404 Scientific Proceedings, Royal Dublin Society. 



which is implied by the term density, does not exist (q) in the ele- 

 mental ether, and that in it the element of volume is the element 

 of mass. There is, accordingly, no such physical quantity as 

 density in the dynamics of the ultimate motions of the elemental 

 ether. It is only when accumulations of these primary motions 

 are lumped together, and where what we are investigating is 

 merely the drifting about of these accumulations — it is only in this 

 branch of dynamics that we find the need and the advantage of 

 the conception of density as a substitute for having to take separately 

 into consideration some of the motions that are really going on. In 

 fact, if any such hypothesis as Sir William Thomson's is true, the 

 density of a lump of iron, i. e. the coefficient by which the elements 

 of its volume have to be multiplied in order to get their masses, is 

 nothing but a mere function of the primary or elemental motions 

 prevailing in that portion of space,[r) and which alone make that 

 portion of space differ from one in which other elemental motions 

 are going on. 



(q) This conclusion is confirmed by an important ontological proposition which is 

 susceptible of demonstration, viz. that nothing that we suppose to exist in nature can 

 be "real," unless it is a syntheton of perceptions actual, potential, or conceivable. 

 Thus, motions and space relations may be "real," for they are such syntheta ; but a 

 "thing to move" is not real except in those cases in -which the motion we are consider- 

 ing is the drifting motion of volumes within which subsidiary motions prevail. In 

 such cases the subsidiary motions are often thought of, and may perhaps without 

 objection be spoken of, as a thing that moves. 



(»•) That is, on the supposition that the luminiferous ether is of uniform texture 

 throughout its whole extent, as seems to be the case. If, however, the fact be other- 

 wise, we must regard the density of the iron as a function both of the elemental 

 motions pervading its volume and of the elemental motions in the adjoining part of the 

 luminiferous ether. The density of the iron would then depend on its situation in the 

 material universe. 



