It is well known that amongst the numerous theories which have dealt with the form of an atom, there is only one which is in accordance with the properties  which we know atoms to possess. It was originated by Sir William Thomson, whose conclusions, based on the researches of Helmholtz on fluid motion, may be briefly summarised.
According to this view the whole of space is filled with a Motionless fluid, and material atoms are portions of this fluid, having a species of rotational motion, which, as Helmholtz proved, must continue for ever.
The best analogy to this universal plenum and to these vortex atoms is the behaviour in an atmosphere of "smoke-rings," such as may be blown from the mouth of the smoker of tobacco, from the funnel of a locomotive, or from the mouth of a cannon. Such smoke-rings have remarkable properties, which are due, not to the smoke, which merely renders them visible, but to their internal motions.
Such " vortex rings " can travel with great rapidity. They can vibrate, they can rebound from each other with perfect elasticity, and, supposing that such action takes place in a frictionless fluid, they would be no less indestructible than uncreatable by mechanical means.
It is also well known that Le Sage of Geneva conceived a kinetic theory of gravitation, which has been adopted by Sir William Thomson. According to Le Sage, the whole of space is filled with small particles, which ho calls ultramundane corpuscles, flying with enormous velocity through every point of space in every direction. These penetrate even the void spaces between atoms, so that of those which shower upon the earth perhaps not more than 1 in 10,000 have their velocity diminished by collision. The others pass right through the earth. Owing to these collisions, however, a smaller number of ultramundane corpuscles are to be found moving in the direction from the earth than towards it. Thus the earth acts as a shield, Protecting surrounding bodies from the shower of ultramundane corpuscles in that direction. Hence the moon, and bodies on the earth's surface, are battered by ultramundane corpuscles most in the direction towards the earth. This force, driving bodies towards the earth, explains terrestrial gravitation. Similarly, all bodies are driven towards each other with a force varying as the product of the masses. Sir William Thomson supposes ultramundane corpuscles to be vortex rings with no hole in the centre and elongated, like a serpent rushing forwards and always turning inside out, spitting its inwards out at its mouth, and absorbing its skin at the other end. Collisions with vortex atoms would not result in a destruction of velocity and consequent enormous generation of heat, but energy of translation is converted into some other form of energy, perhaps energy of vibration.
However artificial these hypotheses may appear at first sight, the more they » re studied the more satisfactory are they. ' They are the only suggestions of the kind which are in any way tenable, and they serve at least the part of working hypotheses. Some remarkable and unforeseen consequences follow from the coexistence of such vortex atoms and ultramundane corpuscles as Thomson has conceived. The following facts seem to follow from the laws of hydro-kinetics : —
1. When a body is heated, and the vortex atoms are rushing about, their mutual collisions originate vibrations in themselves which, when they are free, have a definite period, or periods, depending upon the nature of each vortex atom.
2. When an ultramundane corpuscle passes such a vibrating atom, the successive approaches and recessions of the atom to and from the corpuscle impress upon that corpuscle a wave-form whose dimensions depend partly on the velocity of the corpuscle, partly on the vibrations of the atom.
3. When a corpuscle so stamped continues its progress through the frictionless fluid before mentioned, the position of the wave-marks remain fixed relatively to the corpuscles, without being affected by its internal motions.
4. If such a marked corpuscle in its flight passes the neighbourhood of a cold atom, i.e., one which is not vibrating, and if that atom be capable of vibrating in the same period as the original atom which impressed the wave-trace, then the wave-trace on the corpuscle will, on passing the atom, cause it to vibrate in the вате manner as the original atom.
The phonograph supplies a happy illustration of these processes : —
1. When the membrane, with needle attached, is vibrating we have the analogue of a hot atom. 
2. If during this vibration the tin-foil on the cylinder be passed in front of the needle, the vibrating needle stamps a wave-trace on the tin-foil.
3. The tin-foil preserves this trace during its subsequent motion.
4. If at any subsequent time the stamped tin-foil passes in front of the needle when it is not vibrating (the analogue of a cold atom), the needle is caused to vibrate in the same period as before.
These analogies would be more perfect if the needle were set into vibration by being attached to a tuning-fork of definite period of vibration.
It appears, then, that the co-existence of such vortex atoms and ultramundane corpuscles as Sir William Thomson has devised leads to the conclusion that bot bodies must emit radiations which may be absorbed by cold bodies. The question naturally arises, Can this action be the keystone to a new theory of light ? Can the phenomena of reflection, refraction, interference, diffraction, and polarisation be explained by this kind of action ? In answer to these questions it can at present only be said that the germs of a complete theory of light do exist in this speculation.