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Mr. A. Fleck on the Condensation of 



appear to be more easily condensed than the radium. The 

 above results are therefore not a proof that the two 

 emanations have dissimilar properties of condensation. 



Experiments at Low Pressures with Radium Emanation. 



It was then decided to experiment on the emanations in 

 Hosed tubes to avoid sweeping away the vapour phase. The 

 main idea of the method was to have the emanations 

 enclosed in a tube at very low pressure and to assume that 

 a quantity of gas would diffuse throughout the tube instan- 

 taneously. This assumption was proved to be correct by 

 experiments that will be described later. 



As no work on the condensation of the emanations at low 

 pressures has previously been done, it was thought advisable 

 to work, in the first place, with the single emanations. 



Apparatus. — For this experiment, the apparatus and 

 position of electroscopes is shown in figure 3. A lead 



Fig-. 3. 



Section 

 throu.gk M 



* 



cylinder, M, 44 cm. long and 11 cm. diameter, with a hole 

 2 cm. diameter bored along its axis was used, and it had on 

 top three lead bars of length equal to that of the lead 

 cylinder and each of cross-sectional area of 25 sq. cm. On 

 the right-hand side was a large sensitive 7-ray electroscope, 

 A, while on the left-hand side were (1) the very sensitive 

 /3-ray electroscope, B, described by Soddy * in a recent 

 paper, and (2) the small 7-ray electroscope, C. The electro- 

 scopes were arranged to be slightly above the axis of the 

 lead cylinder, so that if a radioactive tube was in its centre, 

 no rays except those from any portion of the tube purposely 

 exposed could reach an electroscope without passing through 

 considerable thicknesses of lead. The object of this arrange- 

 ment was that experiments could be started with a large 

 quantity of radium emanation, using the small 7-ray electro- 

 scope for measurements, and that w r hen the emanation 

 decayed to such an extent that the effect on this electro- 

 scope was too small, measurements could be continued on 

 the large 7 and finally on the /3-ray electroscope. 



Two thin-walled glass tubes, 60 cm. long and 1*5 cm. 

 * Soddy, Phil. Mag. [6] xxvii. p. 215 (1914). 



