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356 Mr. J. S. Stuart Glennié on the 
sible for a fluid to act on a solid except through a mediate or 
immediate cohesion, but we are led by the Mechanical Theory of 
Heat to conceive every impulse communicated to a body to be 
productive of internal as well as external motion. It is of course 
necessary to make abstraction, out of the infinite number of 
effects, of the particular effect we may desire to consider. But 
an hypothesis of infinitely hard atoms not merely requires, in the 
consideration of the motion of such an atom, abstraction to be 
made of the interior relative motions also consequent on that 
difference of pressures which causes its external relative motion, 
but explicitly denies any internal motion. 
It may be here noted that the Mechanical Theory of Heat 
would lead us to consider as “ ultimate’? no special class of 
bodies or molecules, except simply those, of the internal motions 
of which we do not in any particular theoretical, or cannot in an 
experimental, investigation take account. So any hardness may 
be called “infinite ”’ if we do not consider the internal motions, 
or change of form, consequent on the application of a force which 
causes the translation of the body. But Professor Challis re- 
quires us to concede as physical facts what are properly but con- 
venient mathematical abstractions. 
27. Again (3), the conception of the origination of motion 
under such conditions as a uniform ether and discrete atoms 
therein, all of the same mass, is opposed to the experimental con- 
ception ‘of motion as or iginating in difference in the mutual pres- 
sures of bodies. For these hypotheses give us the conditions of 
an eternal equilibrium. In the theory I propose, it is evident 
that anything short of an absolute equality in the masses and 
distances of the parts of matter implies infinite mutually deter- 
mining motions. 
And Professor Challis speaks of “the existence of the wether 
as the sole source of physical power*.” But in a mechanical 
theory, as 1 have in the introductory, and in the first part of 
this, paper shown, nothing can be accurately spoken of as, of 
itself, ‘a source of power.” ‘A source of power,” a cause of 
motion, or a force, is simply the difference in relation to a third 
body of two resultant pressures upon it. And there can thus be 
no conceivable mechanical power in a fluid of which the elasticity 
is uniform, and on whick the reaction of different solids withm 
it should seem, by this theory, to be either nothing or the same. 
28. Professor Challis further conceives the physical forces to 
be correlated as ‘ modes of action of a single elastic medium.” 
But I shall endeavour in the sequel to distinguish these cor- 
relations, and to show that they are either coexisting, mutually 
* Phil. Mag. February 1861, p. 106; and December 1859, p. 444. 
