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LIIL The Rate of Transformation of the Radium Emanation. 

 1 By G. Rumelin, 'Ph.D.* 



HPHE radium emanation, like all the radioactive products, 

 JL is transformed according to an exponential law. The 

 " period ; " of the emanation, or the time required for the 

 emanation to be half transformed, is an important physical 

 constant, an accurate knowledge of which is required in many 

 experiments. Although observations of its period have been 

 made by a number of investigators, the results obtained have 

 not been very concordant. The period of the radium emana- 

 tion has been determined by Curie |, Rutherford and Soddy J, 

 Bumstead and Wheeler §, and Sackur || ; the results obtained 

 by these observers are given below : — 



Curie 3'99 days 



Rutherford and Soddy ... 3*77 „ 

 Bumstead and Wheeler ... 3'88 „ 



Sackur 3'86 „ 



Two different methods were used for these determinations. 

 Curie, .and Bumstead and Wheeler measured the decay of a 

 quantity of emanation in a closed testing-vessel ; Rutherford 

 and Soddy, as well as Sackur, transferred after different 

 intervals of time into a testing apparatus known volumes of 

 air containing emanation from a large supply which was kept 

 in a gasometer. Since for accurate determinations the 

 measurements have to be extended over several weeks, the 

 first method requires measurements of ionization currents 

 over a wide range of intensity. This is a disadvantage, 

 because the rate of movement of the electrometer-needle, 

 unless very slow, is not accurately proportional to the current, 

 but generally increases more slowly than the latter. The 

 period of the emanation, for this reason, will be found too 

 large. In the second method, this source of error can be 

 eliminated by putting smaller volumes of emanation-air into 

 the testing vessel at the beginning of the experiment than at 

 the end. The drawback of the second method is the liability 

 of escape of emanation, and also of disturbances of the homo- 

 geneous distribution during the process of transference of 

 the emanation into the testing-vessel. 



In the following experiments I endeavoured to get 

 measurements of the period of the radium emanation un- 

 affected by both these sources of error. 



* Communicated by Prof. E, Rutherford, F.R.S. 



t Comptes Rendus, cxxxv. p. 857 (1902). 



X Phil. Mag. April 1903. 



§ Amer. Journ. Science, Feb. 1904. 



I! Ber. d, d. chem. Ges. xxxviii. p. 1754 (1905). 



