77 
REMARKS BY THE REY. PROFESSOR CHALLIS, M.A., F.R.S. 
I concur in all essential respects with the considerations advanced by 
the Lord O’Neill in his paper, On the Action of Will in the Formation and 
Regulation of the Universe , and am induced to offer the subjoined remarks 
only in consequence of the references made in Art. 28 to principles on which 
I have founded a theory of the force of gravitation. The views I hold on 
this and like questions are given in two communications published in the 
Transactions of the Institute (Yol. XI., No. 42, and Yol. XII., No. 45), and 
in various productions contained in the Philosophical Magazine. My present 
purpose is to supply some additional explanations which appeared to me to 
be called for after reading certain statements made in the Lord O’Neill’s 
paper. 
It is true, as he says, that I propose to account for gravitation “on the 
hypothesis of a reaction of the atoms of which matter is composed against 
aethereai pressure,” and for the persistence and constancy of the force by 
supposing that the atoms are always of the same size and shape. But 
according to my views this is not the only condition of the unalterability of 
gravitation. In my researches respecting the characteristics of the physical 
forces, I have uniformly assumed that all active force in nature is exerted by 
the intervention of the ethereal medium, and all passive force is reaction at 
the surfaces of spherical inert atoms of constant magnitude against pressure 
of the aether. On these principles I have endeavoured to account not only 
for gravitation, but generally for the forces concerned in the phenomena of 
light, heat, electricity, galvanism, and magnetism, together with the atomic 
and molecular forces whereby the constitution of sensible masses , as consist- 
ing of an aggregation of atoms, is maintained. In all the reasoning applied 
to these purposes it is assumed that the aether is a homogeneous substance, 
composed of discrete atoms all of the same size, but incomparably smaller 
than the atoms of sensible gross bodies ; also that it is susceptible of varia- 
tion of atomic density, and has the property of pressing against its own 
parts, and against the atoms of all sensible bodies, in exact proportion to its 
atomic density. In other words, the pressure is equal to the density multi- 
plied by a constant factor, as is the case with respect of air of given tempe- 
rature. There is, however, this essential difference, that with respect to air 
the factor is a quantity measurable by experiment ; and a theoretical reason 
for it is derivable, as I have endeavoured to show {Phil. Mag., 1859, pp. 
401-404), from the mutual action between the aether and the aerial atoms. 
But with respect to the aether, the factor must be absolutely constant, in- 
asmuch as it expresses the intrinsic elasticity of the aethereai medium, and 
there are no antecedent physical conditions whereby this elasticity can be 
altered. From this argument I draw the conclusion that the persistence of 
physical force depends wholly on the essential qualities of the atom and on 
the constancy of the elasticity of the aether, and that these are underiv- 
