The Concentration of Induced Radioactivity upon Bodies Charged Negatively.—Rutherford showed that a body exposed to the emanation from thorium became much more active when it was charged negatively than when it was at the same potential as the surrounding objects. On the contrary, it became less excited when it had a positive potential. The same phenomenon is noticed in the case of the excitation by radium and actinium. The nature of this curious phenomenon seems to me not to be well established.
Condensation of the Emanations from Radium and Thorium.—Rutherford and Soddy discovered that the emanations from radium and thorium can be condensed at the temperature of liquid air. A current of air charged with the emanation loses its radioactive properties when passing through a coil of tube plunged in liquid air. The emanations remain condensed in the tube, and can be restored to the gaseous state when it is warmed. The emanation from radium condenses at -150°, while that from thorium condenses between -100° and -150°. The experiment can be performed as follows: Two glass reservoirs, one large and the other small, communicate with one another. They are filled with gas excited by radium. The small reservoir is plunged in liquid air. The large reservoir then rapidly becomes inactive, while all the activity is concentrated in the small reservoir. If the two are then disconnected, and the small one be taken from the liquid air, it is seen that the large reservoir is not at all luminous, while the small one gives off more light than at the beginning of the experiment. The experiment is a very brilliant one if the walls of the reservoirs are coated inside with phosphorescent sulphide of zinc.
When a platinum wire excited by thorium or radium is heated to redness, it loses most of its activity. Fanny Cook Gates showed that this radioactivity is transferred to cold solids placed in the vicinity of the wire. It distils in some way, at a sufficiently high temperature, passing through the intermediate form of a gaseous emanation. The induced radioactivity of solids would thus be analogous to a condensed emanation.
Activity Induced by Remaining in Solution in a Radioactive Liquid. Uranium X. Thorium X.—Certain substances are temporarily excited when they stay in the same solution with radioactive substances. Giesel and Mme. Curie prepared active bismuth by dissolving one of its salts in the solution of a salt of radium. Debierne also excited a salt of barium in a solution of a salt of actinium. The barium salt thus excited presents certain analogies to the salt of radium, and can be fractionated in the same way. By crystallization the active chloride is concentrated in the salt that is deposited.
By various methods of chemical precipitation the activity of uranium has been divided (Crookes, Soddy, Rutherford and Grier, Debierne, Becquerel). For instance, barium chloride is added to a solution of uranyl nitrate, and the barium is then precipitated by adding a little sulphuric acid. The precipitated barium sulphate, when separated and dried, is radioactive. It has carried with it a part of the activity of the uranium, for the uranium salt, when the solution is evaporated to dryness, is less active than before it was submitted to this operation. But after several months the sulphate of barium loses its radioactivity, while the uranium salt has regained its original properties. It is evident that the barium was excited by contact with the uranium, or that it occluded in some special form a part of the activity of the uranium (uranium X of Crookes).
Rutherford and Soddy showed that if nitrate of thorium is precipitated by ammonia, the precipitated oxide of thorium is less active than ordinary thorium. The liquid from which it was precipitated is radioactive, and on evaporating it to dryness the small residue is 2,500 times more active than the thoria (they call the radioactive substance in this residue thorium X). After several weeks the residue has lost its activity, the thorium X has disappeared, and the thoria which was precipitated has, on the contrary, regained its normal activity. Further, while the thorium X existed it emitted the thorium emanation in abundance.
Rutherford and Soddy think that uranium X and thorium X are simply intermediate products of the breaking down of uranium and thorium. Thorium, for example, produces thorium X continuously, which breaks down into the emanation of thorium, that is in its turn transformed into induced activity.
Conductivity of Atmospheric Air. Emanation and Induced Radioactivity at the Surface of the Earth—Elster and Geitel, and also Wilson, showed that atmospheric air conducts electricity to a slight extent, that it is always slightly ionized. This ionization seems to be due to various causes. According to the work of Elster and Geitel, atmospheric air always contains a small proportion of an emanation analogous to that emitted by radioactive substances. Metallic wires suspended in the air and kept at a high negative potential become active under the influence of this emanation. At the summits of high mountains the atmosphere contains more of the emanation than on plains or at the level of the sea. The air of caves and caverns is especially highly charged with the emanation. Air rich in the emanation can also be obtained by aspirating, by means of a tube sunk in the earth the air which is contained in it. The air extracted from certain mineral waters contains the emanation, while the air dissolved in the water of the sea and of the rivers is almost free from it.
The conductivity of the atmosphere is also probably due in part to very penetrating radiations that traverse space, and of which the origin is unknown. Finally, it is probable that all bodies are slightly radioactive, and that those at the surface of the ground render the air around them a conductor of electricity.
Time-Constants that Characterize the Disappearance of Emanations and of Induced Radioactivity.—We have seen that the radioactive emanations and the induced radioactivity of solids disappear spontaneously, and that the law of their disappearance is, in general, a simple exponential law. The intensity of the radiation,