The value of K found by Stefan[[260]] for the diffusion of carbon dioxide into water was 1·36 cm.2/day. These results are thus in harmony with the conclusion drawn from the diffusion of the radium emanation into air, and show that the radium emanation behaves as a gas of high molecular weight.
Condensation of the Emanations.
165. Condensation of the emanations. During an investigation of the effect of physical and chemical agencies on the thorium emanation, Rutherford and Soddy[[261]] found that the emanation passed unchanged in amount through a white-hot platinum tube and through a tube cooled to the temperature of solid carbon dioxide. In later experiments the effects of still lower temperatures were examined, and it was then found that at the temperature of liquid air both emanations were condensed[[262]].
If either emanation is conveyed by a slow stream of hydrogen, oxygen, or air through a metal spiral immersed in liquid air, and placed in connection with a testing vessel as in Fig. 51, no trace of emanation escapes in the issuing gas. When the liquid air is removed and the spiral plunged into cotton-wool, several minutes elapse before any deflection of the electrometer needle is observed, and then the condensed emanation volatilizes rapidly, and the movement of the electrometer needle is very sudden, especially in the case of radium. With a fairly large amount of radium emanation, under the conditions mentioned, a very few seconds elapse after the first sign of movement before the electrometer needle indicates a deflection of several hundred divisions per second. It is not necessary in either case that the emanating compound should be retained in the gas stream. After the emanation is condensed in the spiral, the thorium or radium compound may be removed and the gas stream sent directly into the spiral. But in the case of thorium, under these conditions, the effects observed are naturally small owing to the rapid loss of the activity of the emanation with time, which proceeds at the same rate at the temperature of liquid air as at ordinary temperatures.
If a large amount of radium emanation is condensed in a glass U tube, the progress of the condensation can be followed by the eye, by means of the phosphorescence which the radiations excite in the glass. If the ends of the tube are sealed and the temperature allowed to rise, the glow diffuses uniformly throughout the tube, and can be concentrated at any point to some extent by local cooling of the tube with liquid air.
166. Experimental arrangements. A simple experimental arrangement to illustrate the condensation and volatilization of the emanation and some of its characteristic properties is shown in [Fig. 58]. The emanation obtained from a few milligrams of radium bromide by solution or heating is condensed in the glass U tube T immersed in liquid air. This U tube is then put into connection with a larger glass tube V, in the upper part of which is placed a piece of zinc sulphide screen Z, and in the lower part of the tube a piece of the mineral willemite. The stop-cock A is closed and the U tube and the vessel V are partially exhausted by a pump through the stop-cock B. This lowering of the pressure causes a more rapid diffusion of the emanation when released. The emanation does not escape if the tube T is kept immersed in liquid air. The stop-cock B is then closed, and the liquid air removed. No luminosity of the screen or the willemite in the tube V is observed for several minutes, until the temperature of T rises above the point of volatilization of the emanation. The emanation is then rapidly carried into the vessel V, partly by expansion of the gas in the tube T with rising temperature, and partly by the process of diffusion. The screen Z and the willemite W are caused to phosphoresce brilliantly under the influence of the rays from the emanation surrounding them.
Fig. 58.
If the end of the vessel V is then plunged into liquid air, the emanation is again condensed in the lower end of the tube, and the willemite phosphoresces much more brightly than before. This is not due to an increase of the phosphorescence of willemite at the temperature of the liquid air, but to the effect of the rays from the emanation condensed around it. At the same time the luminosity of the zinc sulphide gradually diminishes, and practically disappears after several hours if the end of the tube is kept in the liquid air. If the tube is removed from the liquid air, the emanation again volatilizes and lights up the screen Z. The luminosity of the willemite returns to its original value after the lapse of several hours. This slow change of the luminosity of the zinc sulphide screen and of the willemite is due to the gradual decay of the “excited activity” produced by the emanation on the surface of all bodies exposed to its action ([chapter VIII]). The luminosity of the screen is thus due partly to the radiation from the emanation and partly to the excited radiation caused by it. As soon as the emanation is removed from the upper to the lower part of the tube, the “excited” radiation gradually diminishes in the upper and increases in the lower part of the tube.
The luminosity of the screen gradually diminishes with the time as the enclosed emanation loses its activity, but is still appreciable after an interval of several weeks.