The only satisfactory mental picture which the facts allow us to form, at present, of the emission of [beta]-rays from radium compounds is that which represents these rays as streams of electrons, that is, particles, each about a thousand times lighter than an atom of hydrogen, each carrying an electric charge, and moving at the rate of about 180,000 miles per second, that is, nearly as rapidly as light. When an electric discharge is passed from a plate of metal, arranged as the kathode, to a metallic wire arranged as the anode, both sealed through the walls of a glass tube or bulb from which almost the whole of the air has been extracted, rays proceed from the kathode, in a direction at right angles thereto, and, striking the glass in the neighbourhood of the anode, produce a green phosphorescence. Facts have been gradually accumulated which force us to think of these _kathode rays_ as streams of very rapidly moving electrons, that is, as streams of extraordinarily minute electrically charged particles identical with the particles which form the [beta]-rays emitted by compounds of radium. The phenomena of radio-activity, and also the phenomena of the kathode rays, have obliged us to refine our machinery of minute particles by including therein particles at least a thousand times lighter than atoms of hydrogen. The term _electron_ was suggested, a good many years ago, by Dr Johnstone Stoney, for the unit charge of electricity which is carried by an atom of hydrogen when hydrogen atoms move in a liquid or gas under the directing influence of the electric current. Some chemists speak of the electrons, which are the [beta]-rays from radium, and the kathode rays produced in almost vacuous tubes, as non-material particles of electricity. Non-material means devoid of mass. The method by which approximate determinations have been made of the charges on electrons consists in measuring the ratio between the charges and the masses of these particles. If the results of the determinations are accepted, electrons are not devoid of mass. Electrons must be thought of as material particles differing from other minute material particles in the extraordinary smallness of their masses, in the identity of their properties, including their mass, in their always carrying electric charges, and in the vast velocity of their motion. We must think of an electron either as a unit charge of electricity one property of which is its minute mass, or as a material particle having an extremely small mass and carrying a unit charge of electricity: the two mental pictures are almost, if not quite, identical.