accuracy of at least 1 part in 100,000; and with apparatus specially designed to test this point an even greater accuracy could be obtained. When I made my experiments with the radium pendulum the close connection between the amounts of uranium and radium in radioactive minerals had not been discovered; this connection makes it exceedingly probable that radium is derived from uranium and that this metal may have weight for weight more electric potential energy, and therefore a larger proportion of its mass in the ether, than radium itself. This points to the conclusion that the proper substance to use for the pendulum experiment is uranium rather than radium, especially since uranium can easily be obtained in sufficiently large quantities to enable us to construct the pendulum of the shape and size which would give the most accurate results, it would not, I think, be impossible to determine the ratio of mass to weight for uranium to an accuracy of 1 part in 250,000.
Though we 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 phenomenon, 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 directions round the sphere. When 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
