570 Prof. Rutherford and Mr. Soddy on 



of the taps and of the mercury-pump were all timed through- 

 out by a stop-watch, aud the observations with the electro- 

 meter were always taken at the same interval after the 

 commencement of the operations. In this way the decay of 

 activity of the emanation was the same in each experiment, 

 and the results obtained in different experiments comparable 

 with one another. One observer took charge of the manipu- 

 lation of the apparatus, the other recorded the lapse of time, 

 the temperature of the spiral at the instant the contents were 

 drawn into the pump by opening the tap F, and the readings 

 of the electrometer after the emanation had been sent into 

 the testing-vessel. The latter could be taken within 90 

 seconds from the time the emanation was removed from the 

 thorium in those cases where it remained in the spiral for a 

 period of 30 seconds. The tap F was a three-way, so that 

 the pump could be cut out of the circuit and experiments 

 in a steady stream of gas carried on without alteration of the 

 apparatus. The amounts of the emanation that remain un- 

 condensed at different temperatures are shown graphically in 

 fig. 6, PL XIV. The different curves represent different 

 series of observations with hydrogen and oxygen respectively, 

 in which the time during which the emanation remained in the 

 spiral was in some cases 30 seconds, in others 90 seconds. 

 Curves A and B illustrate the difference in the condensation- 

 curves for hydrogen and oxygen under similar conditions. 

 The pressure in the spiral after the removal of the uncon- 

 densed emanation corresponded to 19 mm. of mercury. The 

 curves show that a greater proportion of the emanation is 

 condensed for the same temperature in hydrogen than in 

 oxygen. The curves C and D, which were obtained under 

 different conditions of pressure and amount of emanation from 

 curves A and B, show that a greater proportion is condensed 

 in 90 seconds than in 30 seconds. The proportion condensed 

 in a steady stream is less than in any of the experiments by 

 the static method. In all cases, however, condensation com- 

 mences at about the same temperature, viz., —120° C, and 

 there is no doubt that this must be taken as the real con- 

 densation-point of the thorium emanation, and that the 

 identity in the temperatures observed in the earlier experi- 

 ments with a steady stream must be regarded as purely 

 accidental. 



Experiments with the Radium Emanation. 



The apparatus (fig. 4, PI. XIV.) was slightly altered for the 

 determination of the volatilization-point of the radium ema- 

 nation. The thorium tube was replaced by a drying-tube of 



