ies 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, that the
material molecule is some kind of knot or coagulation of ether." [5]

[5] Fortnightly Review, June, 1875, p. 784.

Another interesting consequence of Sir William Thomson's pregnant
hypothesis is that the absolute hardness which has been attributed to
material atoms from the time of Lucretius downward may be dispensed
with. Somewhat in the same way that a loosely suspended chain
becomes rigid with rapid rotation, the hardness and elasticity of the
vortex-atom are explained as due to the swift rotary motion of a soft
and yielding fluid. So that the vortex-atom is really indivisible,
not by reason of its hardness or solidity, but by reason of the
indestructibleness of its motion.

Supposing, now, that we adopt provisionally the vortex theory,--the
great power of which is well shown by the consideration just
mentioned,--we must not forget that it is absolutely essential to the
indestructibleness of the material atom that the universal fluid in
which it has an existence as a vortex-ring should be entirely destitute
of friction. Once admit even the most infinitesimal amount of friction,
while retaining the conception of vortex-motion in a universal fluid,
and the whole case is so far altered t