Aether. [Footnote 15: _Magnetism and Electricity._] [Footnote 16: _Phil. Mag._, 1902.] [Footnote 17: _Ibid._, 1872.] [Footnote 18: _Burnet Lectures._] [Footnote 19: _Burnet Lectures._] [Footnote 20: _Phil. Mag._, 1872.] ART. 78. _The Electro-Magnetic Theory of Light._--We have seen (Art. 71) that light is due to a periodic wave motion of the Aether, and we have previously seen that heat is also due to a periodic wave motion of the Aether. Thus in the phenomena of light and heat, Aether is the medium in which the energy of light is stored, and by which it is transmitted in its passage from a luminous body, as the sun, until it comes into contact with a planet or satellite from which it is reflected, thus giving rise to light and heat. When, however, we come to deal with electro-magnetic phenomena, which are the results and effects produced by electricity and magnetism, we find certain phenomena similar to those that we find in relation to light and heat. Thus, when light is emitted by a luminous body, a certain amount of energy is given out by that body, and if such light is absorbed by another body, the latter becomes heated, a clear proof that it has received energy or motion from some outside source. From the time it left the luminous body till it reached the lighted or absorbing body, it must have existed as energy, that is, motion in the Aether. As we have already seen, Newton thought that the transference of energy was accomplished by the actual transference of certain small corpuscles or atoms given out by the luminous body, which conveyed the energy of the one body to the other. According to the wave theory of light, however, we find that the transference of energy is accomplished by a wave motion in the Aether, which is periodic both in time and space, by which wave motion the energy is transferred from the luminous to the illuminated body. Now every one is familiar with the effects of magnetism and electricity in some form or other, and such familiarity teaches that various kinds of work may be done by electricity. If an electric current be generated and allowed to flow through any circuit, as the ramifications, for example, of an electric-tram system, it can readily be seen that by the action of the current large masses or bodies as trams may be moved. To generate the current requires the expenditure of energy, and for the tram to be moved requires the transmission of that energy from the generating station till it reaches the body to be moved. By what means is such energy transmitted? because if it disappears at one place and reappears at another, it must have passed through a medium during the interval. It has been demonstrated that the medium which conveys the current from place to place is the Aether, so that as light is transmitted through space by the Aether, in a similar manner electric currents are transmitted through space also by the same medium. The discoverer of