Condensation of the Radium Emanation. 723 



they confirm 3*80 as the most probable value for the equili- 

 brium constant of water vapour, hydrogen, and oxygen. 



An attempt was made to determine the lowest temperature 

 at which water vapour could be decomposed by a hot platinum 

 wire. 



For this purpose, the taps B and C were both opened and 

 a steady stream of water vapour was drawn through the 

 bulb A while the temperature of the wire was gradually 

 raised. The wire was first heated to 770°, and then raised 

 about 25° C, each half hour. No oxygen or hydrogen was 

 collected by the pump until the temperature was between 

 1023° and 1043°, when very small quantities of electrolytic 

 gas (containing an excess of hydrogen) were obtained. 



At this temperature the percentage decomposition is only 

 about 28 x TO -6 per cent., so the greater quantity of the 

 electrolytic gas produced would dissolve in the condensed 

 water. 



The fact that it was found possible to collect gas at so low 

 a temperature as 1023° shows that the decomposition of the 

 water vapour must begin at a considerably lower temperature, 

 probably a few degrees above that at which oxygen and 

 hydrogen would combine under similar conditions. 



The University, Manchester. 





LXVII. Condensation of the Radium Emanation. By 

 E. Rutherford, F.R.S., Professor of Physics, University 

 of Manchester*. 



RUTHERFORD and SODDY first showed in 1903 that 

 the radium emanation condensed from the gases with 

 Avhich it was mixed at a temperature of about —150° C. At 

 that time only small quantities of radium preparations were 

 available, so that the partial pressure of the emanation with 

 the gases with which it was conveyed was exceedingly small. 

 Notwithstanding the very minute quantity of emanation 

 present, the temperatures of complete condensation and of 

 complete volatilization were found to be sharply marked, 

 and did not differ from each other by more than a few 

 degrees. Some evidence was obtained that the emanation 

 had a vapour-pressure like an ordinary gas. 



This property of condensation of the emanation in liquid 

 air has proved invaluable in all later researches as a means of 

 separating the emanation from the inactive gases with which 

 it is mixed. 



* Communicated by the Author. A preliminary account of the results 

 was communicated as a letter to ' Nature,' Feb. 18, 1909. 



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