of molecules come under the influence of the electric force in the light. We may suppose that light consists of small transverse pulses and waves travelling along discrete lines of electric force, disseminated throughout the ether, and that the diminution in the intensity of the light as it travels outwards from a source is due not so much the enfeeblement of the individual pulses as to their wider separation from each other, just as on the emission theory the energy of the individual particles does not decrease as the light spreads out; the diminution of the intensity of the light is produced by the spreading out of the particles.
The idea that bodies are connected by lines of electric force with invisible masses of ether has an important bearing on our views as to the origin of force and the nature of potential energy. In the ordinary methods of dynamics a system is regarded as possessing kinetic energy which depends solely upon the velocities of the various parts of which it is composed, and potential energy depending on the relative position of its parts. The potential energy may be of various kinds; thus we may have potential energy due to gravity and potential energy due to stretched springs, or electrified systems, and we have rules by which we can calculate the value of these potential energies corresponding to any position of the system. When we know the value of the potential energy the method known as that of "Lagrange's equations" enables us to determine the behaviour of the system. As a means of calculation and investigation this use of the potential energy works admirably, and is very unlikely to be superseded; but, regarded from a philosophical point of view, the conception of potential energy is much less satisfactory, and stands on quite a different footing from that of kinetic energy. When we recognise energy as kinetic we feel that we know a great deal about it; when we describe energy as potential we feel that we know very little about it, and though it may be objected that from a practical point of view that little is all that
