We have therefore reason to believe, from the phenomena of light and heat, that there is an æthereal medium filling space and permeating bodies capable of being set in motion, and of transmitting that motion to gross matter, so as to heat it and affect it in various ways.... Hence the parts of this medium must be so connected that the motion of one part depends in some way on the motion of the rest; and at the same time these connections must be capable of a certain kind of elastic yielding, since the communication of motion is not instantaneous, but occupies time. The medium is therefore capable of receiving and storing up two kinds of energy--namely, the “actual” energy depending on the motion of its parts, and “potential” energy, consisting of the work which the medium will do in recovering from displacement in virtue of its elasticity. The propagation of undulations consists in the continual transformation of one of these forms of energy into the other alternately, and at any instant the amount of energy in the whole medium is equally divided, so that half is energy of motion and half is elastic resilience. * * * * * In order to bring these results within the power of symbolic calculation, I then express them in the form of the general equations of the electromagnetic field. * * * * * The general equations are next applied to the case of a magnetic disturbance propagated through a non-conducting field, and it is shown that the only disturbances which can be so propagated are those which are transverse to the direction of propagation, and that the velocity of propagation is the velocity _v_, found from experiments such as those of Weber, which expresses the number of electrostatic units of electricity which are contained in one electromagnetic unit. This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat and other radiations, if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws.... Conducting media are shown to absorb such radiations rapidly, and therefore to be generally opaque. [Sidenote: ELECTROMAGNETIC THEORY OF LIGHT.] The conception of the propagation of transverse magnetic disturbances to the exclusion of normal ones is distinctly set forth by Professor Faraday in his “Thoughts on Ray Vibrations.” _The electromagnetic theory of light, as proposed by him, is the same in substance as that which I have begun to develop in this paper_,[52] except that in 1846 there were no data to calculate the velocity of propagation.