RADIOACTIVITY. The name given to the property which uranium, thorium, and other bodies to be described presently have of sending out certain radiations spontaneously. Imme diately after the discovery of X-rays by Profes sor R6ntgen in 1895 many physicists began investigations in order to see whether phosphores cent bodies in general would not emit rays of the same character. Among these was Prof. H. Becquerel of Paris. He, in the first months of 1896, made the great discovery that the salts of uranium emitted spontaneously certain radiations which would affect a photographic plate. He also found that these radiations would, like X rays, discharge electrified bodies, produce phos phorescence, and traverse many bodies which were opaque to ordinary light. The name 'Becquerel rays' was given to these radiations. They were investigated immediately with the greatest care by many others, particularly by E. Rutherford, who was then a student in the laboratory of J. J. Thomson, Cambridge, England; and their proper ties will be described below. In the search for other substances which would emit radiations similar to those described above, it was discov ered, almost simultaneously, by Professor Schmidt and Madame Curie of Paris, in the year 1898, that the salts of thorium emitted rays sim ilar to those of uranium. M. and Mine. Curie began a prolonged investigation of all metals and metaloids, the rare earths, and a great number of rocks and minerals, iu the hope of discovering other radioactive bodies, and were rewarded by discovering that pitchblende, which is a mineral containing the oxide of uranium and other sub stances, was much more active than pure metallic uranium. By a series of chemical separations, they were able to isolate two substances, 'radium' and 'polonium,' which were most intensely radio active, in some cases several thousand times more so than uranium.
Radium is undoubtedly an element with a defi nite atomic weight and is found to accompany the barium which is separated from pitchblende. It is not yet proved that. polonium is an element, hut it is a substance which accompanies the bis muth separated from the pitchblende and is simi lar to it in its chemical properties. The dis
covery of these substances was made in 1898: and in 1899 another radioactive substance was discovered by 11. Debierne which he called 'ac tinium,' and which accompanies certain bodies of the iron group contained in pitchblende and seems to he connected with thorium. It has been shown by recent investigators that almost all substances in nature are to a greater or less ex tent radioactive. Among these may be men tioned the leaves of plants, freshly fallen rain or snow, etc.; and the' hypothesis has been advanced that this radioactivity is due to certain radia tions emitted by the sun itself and which arc connected with the appearance of the aurora borealis and the other phenomena of atmospheric electricity. In the spring of 1903 it was discov ered by Prof. J. J. Thomson that the water ob tained from deep wells contained a radioactive gas; and beyond a doubt other substances will be found which possess this power of radioac tivity.
The properties of the radiations which are ob tained from radioactive bodies may be grouped under various heads: chemical, electrical. fluo rescent, and physiological. Among the chemical properties it may be sufficient to mention the photographic action of the rays and their power to color glass and porcelain, and in certain cases to produce ozone. The electrical properties of the radiations are the most interesting. as they have led to several most important advances in the theories of matter and electricity. These will be discussed more fully in what follows; but, in brief, the most obvious electrical effect of the radiations is to make a gas through which they pa.s a conductor for electricity. In regard to the fluorescent properties nothing need be said except to state that a great number of substances fluoresce under their action. 'The most important physiological actions so far studied are their power to produce luminosity in the human eye, to cause the same kind of peculiar burn of the skin as do X-rays, and in .certain cases to para lyze the nerve centres. It is extremely probable that these radiations have the same therapeutic properties as do X-rays in such diseases as can cer, lupus, etc.