Page:The New International Encyclopædia 1st ed. v. 16.djvu/729

* RADIOACTIVITY. 641 RADIOACTIVITY. same character. Among these was Prof. H. Becquerel of Paris. He, in the first months of 1891), 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 JIme. Curie began a prolonged investigation of all metals and metaloids, the rare earths, and a great number of rocks and minerals, in 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 w'hich is separated from pitchblende. It is not yet proved that polonium is an clement, but it is a substance which accompanies the bis- nuith 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 il. Debierne which he called 'ac- tinium,' and which accompanies certain bodies of the iron group contained in pitchblende and seems to be 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 are 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. j?homson that the water ob- tained from deep wells contained a radioactive gas; and beyond a doubt other substances will be foimd wliieh possess this power of radioac- tivity. The properties of the radiations which are ob- tained from radioactive bodies may be grouped nnder various heads: chemical, electrical, ti^uo- reseent, and physiological. Among the clicmical 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 oljvious electrical effect of the radiations is to make a gas through which they pass a conductor for electricity. In regard to the fluorescent properties nothing need be said exceiJt 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. As stated above, these radiations possess the power of penetrating and passing through many substances which are opaque to ordinary light; but it was soon discovered that there were dif- ferences in this power whicii could only be ex- plained by assuming that the radiations were complex in their nature, being made up of two groups, one very easily absorbed, the other ex- tremely penetrating. The attempt was made to see whether either of these groups of radiations could be reflected, refracted, ditl'racted, or polar- ized, but it was found that they possessed none of these properties. It was discovered, however, that although in this last respect the radiations were similar to X-rays, they dill'ered from them _ in being deviated by a magnetic field. This proves that the radia'tions from radioactive bodies are not disturbances in the ether similar to X- rays or light, but are electrified particles of mat- ter moving at a rapid rate. It was discovered by several observers that the penetrating rays were easily deflected by a magnetic field, and in such a direction as to prove that the.y were car- riers of negative electrical charges, "it has been shown that an electrically charged body, if in rapid motion, is equivalent to an electric cur- rent, and since an electric current, if free to move in an electric field, is acted upon mechanically by the field, therefore an electri- fied particle in motion will have the direction of its path changed if subjected to a strong magnetic field, and the direction of the deflection will depend upon whether the particle is charged positively or negatively. It may be shown that there is a mathematical connection between the mass of the moving particle, its electrical charge, its velocity, the strength of the magnetic field, and the amount of the deflection produced. For obvious reasons, a charged particle in mo- tion will have its direction also changed if it is made to pass through an intense transverse elec- trical field. It was not, however, until 1902 that it was proved, by Rutherford, that the easily ab.sorbed. non-penetrating radiations were de- flected also by a magnetic field, but in such a direction as to prove that they were positively electrified bodies. Thus the foi"mer raj-s are like the cathode rays in an ordinary vacuum tube; the latter, like the so-called canal r.ays. By means of experiments which need not be described here, it has been shown that although the electrical charges carried by the two kinds of radiations are equal and. so far as is known, identical with the charge carried by a hydrogen ion in ordinary elec- trolysis, nevertheless, their masses are quite different. The mass of a particle of the o ra- diations, as the non-penetrating rays are called, is comparable with that of a hydrogen atom; while the mass of a particle of the /3 radiations.