In the first place, we know that one body, such as the sun, can part with its radiant energy to another body, such as the earth, and observation and experiment alike lead us to acknowledge a stage in which the energy has left the one body and has not yet arrived at the other. But we have already seen that energy is always found associated with matter, never by itself. In fact we have spoken of matter as the ‘vehicle of energy.’ Hence it necessarily follows that there is something between the sun and earth capable of moving and transmitting energy, and therefore, from the very conception of energy, possessing mass—this something we agree to call the ethereal medium. Again, we know that different masses of visible matter attract one another apparently at a distance. Our first attempt to analyse the nature of this force leads to the question:—Does it proceed from the surfaces of the attracting bodies, or does it penetrate their entire mass? This question was answered by Newton, who came to the conclusion that every particle of matter attracts every other particle with a force proportional to the product of their masses, and inversely proportional to the square of their distances. But this drives the mystery of gravitation only from the mass to the particle, and here the same sort of questions again occur. A particle as truly as a mass occupies space, and we wish to know whether gravitation force proceeds from the surface of the particle or from its interior. 150. We likewise wish to know how this force is communicated between one particle and another? Before we can solve these questions we must have some definite conception of the nature of a particle and of the constitution of the surrounding medium. Sir W. Thomson, as we have seen, has attempted to advance towards the nature of an atom or particle in his supposition that atoms are vortex-rings generated out of a perfect fluid filling all space. While, however, this conception accounts for some of the properties of an atom it does not at all directly account for anything like gravitation, and hence he adopts in addition the hypothesis of ultra-mundane corpuscles, which he supposes to be only a finer form of vortices.