572 Prof. Rutherford and Mr. Soddy on 



resistance of the copper spiral, and remained so until the 

 latter began to be no longer completely covered. The nitric 

 oxide was not sufficiently pure to enable much weight to be 

 put on this result as a determination of temperature, for from 

 its behaviour it obviously must have contained a considerable 

 quantity of dissolved nitrogen, but it is of interest as being 

 a completely independent check on the thermometer at 

 almost the exact point of condensation, and shows that the 

 value ascribed to the latter cannot be far from the truth. 

 Such a bath of boiling liquid rising very slowly in temperature 

 over the exact range in which volatilization takes place, 

 afforded a means, however, of examining more exactly the 

 progress of the volatilization after the initial point was 

 reached. The Utter occurred in this case at —155° C, a 

 steady current of air being maintained through the spiral. In 

 four minutes the temperature had increased to — 153°*5, and 

 the amount volatilized was about four times as great as at 

 — 153°. In the next 5J minutes the temperature had in- 

 creased to 152°'3, and the whole amount practically had 

 volatilized, which was at least fifty times the amount at the 

 temperature of — 153°'5. Such a result would of course 

 be explained by slight local inequalities in the temperature of 

 the spiral, but since the latter was immersed in a rapidly 

 boiling liquid it is difficult to believe that such could have 

 been the case. It seems more reasonable to attribute it to a 

 true vapour-pressure possessed by the condensed emanation, 

 although great refinement in the experimental methods would 

 be necessary before such a conclusion could be considered 

 established. 



The Explanation of the Anomalous Behaviour of the Thorium 

 Emanations. 

 The results obtained are satisfactory in so far as they show 

 that the two emanations do not condense at the same tempera- 

 ture. The anomalous behaviour of the thorium emanation in 

 condensation, which in the first experiments seemed to indicate 

 that the two emanations condensed at the same temperature, 

 has been shown to be due to an effect, not present in the case 

 of the radium emanation, which depends on the nature of the 

 gas, the concentration of the emanation, and the time that 

 the latter has been left to condense. It remains to develop 

 a view w T hich gives a satisfactory explanation of this be- 

 haviour. In the first place the actual number of particles 

 of emanation that are present must be almost infinitesimally 

 small compared with the number of particles of the gas with 

 which they are mixed. It is difficult to make an accurate 



