been studied so completely as the better known emanations of thorium and radium.
Experiments with large amounts of Radium Emanation.
147. With very active specimens of radium a large amount of emanation can be obtained, and the electrical, photographic, and fluorescent effects are correspondingly intense. On account of the small activity of thorium and the rapid decay of its emanation the effects due to it are weak, and can be studied only for a few minutes after its production. The emanation from radium, on the other hand, in consequence of the slow decay of its activity, may be stored mixed with air in an ordinary gas-holder, and its photographic and electrical actions may be examined several days or even weeks after, quite apart from those of the radium from which it was obtained.
It is, in general, difficult to study the radiation due to the emanation alone, on account of the fact that the emanation is continually producing a secondary type of activity on the surface of the vessel in which the emanation is enclosed. This excited activity reaches a maximum value several hours after the introduction of the emanation, and, as long as it is kept in the vessel, this excited activity on the walls decays at the same rate as the emanation itself, i.e. it falls to half its initial value in about 4 days. If, however, the emanation is blown out, the excited activity remains behind on the surface, but rapidly loses its activity in the course of a few hours. After several hours the intensity of the residual radiation is very small.
These effects and their connection with the emanation are discussed more fully in chapter VIII. Giesel11 has recorded some interesting observations of the effect of the radium emanation on a screen of phosphorescent zinc sulphide. When a few centigrams of moist radium bromide were placed on a screen any slight motion of the air caused the luminosity to move to and fro on the screen. The direction of phosphorescence could be altered at will by a slow current of air. The effect was still further increased by placing the active material in a tube and blowing the air through it towards the screen. A screen of barium platinocyanide or of Balmain’s paint failed to give any visible light under the same conditions. The luminosity was not altered by a magnetic field, but it was affected by an electric field. If the screen were charged the luminosity was more marked when it was negative than when it was positive.
Giesel states that the luminosity was not equally distributed, but was concentrated in a peculiar ring-shaped manner over the surface of the screen. The concentration of luminosity on the negative, rather than on the positive, electrode is probably due to the excited activity, caused by the emanation, and not to the emanation itself, for this excited activity is