84 VISCOSITY OF GASES 213 



The same holds good for the remarkable phenomenon 

 observed in 1825 by Fresnel, 1 which he was inclined to 

 interpret as a repulsion between heated bodies. The essen- 

 tially similar actions which Crookes 2 observed during 

 weighings in rarefied spaces, as well as the motions observed 

 in the apparatus 3 invented by him, and called the radio- 

 meter or light-mill, are to be explained by the same ideas. 



The vanes of the little mill, which are black on one side and 

 white on the other, are warmed by radiated heat, or even by 

 light since a luminous ray is only a heat-ray which is also 

 luminous to the eye but they are warmed unequally, and 

 the black side the more strongly. If, therefore, a particle of 

 air impinges on a black face it carries off more heat, i.e. flies 

 off with greater speed, than if the collision had been against 

 a white face. The reaction which it exerts on the vane in 

 its rebound is, therefore, greater when it leaves the warmer 

 black face than at the colder white side. Consequently, 

 the mill must so turn that the white side of the vanes 

 precedes. 



If we think of the immense speed with which the gaseous 

 molecules move, it seems scarcely necessary to specially 

 prove that the force that results from this unequal heating 

 is really sufficient to bring about this action. But since the 

 proof can be easily given independently of the hypotheses 

 on which the theory of gases rests, we will calculate the 

 magnitude of the energy for a simple case, so chosen that 

 the necessary experimental data are known. Suppose the 

 vanes to be made of aluminium foil of 1 sq. cm. area, and to 

 be blackened with soot on one side, and suppose the heating 

 to be caused, not by a source outside the instrument, but by 

 the glass envelope itself, which we will take to be 1 degree C. 

 warmer than the mill. 



Lehnebach 4 has observed that glass and sooted sur- 

 faces radiate with equal intensity towards a region of rarefied 



1 Ann. Chim. Phys. 1825 [2] xxix. p. 57 ; (Euvres Computes, 1868, ii. 

 p. 667. 



2 Phil. Trans. 1873, clxiii. p. 277. 



3 Quarterly Journal of Science, 1875, xii. p. 337. 



4 Pogg. Ann. 1874, cli. p. 96. 



