Principles of Energetics. 357 
causative, or sequential molecular motions. For the true applica- 
tion of Hydrodynamics would seem to be rather to actual solids 
and fluids, than to such “ hypothetical facts” as form the bases 
of Professor Challis’s Theory of Physics. How much work re- 
mains to be done in that true direction is well known; and the 
greatness of the results in the knowledge that might thereby be 
given of the formation of the solar and other sidereal systems, 
makes every contribution to Hydrodynamics, whatever the 
immediate particular application of the theorems, of peculiar | 
value. 
29. If, therefore, the true application of Hydrodynamics has 
been mistaken, and if a Hydrodynamical Theory of Physics must 
be founded on entities, hypothetical solids and fluids, to which 
such objections as the foregoing can be urged, there remains for 
us only a Molecular Theory of Physics. It is because to such a 
Theory all the most important modern physical researches seem 
to poimt, that I have thought it necessary to examine at such 
length the “fundamental facts” of Professor Challis. For the 
great result of modern science may be said to be the relative con- 
ception it gives of every phenomenon, and hence the demand 
that the fundamental facts of any theory be conceived, not as 
absolute and independent existences, but as expressions of rela- 
tions. Now a Molecular is distinguished from a Hydrodyna- 
mical Theory of Physics in this,—that while in the latter the 
states and motions of bodies are explained by the action on them 
of some hypothetical, uniform, absolute, and all-pervading entity, 
in the former theory, physical states and motions are referred to 
differential conditions of stress and strain among the actual con- 
stituent molecules of bodies. Hence the evident experimental 
advantage of a Molecular Theory is, that its hypotheses being 
as to relative conditions of Molecular pressure and tension, trans- 
mission of motion, &c., they are more or less capable of experi- 
mental proof or disproof; and such a theory will be at least pro- 
lific in the suggestion of experiment. But where one deals with 
the waves or currents of an absolute ether acting on absolute 
atoms, a plausible theory may indeed be made out, but it is 
because its conceptions are fundamentally opposed to, so that 
its minor hypotheses cannot be checked by, experiment. 
30. It is as the new fundamental principle of a Molecular 
Theory of Physics that I venture to suggest the above conception 
of Atoms. It is because, however convinced of the soundness of 
this principle, I am very diffident of my own powers of applying 
it, that I have gone at such length into its illustration, and the 
criticism of the opposed conception, as developed by Professor 
Challis. For should the mechanical explanation which, by 
