2 GO Intelligence and Miscellaneous Articles. 



Were evaporation or the emission of molecules on the irradiated 

 side of the vane the only or at least the chief cause of the radiometer- 

 motion, this would of necessity increase when the rarefaction is 

 continued, since according to experience both evaporation and the 

 emission of molecules must be so much the more vigorous the 

 lower the pressure in the space occupied by rarefied gas. But, as 

 we have learned from experiments by Einkener and Crooks, the 

 moment of rotation exerted by the flame upon the radiometer at 

 first increases, cceteris paribus, with the rarefaction of the gases, 

 reaches its maximum at a certain pressure, and with further rare- 

 faction decreases. This diminution of the radiometric effect, which 

 may fall to -jly of the maximum value, contradicts the above- 

 mentioned consequence of the theories of evaporation and emission. 

 On the other hand, however, the hypothesis that all bodies, in- 

 dependently of their aggregate-condition, evaporate even at 

 ordinary temperatures cannot be called in question ; hence it must 

 be admitted that a force of reaction will also result from the 

 emission of molecules. But, in order to explain the above-mentioned 

 diminution of the radiometric motion, it must be assumed, further, 

 that the reaction-forces resulting from the emission must be either 

 vanishingly small, or so small in comparison with the forces arising 

 from the recoil of the molecules of the gaseous material still present 

 that the motion-phenomena are either exclusively or chiefly con- 

 ditioned by the latter forces. 



On this hypothesis, that decrease of the motion can be ex- 

 plained, according to the kinetic theory of gases, in the following 

 manner : — At the full atmospheric pressure the reaction-force 

 called forth on the irradiated side of the vane is too little to over- 

 come the resistance presented by friction and the air. At sufficient 

 rarefaction the reaction-force overcomes those resistances, and the 

 motion of the vane commences. If the reaction-force, like the in- 

 ternal friction, decreases only very slowly with the pressure, the 

 velocity of the movement attains its maximum at a certain pressure, 

 and decreases when the rarefaction is continued, because not merely 

 the resistance of the air, but also the force of the reaction excited 

 becomes less with the lessened number of the recoiling molecules. 

 In a space absolutely void of air the motion must cease entirely, 

 if no emission of molecules results on the irradiated side of the 

 vane. 



Further, the author describes a radiometer consisting of a fixed 

 cross of mica lamella) blackened on one side, and a very thin 

 cylindrical glass case. The distance of the outermost edges of the 

 vanes from the glass amounted to 2 millims. "When light fell 

 upon it the glass cylinder rotated in the opposite direction to that 

 in which the cross would have been caused to turn. 



The object of this experiment is, to furnish a complete demon- 

 stration that the motions in the radiometer cannot be explained by 

 means of currents of air. — Sitzunrj der maihematiscJi-natarwisscn- 

 scliaftlichen Classe der Icaiserlichen Alcademie der Wissenscliaften in 

 Wim, July 3, 1879. 



