Existence of the Luminiferous JEther. 481 



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

 city/' and the velocity is greatest towards the bright side; 

 and hence the pressure is greatest towards the blackened side. 

 Moreover, " atoms immersed in such a current will be dyna- 

 mically acted on;" and in consequence of this action the vane 

 is pushed on the blackened side. But if the molecules of the 

 blackened side of the vane are pushed, so also are the residual 

 molecules of air which are leaving that side in their passage to 

 the cooler. Now the essential point in the production of 

 Crookes's force is, that the molecules which leave the heater do 

 so with a greater velocity than those which leave the cooler, 

 and thus possess a greater vis viva, The presence of the 

 aether therefore, by exerting this dynamical action, retards and 

 resists the motion of these molecules. We are thus bound to 

 assume that the cause producing Crookes's force is a sufficiently 

 powerful one to overcome the resistance which is offered by 

 the dynamical action of the aether. 



It therefore appears that, if we assume the existence of this 

 aether, the action of the radiometer is caused by the operation 

 of two directly opposite forces— one, the greater, being pro- 

 duced by the movement of the molecules of the residual gas, 

 and the other by the pressure produced by the circulation of 

 the aether. The same cause, viz. the heating of the vane, pro- 

 duces both forces ; but the effect of this heating upon the 

 molecules of the residual gas, we must suppose, is much greater 

 than upon the aether. It appears, therefore, that this mole- 

 cular-pressure theory of the action of the radiometer must be 

 modified in the manner indicated, in order that the disturbing 

 effect of the vehicle of light may be taken into consideration 

 Is it not more probable that there is no aether present ? 



2. Optical Properties of Chemical Substances. 



That the crystalline arrangement of bodies produces a great 

 and decided effect upon a ray of light passing through them 

 is a fact perfectly well known. In fact, to it we owe the 

 phenomena of double refraction. The general bearing of this 

 effect upon the question under consideration has already been 

 considered in the paper referred to above (Phil. Mag. April 

 1879). But there are certain special portions which will re- 

 quire a little further consideration. Some of these are not 

 recent investigations, but are nevertheless very important. 



Phenomena presented by Biaxal Crystals. — It is a well- 

 known fact that the relative positions of the optic axes varies 

 with the temperature; in fact, in some crystals, when heated, 

 not only the inclination of the two optic axes to each other, 

 but also the plane in which they lie is liable to change. Now 



