the mass of the ether bound by the lines of electric force is proportional to this energy, this mass will be greater than when the sphere was at rest. The difference is very small unless the velocity of the spheres approaches the velocity of light, but when it does so the augmentation of mass is very large. Kaufman has succeeded in demonstrating the existence of this effect for the "β" particles emitted by radium; these are negatively electrified particles projected at high speeds from the radium; the velocity of the fastest is only a few per cent. less than the velocity of light; along with these there are others moving much less rapidly. Kaufman determined the masses of the different particles, and found that the greater the speed the greater the mass, the mass of the more rapidly moving particles being as much as three times that of the slower ones. These experiments also led to the very interesting result that the whole of the mass of these particles is due to the charge of electricity they carry. On the view we have been discussing this means that the whole of the mass of these particles is due to the ether gripped by their lines of force.
If lines of electric force grip the ether, then, since waves of light, according to the Electromagnetic Theory of Light, are waves of electric force travelling at the rate of 180,000 miles per second, and as the lines of electric force carry with them some of the ether, a wave of light will be accompanied by the motion of a portion of the ether in the direction in which the light is travelling. The amount of this mass can be easily calculated by the rule that it would possess, if travelling with the velocity of light, an amount of kinetic energy equal to the electrostatic potential energy in the light; as the electrostatic energy is one-half the energy in the light wave, it follows that the mass of the moving ether per unit volume is equal to the energy of the light in that volume divided by the square of the velocity of light. Thus when a body is radiating a portion of the mass of the ether gripped by the body is carried out by the
