Radioactivity

Collier's New Encyclopedia (1921)

the phenomenon of emitting shown by certain substances of en radiation spontaneously. First observed in 1896 by Henri Becquerel, who found that certain uranium salts emitted rays which were capable of affecting a photograph plate through black paper or thin metallic sheets, and were also able to discharge electrified bodies and to produce phosphorescence. Becquerel's discovery was followed in 1898 by those of Schmidt and Madam Curie, who found almost simultaneously that thorium salts also emit rays. Madame Curie, working with her husband, discovered, in the same year, that pitchblende contained in addition to uranium, two radioactive elements, which were named polonium and radium. In the following year, 1899, Debierne discovered actinium in the same mineral. In addition to radiations, Rutherford found that radioactive substances give off gases which are themselves tempoknown as emanations. The emanations from actinium lose their activity very quickly, those from thorium more slowly, while those from radium retain their thousand times as long activity about six as those from actinium. By boiling the solution of a radioactive salt, all the in it can be reemanations contained moved, but the de-emanated salt gradually regains its power of producing emanations. The emanations behave, in every way, way, as ordinary gases, and can cooling to very low tembe liquefied by cooling peratures. The rays emitted are of three kinds, and are known as the A, B, and I rays. They are capable of passing through opaque substances, but their penetrating power varies greatly. A rays are positively charged and have very small penetrating power; B rays are negatively charged and have greater penetrating power than the A rays, but are easily absorbed in comparison with the Frays, which have no electric charge, but are very penetrating. According to Rutherford, a piece of aluminum 8 cm. thick is required to cut off half the F rays, whereas a sheet 0.05 cm. thick will cut off the B rays and 0.0005 cm. the A rays, but these figures must be accepted with reserve, as rays from different elements possess varying penetrating ability. Crooks discovered that when the A rays are allowed to impinge ge upon a screen of zinc sulphide, the fluorescence produced is not a continuous glow but a succession of tiny sparks. An instrument in which this phenomenon is demonstrated is known as the spinthariscope. Madame Curie and Laborde found that radium is always at a temperature above that of surrounding bodies, and it is stated that every hour it generates sufficient heat to raise its own weight of water from freezing to the boiling point. As much energy could be obtained from one gram of radium as from a ton of coal, but since approximately 2500 years would be required for the complete disintegration of the radium, the fact appears to have no practical significance. At present there is no known method of increasing the speed of this disintegration, but in view of the fact that the existence of the phenomenon of radioactivity has been known for scarcely a quarter of a century, it is too soon even to hazard a guess as to the possibilities of its practical application.