an the Mechanical Function of an JEtlier. 42i 



— it exists in the space between separated electric poles, 

 and between separated material particles, but there is no 

 potential energy in the material particles themselves, for 

 the disjoined atoms could not communicate with each other 

 by any except an aetherial process. All the mechanical 

 energy which an atom possesses is kinetic, and this is 

 possessed, so far as it is translatory, by all its electrons alike, 

 irrespective of sign. On Larmor's theory the kinetic energy 

 of matter, including inter-atomic energy and energy of 

 rotation of the atom, depends on nothing but the number 

 of its electrons, the linear dimensions of their nuclei, and 

 their motions through the aether. It is indeed rather difficult 

 to see what other material substratum is necessary, or even 

 what supplementary conception of material substance is 

 possible. The problem of matter, as regards purely physical 

 manifestations, is in that case shifted to an account of the 

 nature and properties of an electron, i. e. of an isolated electric 

 charge with its lines of force and their behaviour when it 

 is in motion. 



Leaving these speculative considerations, I will state the bare 

 result of the familiar experience that the aether is mechanically 

 inoperative, i. e. that it never compensates the momentum of 

 matter, or that the total momentum generated by any stress 

 is zero without including any setherial inertia, by stating that 

 a stress existing in aether always terminates at each end on 

 material bodies, its terminals being the forces which act 

 normally on those bodies, driving them apart or pulling them 

 together ; in other words, that mechanical force is only found 

 at the junction of aether and matter, and acts solely on matter, 

 no force-component of any stress being attributable to the 

 reaction of quiescent aether : — 



5. A Stress extends from one Material Body to another, it 

 does not End in ^Ether [when it is in a steady electric state] . 

 The usual interpretation of Newton's third law is to the 

 same practical effect (without the words in square brackets), 

 since the u reaction " whose constant existence is postulated 

 is silently assumed to be that of another lump of matter. But 

 whereas Newton's law itself can hardly be other than quite 

 general, some exceptional cases where the above usually 

 understood specialization or limitation of it ceases to be true 

 may yet be discovered. In fact Maxwell's "radiation-pres- 

 sure " may perhaps be already such a case. Anyhow, since 

 (5) unqualified represents the usual understanding of the third 

 law, it is better to have it so stated explicitly ; if only that it 

 may be contradicted. The fact is that law iii. has only been 

 established for bodies between which the aether has attained a 

 steady state ; if this condition is not satisfied the law as 



