Page:The New International Encyclopædia 1st ed. v. 16.djvu/730

* HADIOACTIVITT. 642 RADIOACTIVITY. as the penetrating rays are called, is approxi- mately one-thousandth as great. The velocities with which these particles are emitted are ex- tremely great, but vary within wide limits with the substance emitting them and its condition. The velocity may be as great in certain cases as 2 X 10'° cms. per second, which is approximately as great as the velocity of light. It is interesting to note that Prof. J. J. Tliomson has shown from the observations of Kaufmann that the total mass of the ^ radiations may be accounted for as due to the motion of the charge ; because, as explained in the article on ilATTER, an electrified particle in motion has a mass greater than that which it would liave if it were not charged, and therefore electricity, by itself, in motion has the properties of matter, so far as inertia is con- cerned. A most remarkable discovery was made by Paitherford in his investigation of the prop- erties of thorium. He found that there was given off spontaneously by the salts of thorium an emanation which could be blown from one vessel into another, which was radioactive itself, and whidi had the power of producing radio- activity in the walls of the vessel with which it came in contact. Similar emanations are given ofl' by radium, but so far have not been dis- covered with uranium or polonium. Rutherford has shown that the emanation has the proper- ties of an inert gas of a comparatively high atomic weight ; it can be condensed at the tem- perature of liquid air and again vaporized ; it can diffuse through porous partitions, and it can be occluded. The emanation as it leaves the body which produces it has no electrical properties, but it soon emits /3 radiations, and therefore becomes positively charged itself. If, therefore, there are any Tiegatively electrified bodies in the neighborhood, the particles of the emanation will settle upon them ; and it has been observed, as stated above, that these bodies now become radioactive. Certain chemical changes go on, and there is produced a condi- tion, or better, a substance, which has been called 'excited activity.' This in turn is radio- active ; and the radiations or emanations from it produce a secondary excited activity in neigh- boring bodies. In the case of radium this sec- ondary excited activity produces a tertiary ac- tivity ; and it is not known where the process stops. Observations made during the summer of 190.3 show that in these cases, after these slow changes, a gas appears which beyond a doubt is helium. This, then, is the end-product. The method by which the radiations from radioactive bodies produce their effects is not yet established, although many facts are now clear. It has been shown that the a radiations produce almost the entire action of the radia- tion, as might be expected from the fact that, although moving so rapidly and having so large a mass, they are so easily absorbed. The process of making a gas a conductor for electricity, or of 'ionizing' it, as it is called, consists in break- ing up its electrically neutral molecules into electrified parts, or 'ions,' which are therefore acted upon by an electric field. This process of dissociation of the gas is brought about in some manner by the motion through the gas and absorption by it of the rapidly moving a and j3 radiations. The various other actions of the rays, namely, the chemical, physiological. and other ones, can be accounted for roughly from obvious mechanical principles. The explanation of the cause of the emission of these radiations by the various radioactive substances is also not yet clear, although a great step has been made in advance by Ruther- ford and Soddy of ilcGill University. They have shown that if to a salt of thorium (tho- rium nitrate, for instance) ammonia be added, a precipitate is formed; and, if this be se])arated from the solution by filtration, the resulting filtrate, called thorium X, is found to be ex- tremely radioactive, while, on the other hand, the precipitate is at first but slightly radio- active. The thorium X, which can be sepa- rated from the filtrate by evaporating the am- monia, is found to lose its radioactivity with time, and to set free the emanation simul- taneously; while the precipitate referred to above, which is at first radioactive to only a slight degree, rapidly increases in activity. It is found further that, as the precipitate gains in activity, chemical changes go on of such a nature that, if it be now dissolved in ammonia, it will be seen that thorium X has been formed in the process. This means that starting with the almost inactive thorium, owing to the chem- ical changes thorium X is formed in it as a definite substance, which may later be separated from it, and that coincident with these chemical changes the a and |8 radiations are emitted. In short, the radioactivity of thorium, and of the other substances also, is maintained by the continued production of new kinds of matter which have well-defined chemical properties en- abling them to be separated from the original susbtauce, and which possess temporary ac- tivity. In a similar manner the thorium emana- tion is formed from the thorium X, and the thorium excited activity from the thorium emanation, etc. The final product, as stated above, and as was first conjectured by Rutherford, is the gas known as helium. It has been shown that the changes just described do not take place simultaneously, but in the order given, and that the radioactivity of each of the compounds, for instance, thorium X, is connected not with the change in which it was produced, but with that by which the next substance is produced from it. It follows from this that a body which is radioactive must of necessity be undergoing change ; and, therefore, it is not possible for any of the new forms of radioactive matter, for instance, thorium X, the emanations, etc., to be identical with any of the known elements. They remain in existence un- changed for but a short time, and the decay of their radioactivity is an indication of their dim- inishing quantity. As has been described above, there is a definite series of changes by which thorium produces thorium X, thorium X produces thorium emana- tion, and so on. This same series of steps holds to a certain extent with the other radiations so far as studied, although the series is in no other case so complete. Radium has been shown to produce a radium emanation, and this to produce an excited activity, and so on ; the small amount of radium so far available for investigation has made it impossible to learn whether there is an intermediate product, radium X, between radium and its emanation. Uranium has been shown to produce uranium