﻿Thorium and Radium Emanations, 343 



internal diameter, were made, and into one was introduced, 

 with the help of liquid air to get rid of uncondensed gases 

 in the usual way, 14 millicuries of emanation, while in the 

 other 0*07 millicurie was placed. It was estimated that 

 the glass walls of the tube cut down the ft rays to 31 per 

 cent, of the total. The tubes were sealed at a capillary 

 provided for that purpose, when only one bubble was obtained 

 by each stroke of the Topler pump; the sealed end will be 

 referred to as the " liquid air end," while the round end 

 will be spoken of as the "experimental end." 



Method of Experiment. — Whenever the tubes were not in 

 use, the liquid air end always stood in a depth of at least 

 5 cm. of liquid air, so that any time after three hours one 

 could assume that there was no activity at the experimental 

 end. When a determination was to be made, the tubes were 

 withdrawn from the liquid air and the experimental ends 

 quickly placed 5 cm. deep in a bath of pentane which had 

 been previously cooled to some desired low temperature. 

 After an exposure of 40 minutes, during which the tem- 

 perature was kept constant, they were withdrawn and placed 

 alternately under the electroscope for about 20 minutes, 

 while measurements of activity were made, in a standard 

 position in which all the tube, with the exception of 10 cm. 

 of the experimental end, was screened by the heavy lead 

 cylinder. The liquid-air ends were then replaced for at 

 least three hours in liquid air before another experiment at 

 a different temperature was made. Experiments for 40 

 minutes immersion of the experimental ends at liquid-air 

 and room temperatures were also made. The weak tube 

 was measured on the /^-ray electroscope and the strong tube 

 initially on the small 7-ray electroscope. 



A series of such determinations for these two tubes at 

 different temperatures were simultaneously made, and then 

 the strong tube was laid aside for 10 days. Another set of 

 experiments were made with it alone, measurements being- 

 made this time on the large 7-ray electroscope, after which it 

 was laid aside for the same period, and then redetermined. 

 This time the /3-ray electroscope was used for measurements. 



It was proved in the first place that all the emanation was 

 condensed by liquid air by removing the liquid-air end from 

 liquid air and immediately measuring the activity of the 

 experimental end, when it was found that after extrapolation 

 backwards, the activity at the instant of withdrawal was 

 only 0*3 d.p.m., while after 40 minutes at the temperature 

 of the room the activity was 18*8 d.p.m. 



The activities obtained for a low temperature experiment 



