205 
be treated of in a subsequent portion of this essay. I shall now 
only make the remark that this philosophy altogether excludes any 
action at a distance of one body on another which is not produced 
by means of an intermediate substance. . _ 
22. Newton, who regarded li the action at a distance as a 
physical absurdity, has left on record his opinion that the substance 
by tlie intervention of which visible and tangible bodies act on each 
other is a fluid ( u a certain very subtile spirit ), in all respects 
like air of given temperature, but exceedingly more elastic. This 
is the fluid which is now generally called the ether, — a term which, 
like atom , has come to us from a remote philosophical age. As 
the principles laid down in art. 21 do not allow of any othei form 
of force than pressure, we have to assume that the ether has the 
property of pressing, and that, like air, it is susceptible of vana- 
tions of density and pressure, as being in like manner atomically 
constituted. For the purpose of laying a foundation for mathe- 
matical reasoning, we may make the hypothesis (to be subsequently 
tested by results), that the variations of pressure are in exact pro- 
portion to the variations of density. After this there is no more 
occasion to refer to the atomic constitution of the ether, the 
calculation of the effects of its pressure being made on the sup- 
position of its being a continuous fluid, and in exact accordance 
with the ordinary principles of Hydrodynamics. 
23. Let us here briefly recapitulate what has been said respecting 
the foundations of Natural Philosophy. On the one hand we have 
the ultimate atom, endowed with inertia, spherical in form, and of 
constant and extremely small magnitude ; on the other we have 
the ether, a perfect fluid, susceptible of variation as to density, 
and endowed with the faculty of pressing in exact proportion to its 
density. The ether is the active substance, the atom is the passive 
substance. But although the ether, regarded as being of uniform 
and constant elasticity and unlimited in extent, might be taken to 
be the source from which all active physical force emanates, it may 
still be true that there exists in the universe another hind of 
physical force having a different origin. The property of constancy 
of form and magnitude, with which we have supposed the atom to 
be endowed, would act as resistance to any pressure tending to 
change its form or dimensions, and, pro tanto , would be a real 
physical force. Such force could only have its origin, and be 
maintained, by the Will and Power that originally brought the 
atom into existence and affixed its properties. If Dr. Tyndall 
attributed to atoms only this passive and delegated “ potency, I 
could agree with him ; but I am not prepared to assign to them 
any active agency. 
24. After the foregoing discussion of the principles of the 
