Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/346

 and cannot be destroyed; so that if, as Thomson suggested in 1867, the atoms of matter are constituted of vortex-rings in a perfect fluid, the conservation of matter may be immediately explained. The mutual interactions of atoms may be illustrated by the behaviour of smoke-rings, which after approaching each other closely are observed to rebound: and the spectroscopic properties of matter may be referred to the possession by vortex-rings of free periods of vibration.

There are, however, objections to the hypothesis of vortex-atoms. It is not easy to understand how the large density of ponderable matter as compared with aether is to be explained; and further, the virtual inertia of a vortex-ring increases as its energy increases; whereas the inertia of a ponderable body is, so far as is known, unaffected by changes of temperature. It is, moreover, doubtful whether vortex-atoms would be stable. "It now seems to me certain," wrote W. Thomson (Kelvin) in 1905, "that if any motion be given within a finite portion of an infinite incompressible liquid, originally at rest, its fate is necessarily dissipation to infinite distances with infinitely small velocities everywhere; while the total kinetic energy remains constant. After many years of failure to prove that the motion in the ordinary Helmholtz circular ring is stable, I came to the conclusion that it is essentially unstable, and that its fate must be to become dissipated as now described."

The vortex-atom hypothesis is not the only way in which the theory of vortex-motion has been applied to the construction of models of the aether, It was shown in 1880 by W. Thomson that in certain circumstances a mass of fluid can exist in a state in which portions in rotational and irrotational