34 Proceedings of Royal Society of Edinburgh. [sess. 
neutralised by an equal motion from the opposite. Hence the 
pressure in the medium which Sir J. Herschel found must exist.* 
Further, to show that such pressure, or something equivalent to it, 
must exist in space, it has only to be pointed out that if the earth 
fell from its present distance into the sun, under the action of 
gravity, the amount of heat developed by the impact would cover 
the solar emission for about 95 years. The impact of Jupiter, if 
he fell from his present distance, would cover it for 32,240 years. 
This enormous amount of heat, or its equivalent in some other form 
of energy , must reside in the space separating the bodies, or in the 
bodies themselves. We cannot suppose it to be created out of 
nothing. It is there, and we cannot get away from it. But we 
see a practically infinite amount of heat (the radiation of the stars) 
apparently wasted. What, then, more natural to conceive than that, 
instead of being wasted, it produces the intense pressure, other- 
wise not accounted for, mathematically demonstrated by Sir J. 
Herschel ? 
Amongst other reasons for Sir J. Herschel’s conception of a 
solid constitution for the ether, was the necessity of conceiving of 
some bounding envelope to restrain the pressure; for he says — 
that under no conception but that of a solid can an elastic and, 
expansible medium be self-contained. If free to expand in all 
directions, it would require a bounding envelope of sufficient 
strength to resist its outward pressure. And to evade this by 
supposing it infinite in extent, is to solve a difficulty by words 
without ideas — to take refuge from it in the simple negation 
of that which constitutes the difficulty.” How, it seems to me 
that the solid conception does not rid us of this difficulty ; for, 
granted that the solid constitution does prevent its constituent 
particles from flying off into space, the mere statement of this fact 
does not at all explain how it is brought about. But if the theory 
I am dealing with can give a reasonable and mechanical ex- 
planation of the fact, it ought to be a great point in its favour. 
And first as to the ether, — other solid bodies being dealt with in 
the chapter on the constitution of matter. Although I hold with 
Sir J. Herschel that the ether is a solid in the sense that he limits 
* It goes without saying that I conceive this pressure as the source of all the 
so-called “Forces.” 
