M. E. Pringsheirn on the Radiometer . 121 



o 



Here the question immediately presses, whether a similar 

 force will not arise at an element of the surface which, instead 

 of giving heat to the gas, takes heat from it. This question is 

 answered Jby proposition 3 (p. 119), if it be considered from the 

 point of view of heat-conduction which we have gained for radio- 

 meter-motion universally. For the heat which is given out to 

 the gas by the first warmed surface simply escapes from the gas 

 again at the surface of the vane; and as it passes to the vane it 

 imparts to it a receding motion. After the analogy of the 

 above procedure, we can here also conclude that the repellent 

 force is proportional to the quantity of heat passing. This 

 agrees also with the experience that this force increases with 

 the difference of temperatures between the heated surface and 

 the gas and with the approach of the two surfaces to one 

 another. We can therefore extend the above proposition and 

 say:— 



A surface-element at which heat enters or leaves the rarefied 

 air undergoes a repulsion, of which the amount is proportional 

 to the intensity of the heat-current. 



In this proposition are contained not only the three rules 

 above given, but all the propositions that can be enunciated 

 on radiometer-motion. Thus, from it is directly inferred, as 

 a consequence, the influence which a favourable position of 

 the vane with respect to the glass case has upon the motion. 

 For the nearer the vane confronts the glass, the more quickly 

 do the air-particles give up their heat to the glass, and there- 

 fore the more quickly the conduction goes on, and hence the 

 more heat is withdrawn from the vanes in the unit of time. 

 Since Stokes, as already mentioned (p. 117), traced the phe- 

 nomena occurring in radiometers with curved vanes to the 

 favourable position of the convex side with respect to the glass 

 case, our proposition contains also rule 2 (p. 120). 



Theory of Kadiometer-motion. 



Till now we have sought to fathom the laws of radiometer- 

 motion from the point of view that that motion is generated 

 by the passage of heat between a surface and a gas, without 

 forming any theoretical notion as to how such a passage of 

 heat is capable of producing the motion. Now there are two 

 quite different ways of explaining this — namely, the theory of 

 air-currents, and those theories of the radiometer which rest 

 upon the kinetic theory of gases. 



The view that the motions are due to air-currents* can 

 hardly be justified; for as the analogous phenomena under 



* Neesen, Pogg. Ann. clvi. pp. 144-156 (1875), clx. pp. 143-153 (1877). 

 0. E. Meyer, Kinetische Theorie der Gase, Breslau, p. 154 (1877). 



Phil Mag. S. 5. Vol. 15. No. 92. Feb. 1883. K 



