THE LAWS OF MOTION 283 



that the physicist is able to test the truth of the laws of 

 motion he propounds. 



In the first place he postulated these laws for particles, 

 and demonstrated their validity by showing that they 

 enabled him to describe the routine of his sense-impressions 

 with regard to physical " bodies." But with the growth 

 of our ideas as to the nature of ether and gross " matter," 

 we naturally begin to question whether the laws which 

 describe the relative motion of two particles are to be 

 conceived as holding for two molecules, two chemical 

 atoms, two prime-atoms, and ultimately for two ether- 

 elements. Or, what may possibly be still more important, 

 are they to hold for the relative motion of a prime-atom 

 and adjacent ether-elements ? How far are we to consider 

 the laws of motion as applied to particles of gross "matter" 

 to result from the manner in which particles are built up 

 from molecules, molecules from atoms, and ultimately 

 atoms probably from ether-elements ? Now this is a 

 very important issue, and one which does not appear to 

 have always been sufficiently regarded. If we assume 

 that the particle is ultimately based on a certain type of 

 ether-motion, then we must admit the existence of other 

 types of ether-motion which do not constitute gross 

 " matter." In this case it will by no means follow that 

 the relative motion of two particles, or of two prime-atoms, 

 will follow the same laws as the relative motion of two 

 ether-elements. It is quite clear, of course, that modes 

 of motion peculiar to gross " matter " must arise from its 

 special structure, and not be assumed to flow from laws 

 applying to all moving things. For example, gravi- 

 tation, magnetisation, electrification, the absorption and 

 emission of heat and light are all phases of sense-impression 

 which we associate with gross " matter," and therefore they 

 must be described by modes of motion characteristic of 

 gross " matter," or modes which flow from its peculiar 

 constitution. As kinetic formulae or special laws of motion 

 they cannot be extended to the ether in general. But 

 there are still more general laws of motion, which we may 

 describe as the Newtonian laws, and which certainly 



