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