effects at the several surfaces. 



In order to investigate the question thus presented, I placed the 

 radiometer before a common kerosene-lamp, and observed, with a stop- 

 watch, the number of seconds elapsed during ten revolutions of the 

 little wheel. Finding that this number was absolutely constant, I next 



