236 RADIO-ACTIVE EMANATIONS [CH. 
Condensation of the Hmanations. 
156. Condensation of the emanations. During an in- 
vestigation of the effect of physical and chemical agencies on 
the thorium emanation, Rutherford and Soddy! found that the 
emanation. passed unchanged im amount through a white-hot 
platinum tube and through a tube cooled to the temperature 
of solid carbon dioxide. In later experiments, the effects of still 
lower temperatures were examined, and it was then found that at 
the temperature of liquid air both emanations were condensed’. 
If either emanation is conveyed by a slow stream of hydrogen, 
oxygen or air through a metal spiral immersed in liquid air, and 
placed in connection with a testing vessel as in Fig. 37, no trace 
of emanation escapes in the issuing gas. When the liquid air is 
removed and the spiral plunged into cotton-wool, several minutes 
elapse before any deflection of the electrometer needle 1s observed, 
and then the condensed emanation volatilizes rapidly, and the 
movement of the electrometer needle is very sudden, especially 
in the case of radium. With a fairly large amount of radium 
emanation, under the conditions mentioned, a very few seconds 
elapse after the first sign of movement before the electrometer 
needle indicates a deflection of several hundred divisions per 
second. It is not necessary in either case that the emanating 
compound should be retained in the gas stream. After the 
emanation is condensed in the spiral, the thorium or radium 
compound may be removed and the gas stream sent directly — 
into the spiral. But in the case of thorium under these condi- 
tions, the effects observed are naturally small owing to the rapid 
loss of the activity of the emanation with 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 tempera- 
ture allowed to rise, the glow diffuses uniformly throughout the 
1 Phil, Mag. Nov. 1902. 
2 Phil. Mag. May 19038. 
