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 decay of the "excited activity" produced by the emanation on the surface of all bodies exposed to its action (chapter ). 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.

An apparatus of a similar character to illustrate the condensation of the radium emanation has been described by P. Curie.

Fig. 59.

167. Determination of the temperature of condensation. A detailed investigation was made by Rutherford and Soddy (loc. cit.) of the temperatures at which condensation and volatilization commenced for the two emanations. The experimental arrangement of the first method is shown clearly in Fig. 59. A slow constant stream of gas, entering at A, was passed through a copper spiral S, over 3 metres in length, immersed in a bath of liquid ethylene. The copper spiral was made to act as its own thermometer by determining its electrical resistance. The