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 which 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, diffracted, 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 differed from them in being deviated by a magnetic field. This proves that the radiations 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 they 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 absorbed, non-penetrating radiations were de fleeted also by a magnetic field, but in such a direction as to prove that they were positively electrified bodies. Thus the former rays are like the cathode ray: in an ordinary vacuum tube; the latter. like the so-called canal rays. 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 a 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 p radiations, as the penetrating rays arc called, is approxi mate]; one-thousandth as great. The velocities with which there particles are emitted are ex tremely great, but vary within wide with the substance emitting them and its condition. The velocity may be as great in certain eases as 2 X 10`° ems. per second, which is approximately as great as the velocity of light. It is interesting
to note that Prof. J. J. Thomson 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 MATTER, an electrified particle in motion has a mass greater than that which it would have 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 Rutherford 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 which had the power of producing radio activity in the walls of the vessel with which it came in contact. Similar emanations are given off 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 negatively 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 1903 show that in these eases, 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 gai is brought about in some manner by the motion through the gas and absorption by it of the rapidly moving a and # radiations. The various other actions of the rays, namely, the chemical, physiological, and other ones, can be accounted for roughly from obvious mechanical principles.