We are now brought to one of the profoundest speculations of modern times, the vortex-atom theory of Helmholtz and Thomson, in which the evolution of ordinary matter from ether is plainly indicated. The reader first needs to know what vortex-motion is; and this has been so beautifully explained by Professor Clifford, that I quote his description entire: "Imagine a ring of india-rubber, made by joining together the ends of a cylindrical piece (like a lead-pencil before it is cut), to be put upon a round stick which it will just fit with a little stretching. Let the stick be now pulled through the ring while the latter is kept in its place by being pulled the other way on the outside. The india-rubber has then what is called vortex-motion. Before the ends were joined together, while it was straight, it might have been made to turn around 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