644 Dr. C. V. Burton on the 



an aetherial pressure-gradient has no tendency to displace 

 material bodies one way or the other with respect to the 

 aether. Though (pressural) forces of the kind just referred! 

 to may conceivably be concerned in gravitation, it seems- 

 probable that the mechanism of the action of electrically 

 charged bodies and of magnets on one another are of types 

 entirely distinct. 



5. Again, when dealing with exchanges of momentum 

 between the aether and ''ordinary'"' matter, we have no right 

 to treat the momentum of a material particle, measured by 

 the product of its mass and velocity, as equivalent to aetherial 

 momentum similarly measured as a product of mass and 

 velocity. Experience teaches us that, through a wide range 

 of phenomena, the principle of conservation of momentum 

 holds good with great exactitude for ordinary material 

 systems, every particle or element of matter being regarded 

 merely as an entity endowed with definite inertia and with 

 perfect mobility, except in so far as its motion may suffer 

 interference from the neighbourhood of other particles. On 

 such a basis the dynamics of matter reached a high degree 

 of development before the existence of an aether was realized; 

 but when the interaction of matter and aether is in question, 

 this primitive conception of material inertia and momentum 

 is no longer adequate. The motion of a material particle 

 with respect to the aether represents kinetic energy (T) which 

 is a function of the velocity (Se, y, z) of the particle. This 

 kinetic energy we suppose to be due to some type of aetherial 

 motion, since the motion of the particle through the aether 

 amounts to a progressive readjustment of those aetherial 

 strains, the aggregate of which constitutes the particle. The 

 aetherial motion in question may not (and according to §§ 9-21 

 below does not) involve any bodily drift in the direction of 

 motion of the particle; though in any case the components 

 of momentum of the particle corresponding to the coordinates 

 (x, y, z) are, in the generalized dynamical sense (dTfdJ-, 

 BT/dy, dT/dir) ; which together also express the momentum 

 of the particle as ordinarily understood. 



6. The implicit assumption referred to in § 3 above leads 

 further to the conclusion that the aether is indefinitely com- 

 pressible. For consider a condenser made up of an inner 

 conducting sphere and a concentric spherical cavity in an 

 outer conducting body, the interspace being vacuous. Let 

 the inner sphere be of radius r { and the concentric cavity of 

 radius r 2 ; also let the inner sphere carry a charge e, the 

 charge upon the hollow spherical surface of the outer con- 

 ductor being therefore —e. As we pass across from the 



