vortex-ring would do in an incompressible frictionless fluid.
Thus the startling question is suggested, Why may not the
ultimate atoms of matter be vortex-rings forever existing in such
a frictionless fluid filling the whole of space? Such a
hypothesis is not less brilliant than Huyghens's conjectural
identification of light with undulatory motion; and it is
moreover a legitimate hypothesis, since it can be brought to the
test of verification. Sir William Thomson has shown that it
explains a great many of the physical properties of matter: it
remains to be seen whether it can explain them all.

Of course the ether which conveys thermal and luminous
undulations is not the frictionless fluid postulated by Sir
William Thomson. The most conspicuous property of the ether is
its enormous elasticity, a property which we should not find in a
frictionless fluid. "To account for such elasticity," says
Professor Clifford (whose exposition of the subject is still more
lucid than that of our authors), "it has to be supposed that even
where there are no material molecules the universal fluid is full
of vortex-motion, but that the vortices are smaller and more
closely packed than those of [ordinary] matter, forming
altogether a more finely grained structure. So that the
difference between matter and ether is reduced to a mere
difference in the size and arrangement of the component
vortex-rings. Now, whatever may turn out to be the ultimate
nature of the ether and of molecules, we know that to some extent
at least they obey the same dynamic laws, and that they act upon
one another in accordance with these laws. Until, therefore, it
is absolutely disproved, it must remain the simplest and most
probable assumption that they are finally made of the same stuff,