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In 1896 H. Becquerel discovered that compounds of the metal uranium continually emitted rays capable of penetrating opaque screens and affecting photographic plates. Like cathode and Rontgen rays, the rays from uranium make the air through which they pass a conductor of electricity, and this property gives the most convenient method of detecting the rays and of measuring their intensity. An electroscope may be made of a strip of gold-leaf attached to an insulated brass plate and confined in a brass vessel with glass windows. When the gold-leaf is electrified, it is repelled from the similarly electrified brass plate, and the angle at which it stands out measures the electrification. Such a system, if well insulated, holds its charge for hours, the leakage of electricity through the air being very slow. But, if radio-active radiation reach the air within, the gold-leaf falls, and the rate of its fall, as watched through a microscope with a scale in the eye-piece, measures the intensity of the radiation. With some form of this simple instrument, or with the more complicated quadrant electrometer, most radio-active measurements have been made.

It was soon discovered that the activity of uranium compounds was proportional to the amount of uranium present in them. Thus radio-activity is an atomic property dependent on the amount of an element and independent of its state of chemical combination.

In a search for radio-activity in different minerals, M. and Mme Curie found a greater effect in pitch-blende than its contents of uranium warranted, and, led by the radio-active property alone, they succeeded, by a long series of chemical separations, in isolating compounds of a new and intensely radio-active substance which they named radium.

Radium resembles barium in its chemical properties, and is precipitated with barium in the ordinary course of chemical analysis. It is separated by a prolonged course of successive crystallisation, the chloride of radium being less soluble than that of barium, and therefore sooner separated from an evaporating solution. When isolated, radium chloride has a composition, which, on the assumption that one atom of metal combines with two of chlorine as in barium chloride, indicates that the relative weight of the atom of radium is about 225. As thus prepared, radium is a well-marked chemical element, forming a series of compounds analogous to those of barium and showing a characteristic line spectrum. But, unlike most other chemical elements, it is intensely radio-active, and produces effects some two million times greater than those of uranium.

In 1899 E. Rutherford, then of Montreal, discovered that the radiation from uranium, thorium and radium was complex. (Rutherford, "Radio-activity"(2nd edition), Cambridge, 1905.) Three types of rays were soon distinguished. The first, named by Rutherford alpha-rays, are absorbed by thin metal foil or a few centimetres of air. When examined by measurements of the deflections caused by magnetic and electric fields, the alpha-rays are found to behave as would positively electrified particles of the magnitude of helium atoms possessing a double ionic charge and travelling with a velocity about one-tenth that of light. The second or beta type of radiation is much more penetrating. It will pass through a considerable thickness of metallic foil, or many centimetres of air, and still affect photographic plates or discharge electroscopes. Magnetic and electric forces deflect beta-rays much more than alpha-rays, indicating that, although the speed is greater, approaching in some cases within five per cent. that of light, the mass is very much less. The beta-rays must be streams of particles, identical with those of cathode rays, possessing the minute mass of J.J. Thomson's corpuscle, some eight-hundredth part of that of a hydrogen atom. A third or gamma type of radiation was also detected.

More penetrating even than beta-rays, the gamma-rays have never been deflected by any magnetic or electric force yet applied. Like Rontgen rays, it is probable that gamma-rays are wave-pulses in the luminiferous aether, though the possibility of explaining them as flights of non-electrified particles is before the minds of some physicists.

Still another kind of radiation has been discovered more recently by Thomson, who has found that in high vacua, rays become apparent which are absorbed at once by air at any ordinary pressure.

The emission of all these different types of radiation involves a continual drain of energy from the radio-active body. When M. and Mme Curie had prepared as much as a gramme of radium chloride, the energy of the radiation became apparent as an evolution of heat. The radium salt itself, and the case containing it, absorbed the major part of the radiation, and were thus maintained at a temperature measurably higher than that of the surroundings. The rate of thermal evolution was such that it appeared that one gramme of pure radium must emit about 100 gramme-calories of heat in an hour. This observation, naturally as it follows from the phenomena previously discovered, first called attention to the question of the source of the energy which maintains indefinitely and without apparent diminution the wonderful stream of radiation proceeding from a radio-active substance.

In the solution of this problem lies the point of the present essay.

In order to appreciate the evidence which bears on the question we must now describe two other series of phenomena.