RADIUM (from Lat. radius, ray), a metallic chemical element obtained from pitchblende, a uranium mineral, by P. and Mme. Curie and G. Bemont in 1898; it was so named on account of the intensity of the radioactive emanations which it yielded. Its discovery was a sequel to H. Becquerel's observation in 1896 that certain uranium preparations emitted a radiation resembling the X rays observed by Röntgen in 1895. Like the X rays, the Becquerel rays are invisible; they both traverse thin sheets of glass or metal, and cannot be refracted; moreover, they both ionize gases, i.e. they discharge a charged electroscope, the latter, however, much more feebly than the former. Characteristic, also, is their action on a photographic plate, and the phosphorescence which they occasion when they impinge on zinc sulphide and some other salts. Notwithstanding, these resemblances, these two sets of rays are not indentical. Mme. Curie, regarding radioactivity - i.e. the emission of rays like those just mentioned - as a property of some undiscovered substance, submitted pitchblende to a most careful analysis. After removing the uranium, it was found that the bismuth separated with a very active substance - polonium; this element was afterwards isolated by Marckwald, and proved to be identical with his radiotellurium; that the barium could be separated with another active substance - radium; whilst a third fraction, composed mainly of the rare earths (thorium, &c.), yielded to Debierne another radioactive element - actinium, which proved to be identical with the emanium of Giesel. Another radioactive substance - ionium - was isolated from carnotite, a uranium mineral, by B. B. Boltwood in 1905. Radioactive properties have also been ascribed to other elements, e.g. thorium and lead. There is more radium than any other radioactive element, but its excessive rarity may be gauged by the facts that Mme. Curie obtained only a fraction of a gramme of the chloride and Giesel 2 to 3 gramme of the bromide from a ton of uranium residues.
There is a mass of evidence to show that radium is to be regarded as an element, and in general its properties resemble those of the metals of the alkaline earths, more particularly barium. To the bunsen flame a radium salt imparts an intense carmine-red colour (barium gives a green). The spectrum, also, is very characteristic. The atomic weight, 226.4, places the element in a vacant position in group II. of the periodic classification, along with the alkaline earth metals.
Generally speaking, the radiation is not simple. Radium itself emits three types of rays: (1) the a rays, which are regarded as positively charged helium atoms; these rays are stopped by a single sheet of paper; (2) the s rays, which are identified with the cathode rays, i.e. as a single electron charged negatively; these rays can penetrate sheets of aluminium, glass, &c., several millimetres thick; and (3) the 'y rays - which are non-electrified radiations characterized by a high penetrating power, i % surviving after traversing 7 cm. of lead or 150 cm. of water. In addition, radium evolves an "emanation" which is an extraordinarily inert gas, recalling the "inactive" gases of the atmosphere. We thus see that radium is continually losing matter and energy as electricity; it is also losing energy as heat, for, as was observed by Curie and Laborde, the temperature of a radium salt is always a degree or two above that of the atmosphere, and they estimated that a gramme of pure radium would emit about 100 gramme-calories per hour.
The Becquerel rays have a marked chemical action on certain substances. The Curies showed that oxygen was convertible into ozone, and Sudborough that yellow phosphorus gave the red modification when submitted to their influence. More interesting are the observations of D. Berthelot, F. Bordas, C. Doelter and others, that the rays induce important changes in the colours of many minerals. (See Radioactivity.) The action of radium on human tissues was unknown until 1901, when, Professor Becquerel of Paris having incautiously carried a tube in his waistcoat pocket, there appeared on the skin within fourteen days a severe inflammation which was known as the famous "Becquerel burn." Since that time active investigation into the action of radium on diseased tissues has been carried on, resulting in the establishment in Paris in 1906 of the "Laboratoire biologique du Radium." Similar centres for study have been inaugurated in other countries, notably one in London in 1909. The diseases to which the application has been hitherto confined are papillomata, lupus vulgaris, epithelial tumours, syphilitic ulcers, pigmentary naevi, angiomata, and pruritus and chronic itching of the skin; but the use of radium in therapeutics is still experimental. The different varieties of rays used are controlled by the intervention of screens or filtering substances, such as silver, lead or aluminium. Radium is analgesic and bactericidal in its action.
See Radiumtherapie, by Wickham and Degrais (1909); Die therapeutische Wirkung der Radiumstrahlen, by O. Lassar, in Report of Radiology Congress, Brussels, 1906; E. Dorn, E. Baumann and S. Valentiner in Physische Zeitung (1905); Abbe in Medical Record (October 1907).
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