30 
Proceedings of Royal Society of Edinburgh. 
The two equations (1) and (2) admit of two, and only two, 
solutions, viz . — u = a, v — b; and u — b , v = a. 
The first leads to a dynamic paradox, which it is needless to 
discuss; the second, viz., that the particles simply exchange 
velocities, is the well known result of supposing the coefficient of 
restitution of the particles to he unity. (See Thomson and Tait, 
l.c.) The dynamics of the more complicated cases that arise 
when we take the masses of the particles to he unequal and the 
impact to be indirect is merely a particular case of the usual 
formulae worked out from Newton’s fundamental law. 
As our ether particles are assumed to have no internal structure, 
and to he absolutely undeformable, we are not to expect to find 
any exact counterpart in ordinary tangible bodies. The nearest 
approach which Newton found in his experiments on impact was 
the case of balls of glass, for which he found the coefficient of 
restitution to be y|-. As a mere matter of logic, we are entitled 
to attribute to our ethereal particles any properties which do not 
contradict the laws of motion, and in particular the Law of the 
Conservation of Energy. This having been seen to, it is merely a 
matter of phraseology whether we speak of them as perfectly rigid 
or perfectly elastic. The point is, that their impacts are to be 
infinitely short in duration, to produce no deformation, to involve 
no loss of energy.*] 
The Constitution of the Ether. 
I now proceed to introduce my theory of the constitution of 
matter. And first as to the ether : — I regard the ether, as I have 
already hinted, as the substratum of all matter, and to consist of 
perfectly hard, globular, smooth, and inconceivably small bodies, 
of equal mass, and equal in every other respect. It seems to be 
everywhere present in space and in the pores of other bodies ; 
and as for its density, instead of being, as is generally conceived, 
exceedingly tenuous, I, for my part, am of the opinion of Sir J. 
Herschel, viz., that its density is like that of “an adamantine 
solid.” In fact, that it is far denser than the densest metal. 
*The part within brackets has been written by a mathematician, who 
advised me that it was better put in this way than in the way in which I 
originally put it. It is, at any rate, far more concise. 
