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, 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.