M. E. Pringsheim on the Radiometer. Ill 



emanates a pressure which increases with the temperature of the 

 surface. 



We have (p. 106) made the supposition that in our expe- 

 riments absorption of heat within the rarefied air filling the 

 radiometer has no influence upon the motion. Since, how- 

 ever, on lateral irradiation this air is illuminated as well as 

 the glass side, it might still be possible to refer the generated 

 motion to a rise of temperature of the residual air, produced 

 by absorption. That this would be erroneous we will demon- 

 strate in the following section. 



II. Action of the Residual Gas. 



It must first of all be premised that, according to all theories 

 of the radiometer that rest on the kinetic theory of gases, any 

 one-sided conveyance of heat to the gas must cause a motion 

 of the vane; and therefore a local rise of temperature produced 

 by absorption in the rarefied air must do so also. Yet, at the 

 high degree of rarefaction at which the air in the radiometer 

 is, it is in itself very unlikely that that air is capable of absorb- 

 ing any considerable amount of heat, especially as the rays 

 from the source of heat must, before reaching the apparatus, 

 pass through so great a thickness of atmospheric air that they 

 are doubtless completely cleared of all the rays which can be 

 absorbed by air. 



In relation to this I made some experiments with sunlight 

 and gas-light, by throwing close before the translucent mica 

 disk of my apparatus, arranged as in the experiments described 

 above, a focus in the rarefied air. 



In order to avoid the influence of the heating of the glass 

 side, I caused the light to fall from above obliquely into the 

 apparatus, so that the glass was struck by the light at a place 

 obliquely above the mica plate at a greater distance from it. 

 There was then not the slightest perceptible deflection of the 

 vane. This proved that in the above experiments an absorp- 

 tion of heat by the rarefied air had no influence on the result. 



Yet, in order to prove by experiment the theoretically im- 

 portant assumption that absorption of heat by the gas might 

 produce motion of the vane, I filled my apparatus with illu- 

 minating-gas instead of atmospheric air, by connecting the 

 second side tube with a drying-apparatus which was connected 

 with the gas-pipe supplying the house. After filling and ex- 

 hausting the apparatus five times, so as to be certain that I 

 really had in it rarefied illuminating-gas, I repeated the ab- 

 sorption-experiments which had been made with atmospheric 

 air, but with the same negative result. It appears, therefore, 



