THE LAWS OF MOTION 311 



cule, or a particle ? This seems very doubtful indeed. 

 If the ratios of the mutual accelerations of two ether- 

 elements, of two atoms and of two particles be each in 

 themselves constant and capable of leading us to a clear 

 definition of mass for each type, it is still by no means 

 certain whether the ratio of the mutual accelerations of 

 an ether-element and a particle are inversely as the ratio 

 of the ether-element mass to the particle mass. Possibly 

 we cannot conceive these masses measurable by the same 

 standard. 



If the prime-atom consist of ether in motion, then its 

 mass would certainly vanish with this motion ; but the 

 ether-elements which formed the prime-atom would still 

 retain their ether-mass. Hence it seems likely that the 

 possibility of a velocity entering into the mass of gross 

 " matter " may hinder us from asserting that the ratio of 

 the mutual accelerations of ether-element and particle is 

 " inversely as their masses." Thus the idea of mechanical 

 action and reaction between ether and gross " matter " 

 becomes very obscure. Of the validity of postulating 

 these laws for particles there can be small doubt ; they 

 may possibly suffice to describe the relation of ether- 

 elements to each other, but they cannot be dogmatically 

 asserted of the action between ether and gross " matter." 

 I have purposely led the reader to these difficult and 

 still unsettled points, because physicists, finding that 

 certain laws of motion applied to particles will suffice to 

 describe our perceptual experience of physical bodies 

 (which they represent by systems of particles), are, I 

 venture to think, too apt to assert that these same laws 

 hold throughout the whole of the conceptual model by 

 which they describe the universe. They would admit 

 that special modes of acceleration like gravitation, 

 magnetisation, etc., probably flow from the manner in 

 which the prime-atom and the particle are to be con- 

 ceived as constituted. But there may be more than this 

 to be admitted — the greater part of the laws of motion as 

 we state them for particles may also flow from the 

 peculiar structure of the particle. They may largely 



