724: Prof. E. Rutherford on Condensation 



Using large quantities of radium, Sir William Ramsay and 

 Cameron observed that the emanation, condensed in a glass 

 tube surrounded by liquid air, could be gradually removed by 

 continuous pumping, indicating that the emanation exerted a 

 sensible vapour-pressure even at that low temperature. The 

 writer has found that the rate of removal of the emanation 

 by pumping increases rapidly as the temperature of the 

 emanation-tube approaches the temperature of complete 

 volatilization of the emanation. 



The temperature of condensation, viz. — 150 c C, found 

 by Rutherford and Soddy corresponded to the liquefaction- 

 point of the emanation under a very low pressure. If the 

 emanation behaves like an ordinary gas, the temperature of 

 initial condensation should rise with increase of pressure 

 of the emanation. It was consequently of interest to 

 examine how the condensation-point of the emanation varied 

 with pressure, and to fix its boiling-point under atmospheric 

 pressure. 



Special experimental methods are necessary in order to 

 determine the vapour-pressure of the very small volume of 

 emanation available. It has been shown in the experiments 

 of the writer * that the volume of pure emanation from one 

 gram of radium in equilibrium is about 0'6 cubic mm. at 

 normal pressure and temperature. This is in good agreement 

 with the calculated value, viz. 0*585 cubic mm., which has 

 been deduced by Rutherford and Geiger f- In the present 

 experiments, the amount of emanation, available after the 

 process of purification, was equivalent to the equilibrium 

 amount from 140 mgs. of radium. Taking the calculated 

 volume of the emanation, this corresponds to a volume o£ 

 pure emanation of 0*082 cubic mm. 



In order to obtain a column of gas of several centimetres 

 length at atmospheric pressure, it was consequently necessary 

 to employ capillary tubes of fine bore. In the experiments 

 recorded later, glass capillary tubes were employed of diameter 

 varying between about 0*05 mm. and 0'15 mm. 



After purification of the emanation in the manner described 

 in a previous paper "f, the emanation was allowed to expand 

 into a vertical glass reservoir and then compressed by raising 

 the mercury into the capillary tube fixed at the top. This 

 tube of length nearly 20 cms. was bent twice at right angles, 

 so that the free end of length about 8 cms. was vertical and 

 dipped downwards. The end of the capillary, of external 



* Phil. Mag. Aug. 1908. 



f Proc. Roy. Soc. A.lxxxi.p. 162 (1908). 



+ Rutherford, Phil. Mag. Aug. 1908. 



