562 Prof. Rutherford and Mr. Soddy on 



dioxide. The acquisition of a liquid-air machine by the 

 laboratory has enabled us to investigate the effect of lower 

 temperature on the emanations from both thorium and 

 radium. The result has been to show that both emanations 

 condense at the temperature of liquid air, and possess sharply 

 defined points of volatilization and condensation. 



If either emanation is conveyed by a slow stream of 

 hydrogen, oxygen, or air through a metal spiral immersed 

 in liquid air, 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 deflexion 

 of the electrometer-needle is observed, and then the condensed 

 emanation volatilizes as a whole, 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 deflexion of several hundred divisions of 

 the scale per second. It is not necessary in either case that 

 the emanating compound itself 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 conditions the effects observed are natu- 

 rally small, owing to the very rapid loss of activity of the 

 emanation with time, which experiment showed occurs at the 

 same rate at the temperature of liquid air as at ordinary 

 temperatures. As a matter of fact, in the case of radium the 

 salt itself was seldom used. It was convenient to obtain the 

 emanation from the solution and store it mixed with air in 

 small gas-holders, the loss of activity during the course of a 

 day's experiments being only a small part of the whole. 



If the radium emanation is condensed in a glass U-tube, 

 the progress of the condensation can be followed by the eye 

 by means of the fluorescence which the radiations excite in 

 the glass. With a sufficiently slow gas-stream the fluores- 

 cence is confined to the limb where the gas enters. If the 

 ends of the tube are sealed and the temperature allowed to 

 rise, the glow diffuses throughout the tube, and can be again 

 concentrated at any point to some extent by application of a 

 pad of cotton-wool soaked in liquid air. The U-tube can be 

 made to impress its own image on a photographic plate 

 through aluminium-foil, and the impression is uniformly 

 dense throughout the length of the tube. It retains its lumi- 

 nosity to a feeble extent after several days. 



The suddenness of the volatilization-point of the condensed 



