VIII] RADIO-ACTIVE EMANATIONS 237 
tube, and can be concentrated at any point to some extent by 
local cooling of the tube with hquid air. 
157. Experimental arrangements. A simple experimental 
arrangement to illustrate the condensation and volatilization of the 
emanation and some of its charac- 
teristic properties is shown in Fig. 
44. The emanation obtained by 
solution or heating, from a few milli- 
grams of radium bromide, is con- 
densed in the glass U tube T im- 
mersed 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 1s ee 
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 7’ 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 7’ rises 
above the point of volatilization of the emanation. The emana- 
tion is then rapidly carried into the vessel V, partly by expansion 
of the gas in the tube 7 with rismg temperature, and partly by 
the process of diffusion. The screen Z and the willemite are 
caused to phosphoresce brilliantly under the influence of the rays 
from the emanation surrounding them. 
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 etfect of the rays from 
the emanation condensed around it. At the same time the lumin- 

