Becquerel, the French physicist, discovered, soon after Roentgen’s discovery, that certain substances associated with the element uranium emitted electrons, both negative and positive, without being in a vacuum-tube and submitted to the action of a great electrical force. Madame Curie isolated the most active of these substances and called it _radium_. The action of electron emission discovered by Becquerel was called radioactivity. Three distinct things, it was found, were emitted by radium: negative electrons, the so-called beta rays, some of which were moving with enormous velocities; positive electrons, the so-called alpha rays, moving with smaller velocities; and, finally, an emission which had the same physical properties as the X-rays. The beta rays, some of which move with a velocity nearly equal to the velocity of light, enabled the physicists to determine experimentally, employing J. J. Thomson’s method, the relation between the mass of the electron and its velocity, and lo and behold, it was found that, in all probability, the negative electron contained no other mass except the mass due to its electromagnetic energy. In other words, a negative electron is concentrated electricity and nothing else. Similar experiments with positive electrons led to similar conclusions. Another most remarkable result was the revelation of the great difference between the masses, and, therefore, between the electrical energies residing in the negative and in the positive electron. The mass of a positive electron was found to be very nearly equal to the mass of a hydrogen atom, and the mass of a negative electron was found to be only about one two-thousandth part of the mass of the positive electron, and this meant that if the electrons are of spherical shape then the diameter of the positive electron is only one two-thousandth part of the diameter of the negative electron, since the energies and therefore the masses are inversely proportional to the diameters. In other words, there is in the positive electron a much bigger concentration of electricity than in the negative and, therefore, much more work was used up to produce that concentration. Experimental data and calculation gave for the diameter of a negative electron one ten-thousandth part of the diameter of the smallest atom, that is, of the hydrogen atom, and therefore the diameter of the positive electron should be only one twenty-millionth part of the diameter of a hydrogen atom. A most bewildering revelation!