ifferent thing from the destruction of a mode of
motion. In the latter, only the form of the motion need be destroyed to
completely obliterate every trace of the atom. In the former, there
would need to be the destruction of both substance and energy, for it is
certain, for reasons yet to be attended to, that the ether is saturated
with energy.

One may, without mechanical difficulties, imagine a vortex-ring
destroyed. It is quite different with the ether itself, for if it were
destroyed in the same sense as the atom of matter, it would be changed
into something else which is not ether, a proposition which assumes the
existence of another entity, the existence for which is needed only as a
mechanical antecedent for the other. The same assumption would be needed
for this entity as for the ether, namely, something out of which it was
made, and this process of assuming antecedents would be interminable.
The last one considered would have the same difficulties to meet as the
ether has now. The assumption that it was in some way and at some time
created is more rational, and therefore more probable, than that it
either created itself or that it always existed. Considered as the
underlying stratum of matter, it is clear that changes of any kind in
matter can in no way affect the quantity of ether.

17. MATTER HAS INERTIA.

The resistance that a mass of matter opposes to a change in its position
or rate and direction of movement, is called inertia. That it should
actively oppose anything has been already pointed out as reason for
denying that matter is inert, but inertia is the measure of the reaction
of a body when it is acted upon by pressure from any source tending to
disturb its condition of either rest or motion. It is the equivalent of
mass, or the amount of matter as measured by gravity, and is a fixed
quantity; for inertia is as inherent as any other quality, and belongs
to the ultimate atoms and every combination of them. It implies the
ability to absorb energy, for it requires as much energy to bring a
moving body to a standstill as was required to give it its forward
motion.

Both rotary and vibratory movements are opposed by the same property. A
grindstone, a tuning-fork, and an atom of hydrogen require, to move them
in their appropriate ways, an amount of energy proportionate to their
mass or inertia, which energy is again transformed through friction into
heat and radiated away.

One may say that inertia