of the Radium Emanation. 725 



diameter about 1 mm., was immersed in liquid pentane con- 

 tained in a small unsilvered Dewar cylinder. The tempe- 

 rature of the bath could be varied by circulating liquid air 

 through a glass U-tube placed in the liquid. The temperature 

 of the bath, which was kept well stirred, was determined by 

 means of a nickel-iron thermo- junction in series with a 

 D' Arson val galvanometer. The deflexions on the scale were 

 calibrated by keeping one junction in a freezing-mixture and 

 immersing the other successively in (1) a paste of solid 

 carbon-dioxide and ether ( — 78°*2), (2) boiling ethylene 

 (— -103°*5), and (3) liquid air whose percentage of oxygen 

 was determined. 



As the complete purification of the emanation is a long 

 and tedious process, and it is difficult to keep it pure over a 

 wide range of observations, many of the experiments were 

 made with emanation of about 50 to 60 per cent, purity. The 

 true volume of the emanation present was determined by 

 comparing its 7 ray activity by means of an electroscope with 

 that of a standard radium preparation, assuming that the true 

 volume of the emanation from one gram of radium is 0'585 

 cubic mm. The actual volume occupied by the emanation 

 and impurities was measured in the capillary at atmospheric 

 pressure. By comparison of the calculated with the ob- 

 served volume, the percentage of impurity was determined 

 and also the correction to be applied to the observed pressure 

 to give the true partial pressure of the emanation. 



In the experiments on the condensation-point of the 

 emanation for pressures above 5 cms., a capillary tube of 

 mean diameter *05 mm. was used. The cross-section of the 

 capillary was found to be elliptical in shape, the axes being 

 048 mm. and *052 mm. respectively. In this capillary the 

 emanation from 100 mgrs. of radium would occupy a length 

 of 3*0 cms. It was found necessary to use such a fine bore 

 in order to be able to cool down the end of the capillary to 

 the temperature of the pentane bath and yet to keep the 

 mercury column from freezing. 



The point of condensation was found in most cases to be 

 well defined. At the moment of condensation a brilliant 

 phosphorescent point of light due to condensed emanation 

 appeared at the extreme end of the capillary. The tempe- 

 rature of the pentane bath was noted at the moment the 

 phosphorescent point disappeared. This corresponded to the 

 temperature of initial condensation at that particular pressure 

 of the emanation. 



Preliminary experiments made in this way showed that the 

 condensation temperature of the emanation like that of all 



