480 Mr. E. H. Cook on the 



for us that a molecular motion of a particular kind produces 

 the rotation of the fly. That this is the true explanation of the 

 phenomena has been abundantly proved by the experiments 

 of Mr. Crookes and others. Now, as the luminiferous aether 

 is all-pervading, and is therefore present in the exhausted 

 cases of the radiometers, we ask where does its influence come 

 in in the phenomenon? Does not the acceptance of this 

 theory virtually ignore the existence of the aether ? Only one 

 way out of the dilemma is left, viz. to say that the aether is 

 there, but that it does not affect the motion of the molecules 

 in the least. Here, as in every other case where its influence 

 is likely to be felt, we find it exerting no influence, and beha- 

 ving like an inert substance. But are we, when, as in this 

 case, we call upon the believers in the existence of this sub- 

 stance to tell us how it will affect a certain action, to be con- 

 tent with the bare statement that it exerts no action ? We 

 are entitled to ask Why ? And what are the properties of the 

 substance in virtue of which it is inert ? But if the aether be 

 present, it must make its presence felt in some way or other. 

 In fact, Prof. Challis has attempted to explain the phenomena 

 of the radiometer simply by the motion of an aethereal cur- 

 rent. He says: — "I assume that the radiant light or heat 

 which is incident on the vanes, being thereby converted into 

 heat of temperature, causes the atoms in a thin superficial 

 stratum to be displaced from their neutral positions, and in a 

 greater degree on the blackened side than on the other, on 

 account of the greater accession of temperature on that side. 

 .... This is a real change of condition of the vanes. .... 

 Every such superficial disturbance, however caused, gives 

 rise to a steady circulating setherial current, in which the 

 pressure varies so as to be always less the greater the velo- 

 city; and consequently atoms immersed in such a current 

 will be dynamically acted upon by reason of the variation 

 of pressure. In this instance the course of the current is 

 from the blackened to the opposite surface, because, on account 

 of the greater expansion of the vane on the warmer side, the 

 channel for the currents diminishes and the velocity increases 

 towards the bright side, and accordingly the dynamical action 

 is the same as if the vane were pushed on the blackened side "*. 

 Here, then, we have very clearly expressed the action which 

 the aether would exert if present. Taking now the mole- 

 cular-pressure theory, let us see how this action affects the 

 movements of the molecules. In the aethereal current " the 



* "A Theory of the Cup-shaped Radiometer with both 

 By Prof. Challis, M.A., F.R.S. Phil. Mag. April 1877. 



Sides Bright." 



