RADIOACTIVITY, the phenomenon shown by certain substances of emitting radiation spontaneously. First observed in 1896 by Henri Becquerel, who found that certain uranium salts emitted rays which were capable of affecting a photo graph plate through black paper or thin metallic sheets, and were also able to dis charge 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 addi tion to uranium, two radioactive ele ments, 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 tempo rarily radioactive. These gases are known as emanations. The emanations from actinium lose their activity very quickly, those from thorium more slowly, while those from radium retain their activity about six thousand times as long as those from actinium. By boiling the solution of a radioactive salt, all the emanations contained in it can be re moved, but the de-emanated salt grad ually regains its power of producing em anations. The emanations behave, in every as ordinary gases, and can be liquefied by cooling to very low tem peratures. The rays emitted are of three kinds, and are known as the A, B, and r rays. They are capable of pass ing 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 I' rays, 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 r 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 ac cepted with reserve, as rays from differ ent elements possess varying penetrating ability. Crooks discovered that when the A rays are allowed to impinge upon a screen of zinc sulphide, the fluores cence produced is not a continuous glow but a succession of tiny sparks. An in strument in which this phenomenon is demonstrated is known as the spinthari scope.
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 suf ficient 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 dis integration of the radium, the fact ap pears to have no practical significance. At present there is no known method of increasing the speed of this disintegra tion, but in view of the fact that the ex istence of the phenomenon of radioac tivity 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.