MATTER AND ETHER THOMSON. 239 



Though "sve have not been able to get direct experimental evidence 

 of the existence of the part of the mass in the ether in this way, we 

 are in a more fortunate position in respect to a closely allied phe- 

 nomenon, viz, the effect of the speed of a body on its apparent mass. 

 We have seen that the mass of the ether bound by any electrical 

 system is .proportional to the electric potential energy of that system. 

 Now let us take the simplest electrical system we can find — a charge 

 of electricity concentrated on a small sphere. "When the sphere is at 

 rest the lines of electric force are uniformly distributed in all direc- 

 tions round the sphere. Wien the lines are arranged in this way 

 the electric potential energy is smaller than for any other possible 

 distribution of the lines. Now, let us suppose that the sphere is set 

 in rapid motion, the lines of electric force have a tendency to set 

 themselves at right angles to the direction in which they are moving; 

 they thus tend to leave the front and rear of the sphere and crowd 

 into the middle. The electrical potential energy is increased by this 

 process, and since the mass of the ether bound by the lines of electric 

 force is proportional to this energ}^, this mass will be greater than 

 when the sphere was at rest. The diiference 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 suc- 

 ceeded in demonstrating the existence of this elfect for the /? 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 experi- 

 ments 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 traveling 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 traveling. The amount of this 

 mass can be easily calculated by the rule that it would possess, if 

 traveling with the velocity of light, an amount of kinetic energy 

 equal to the electrostatic potential energy in the light ; as the electro- 

 static 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 



