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. [Illustration: Fig. 58.]