238 RADIO-ACTIVE EMANATIONS [CH. 
osity 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 several weeks interval. 
An apparatus of a similar character to illustrate the condensa- 
tion of the radium emanation has been described by P. Curie!. 
158. Determination of the temperature of condensa- 
tion. <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 experi- 
mental arrangement of the first method is shown clearly in Fig. 45. _ 
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 
resistance temperature curve was obtained by observation of the 
resistances at 0°, the boiling poimt of liquid ethylene — 103°5°, 
the solidification point of ethylene — 169° and in liquid air. The 
temperature of the liquid air was deduced from the tables given 
by Baly for the boiling point of liquid air for different percentages 
of oxygen. The resistance temperature curve, for the particular 
spiral employed, was found to be nearly a straight line between 
1 Société de Physique, 1903. 
