15 
which have to be accounted for by reasoning from ulterior 
principles. It is, however, true that to attribute to the atom 
the quality of unlimited resistance to change of form is, in 
fact, to postulate the existence of a real physical force, distinct 
from that which is supposed, in hypothesis III, to be resident 
in the ether. The latter is an active force ; the other is simply 
reaction, called into operation only so far as the surface of the 
atom is pressed by the ether. The theories of the different 
kinds of physical force, and of their laws, which I have pro- 
posed in various scientific publications, depend in an essential 
manner on the co-existence of this force of reaction at the 
surfaces of atoms with the active force of the ether. Similarly 
the force of gravity causes a planet to move in an elliptic 
orbit, only in consequence of the co-existing passive resistance 
to change of the direction and amount of the motion, which 
is due to the planet's vis inertice ; the resistance to change of 
the motion being an actual intrinsic quality of the atom, ana- 
logous to its hypothetical intrinsic quality of resisting change 
as to form. 
22. It remains to consider by what reasoning it may be 
ascertained whether the three hypotheses are true or false. 
I have already said that the only possible process is to compare 
results mathematically deduced from them with quantities 
derived from experiment, for the purpose of determining 
whether the calculated quantities are in such accordance with 
the experimental values as the verification of the hypotheses 
demands. By a known rule of philosophy, a large number of 
accordances will only establish a presumption of the truth 
of an hypothesis, whereas a single instance of positive con- 
tradiction is conclusive proof of its being untrue. It would 
extend this essay to an unreasonable length to cite all available 
instances of such comparisons, for the purpose of estimating 
the amount of evidence they give of the truth of the three 
hypotheses ; I can do no more than refer for the evidence in 
full which has been brought to bear on this inquiry, to the 
various discussions of physical problems contained in my pub- 
lished philosophical writings. There is, however, one charac- 
teristic of these hypotheses which may be considered to be 
important evidence of their truth, and may suitably be treated 
of here ; namely, the facility with which, although few in 
number, they admit of being applied in the whole range of 
experimental physics. This point, as giving primd facie proof 
of the adequateness of the hypotheses to constitute the founda- 
tion of physics, is dwelt upon at considerable length in my 
paper on “ The Metaphysics of Scripture," especially in secs. 
24-28, and, consequently, I do not think it necessary to go 
