But what if these molecules, indestructible as they are, turn out to
be not substances themselves, but mere affections of some other
substance?
According to Sir W. Thomson's theory of Vortex Atoms, the substance of
which the molecule consists is a uniformly dense _plenum_, the
properties of which are those of a perfect fluid, the molecule itself
being nothing but a certain motion impressed on a portion of this
fluid, and this motion is shewn, by a theorem due to Helmholtz, to be
as indestructible as we believe a portion of matter to be.
If a theory of this kind is true, or even if it is conceivable, our
idea of matter may have been introduced into our minds through our
experience of those systems of vortices which we call bodies, but
which are not substances, but motions of a substance; and yet the idea
which we have thus acquired of matter, as a substance possessing
inertia, may be truly applicable to that fluid of which the vortices
are the motion, but of whose existence, apart from the vortical motion
of some of its parts, our experience gives us no evidence whatever.
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