d without changing position, by rolling it between the
hands. Just the same motion of rotation it has on the stick, only that
the ends are now joined together. All the inside surface of the ring is
going one way, namely, the way the stick is pulled; and all the outside
is going the other way. Such a vortex-ring is made by the smoker who
purses his lips into a round hole and sends out a puff of smoke. The
outside of the ring is kept back by the friction of his lips while
the inside is going forwards; thus a rotation is set up all round the
smoke-ring as it travels out into the air." In these cases, and in
others as we commonly find it, vortex-motion owes its origin to friction
and is after a while brought to an end by friction. But in 1858 the
equations of motion of an incompressible frictionless fluid were first
successfully solved by Helmholtz, and among other things he proved
that, though vortex-motion could not be originated in such a fluid, yet
supposing it once to exist, it would exist to all eternity and could
not be diminished by any mechanical action whatever. A vortex-ring, for
example, in such a fluid, would forever preserve its own rotation, and
would thus forever retain its peculiar individuality, being, as it were,
marked off from its neighbour vortex-rings. Upon this mechanical truth
Sir William Thomson based his wonderfully suggestive theory of the
constitution of matter. That which is permanent or indestructible in
matter is the ultimate homogeneous atom; and this is probably all that
is permanent, since chemists now almost unanimously hold that so-called
elementary molecules are not really simple, but owe their sensible
differences to the various groupings of an ultimate atom which is alike
for all. Relatively to our powers of comprehension the atom endures
eternally; that is, it retains forever unalterable its definite mass
and its definite rate of vibration. Now this is just what a 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 propert