RADIOACTIVITY. The subject of radioactivity deals with phenomena exhibited by a special class of bodies of high atomic weight of which uranium, thorium, radium and actinium are the best known examples. These substances possess the properties of spontaneously emitting radiations of a special character which are able to penetrate through matter opaque to ordinary light. The study of the radioactive bodies has proved of great importance in the development of modern Physics and has thrown a flood of light on the structure of the atom and the processes which may occur within it. It showed for the first time that the atoms of certain heavy elements are not permanently stable but break up with ex plosive violence and great emission of energy, leading to the pro duction of a whole series of new elements which have a limited life. The disintegration of these atoms is accompanied by the ex pulsion of special types of radiation of great individual intensity which have been of great service in throwing light on the processes which take place in their absorption by matter. The alpha-particles have the greatest individual energy of any particle known to sci ence and have proved invaluable in probing the inner structure of atoms and in effecting the artificial disintegration of certain light elements. A study of the scattering of alpha-particles by matter first disclosed the nuclear structure of atoms and provided us with methods of determining the charge carried by the nuclei of the elements. The examination of the chemical behaviour of the radioactive element first indicated the isotopic constitution of certain elements, i.e., the existence of elements with almost identi cal physical and chemical properties but yet with different atomic weights and radioactive properties. This has led to the proof of the isotopic constitution of the ordinary elements. In many respects the wonderful series of transformations occurring in uranium and thorium provide data which cannot but prove of great value in attacking that most fundamental of problems—the constitution of atomic nuclei.
The beginning of Radioactivity dates from 1896, and was an indirect consequence of the discovery of the X-rays made a few months before by Röntgen. It was known that the production of X-rays in a vacuum tube was accompanied by a strong phosphor escence of the glass, and it occurred to several investigators that ordinary substances made phosphorescent by visible light might emit a penetrating radiation similar to X-rays. Following out this
idea, H. Becquerel, a distinguished French physicist, exposed amongst other substances a phosphorescent compound of uranium, uranium-potassium sulphate, enveloped in paper beneath a photo graphic plate. A weak photographic effect was obtained. This was shown to be due to a penetrating radiation capable of passing through sheets of matter opaque to ordinary light. Further investi gation showed that this photographic action was exhibited by all compounds of uranium and by the metal itself, and had nothing to do with phosphorescence. It was shown if the uranium were kept in darkness it did not vary appreciably with time. Becquerel showed that the rays from uranium like X-rays were capable of discharging a body whether positively or negatively electrified. A uranium compound brought close to the charged plate of a gold leaf electroscope causes a rapid collapse of the gold leaves. This property of uranium, and also of the radioactive bodies in general, has supplied a delicate and quantitative method of accurate com parison of the intensity of the radiations from substances under varying conditions. A modified form of gold leaf electroscope (see INSTRUMENTS, ELECTRICAL) has come into general use for comparison of the radioactivity of substances. Rutherford made a systematic examination of the discharging effect produced by the rays from uranium and showed that it was due to the production of charged carriers or ions in the volume of the gas through which the radiations pass. In an electric field, the positive ions travel to the negative electrode and vice versa, thus causing a discharge of the electrified body. If a sufficiently strong field is used, the ions are all swept to the electrodes before appreciable loss of their number can occur by recombination. The rate of discharge then reaches a steady maximum value which is not altered by a large increase in voltage. This maximum current through the gas is called the saturation current. The ions produced in gases by the .rays from uranium and other radioactive substances are in gen eral identical with those produced by X-rays, and the mechanism of conductivity of the gas is very similar in both cases (see ELEC TRICITY CONDUCTION, Through Gases).