72 G Prof*. E. Rutherford on Condensation 



gases rose with increase of pressure of the emanation. If the 

 emanation started condensing at a particular temperature, 

 lowering of the pressure at once caused a rapid volatilization 

 of the condensed emanation. 



A number of experiments were made of the initial con- 

 densation-point of the emanation at atmospheric pressure. 

 This was found to be about — 65° C, the temperature of the 

 pentane bath being measured, both by the thermo-couple and 

 u pentane thermometer, and the pressure being kept constant. 

 This fixes the true boiling-point of the emanation as —65° CL 

 or 208° absolute. Since the emanation gives out heat, the 

 temperature of the inside of the capillary was no doubt slightly 

 higher than that of the pentane bath. The experiments are 

 not, however, of sufficient precision to introduce small cor- 

 rections of this kind. 



A number of experiments were made on the vapour-pressure 

 of the emanation, using a paste of solid carbon dioxide and 

 ether to give a constant temperature ( — 78°'2 C). As it was 

 difficult to view the end of the capillary through the opaque 

 paste, it was found convenient to remove rapidly the refri- 

 gerant, and observe at the moment of removal whether the 

 emanation was condensed in the tube. It was found in this 

 way possible to fix the point of initial condensation with con- 

 siderable accuracy. An increase of one per cent, in the 

 pressure was sufficient to cause a transition from no conden- 

 sation to well-marked condensation. 



The mean of experiments showed that the emanation com- 

 menced to condense at — 78°*2 at a pressure of 23 cms. 

 It was found that the amount of impurity with the ema- 

 nation had no influence on its condensation-point when the 

 true partial pressure of the emanation was deduced in 

 the manner previously discussed. An interesting effect was 

 noticed in these experiments. On removal of the refrigerant, 

 occasionally some of the paste adhered to the capillary. The 

 rapid evaporation of this caused a local lowering of the tem- 

 perature sufficient to cause a marked condensation of the 

 emanation at points on the tube, even though no condensation 

 was observed in the tube when in the bath itself. 



The following table gives the vapour-pressure of the 

 emanation at various temperatures. 



our-Pressme. 



Temperature 



76 cms. 



- 65° C. 



25 „ 



- 78° 



5 „ 



-101° 



•9 „ 



-127° 



