No sooner had Perrin, a French physicist, demonstrated that the cathode rays were negative electricity moving from the negative electrode of a vacuum-tube to the positive electrode than Professor John Joseph Thomson, of the University of Cambridge, proved that this negative electricity is concentrated in small corpuscles, called electrons to-day, which move with great velocities, and that the ratio of the electrical charge to the mass of each electron is experimentally determinable, and is, under ordinary conditions, a definite and invariable quantity. This learned man, when a youth of only twenty-five, had predicted in 1881, fourteen years before Roentgen’s discovery, that the cathode rays were small negatively charged bodies, moving with great velocities. Assuming them to be spherical, he calculated, by the Faraday-Maxwell electromagnetic theory, the ratio of their charge to their mass. He showed theoretically that their mass consisted of two parts, one of which is the ordinary gravitational or material mass, and the other a new mass which is proportional to the electrical energy in the electron, and that this mass also depended upon the velocity of the motion in a definite way. He devised and employed an experimental method to determine this ratio. The most remarkable feature of this interrelation between the electromagnetic mass and the velocity of motion was the fact that when the velocity approached the velocity of light the mass approached an infinitely large value. But no such extremes of velocity of motion of the electrons in a vacuum-tube were found at that time.