of Maxwell's Electrical Theory. 179 



Before considering this case a word or two of general import 

 will be given. 



I cannot see that Maxwell has shown his right to ignore, 

 as he does in considering the propagation of waves, the 

 equation of motion of the medium . It is not hard to show 

 that we may disregard it if we accept his equations as literally 

 true for rapid waves in a medium of ordinary density. But 

 may we do so for the aether, and may we do so in the case of 

 gases where the gross matter present has practically no effect 

 on electrical actions ? 



By ignoring the equation Maxwell is led to the conclusion 

 that the aether when it is excited by electromagnetic waves 

 exerts a pressure on gross matter. "What is the effect of this 

 pressure after it has left the sun and before it has again 

 reached gross matter ? It must have some effect on the aether, 

 but the equations expressing that effect are wanting. If we 

 tie ourselves down to a strict dynamical theory these equa- 

 tions appear, and we have to decide what is their meaning. 

 In case I., on the present hypothesis, there is no doubt at all 

 about the meaning. It can be expressed either by the equa- 

 tion 



V(4ttDVE' + BVH') = ; . . . . (14) 



or by saying that the Poynting intensity remains constant 

 for every element of the aether. It follows also that there is 

 no ponderomotive action and reaction between the aether and 

 gross matter, which has an obvious bearing on the question of 

 whether or not the aether can exert a pressure on gross matter. 



Are these results consistent with an electromagnetic 

 theory of light ? I think they are not, but this is not on 

 account of the most obvious apparent consequences. Take, 

 for instance, the fact of the constancy of the Poynting in- 

 tensity. From this we deduce that if any part of the aether 

 were ever electrically at rest the actual displacement and 

 magnetic force for that part would always be parallel, and if 

 the displacement and magnetic force were for any part of the 

 aether ever not parallel that part could never come electrically 

 to rest. 



8. These results are not necessarily inconsistent with a 

 theory of light, for it must be remembered that the aether now 

 contemplated is capable of indefinite expansion or condensa- 

 tion, and that whatever be the amount of this expansion or 

 condensation its electrical constants remain absolutely un- 

 affected. But by indefinite expansion the volume distribution 

 of Poynting intensity is indefinitely diminished,, and con- 

 versely for condensation. In fact, the aether is capable of 



