Radium compounds send forth three kinds of rays, distinguished as alpha, beta, and gamma rays.  Experiments have made it extremely probable that the [alpha]-rays are streams of very minute particles, somewhat heavier than atoms of hydrogen, moving at the rate of about 18,000 miles per second; and that the [beta]-rays are streams of much more minute particles, the mass of each of which is about one one-thousandth of the mass of an atom of hydrogen, moving about ten times more rapidly than the [alpha]-particles, that is, moving at the rate of about 180,000 miles per second.  The [gamma]-rays are probably pulsations of the ether, the medium supposed to fill space.  The emission of [alpha]-rays by radium is accompanied by the production of the inert elementary gas, helium; therefore, the [alpha]-rays are, or quickly change into, rapidly moving particles of helium.  The particles which constitute the [beta]-rays carry electric charges; these electrified particles, each approximately a thousand times lighter than an atom of hydrogen, moving nearly as rapidly as the pulsations of the ether which we call light, are named electrons.  The rays from radium compounds discharge electrified bodies, ionise gases, that is, cause them to conduct electricity, act on photographic plates, and produce profound changes in living organisms.

The radium emanation is a gas about 111 times heavier than hydrogen; to this gas Sir William Ramsay has given the name niton.  The gas has been condensed to a colourless liquid, and frozen to an opaque solid which glows like a minute arc-light.  Radium emanation gives off [alpha]-particles, that is, very rapidly moving atoms of helium, and deposits exceedingly minute quantities of a solid, radio-active substance known as radium A. The change of the emanation into helium and radium A proceeds fairly rapidly:  the half-life period of the emanation is a little less than four days.  This change is attended by the liberation of much energy.

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.