G. C. Ashman — Determination of Radium Emanation. 119 



Art. XIII. — A Quantitative Determination of the Radium 

 Emanation in the Atmosphere • by George C. Ashman. 



It was shown by Elster and Geitel* in 1902 that a nega- 

 tively charged wire exposed for a few hours in the air receives 

 a radio-active deposit similar in character to the quick-changing 

 radium products. The first attempt to measure the amount of 

 radio-active matter in the atmosphere was made by Evef in 

 1905 in Montreal. The method used consisted in collecting 

 the active deposit on a charged wire placed in a cylinder of 

 known volume. The results were apparently not very satis- 

 factory, since the estimated amount of pure radium necessary 

 to keep the emanation constant in one cubic meter of air 

 varied from 82X10" 12 to 287 XlO" 12 gram. The smaller value 

 was obtained from a relatively small cylinder out of doors, 

 the other from a large abandoned water tank indoors. The 

 same higher value was reached by both cylinders indoors. 

 Subsequent experiments by the same author;}; emphasized the 

 objections to the active deposit method.- The maximum values 

 were sixteen times the minimum, and furthermore this method 

 does not furnish direct proof of the presence of radium ema- 

 nation. Eve has recently§ described a rather complicated 

 new method of determining . directly the amount of radium 

 emanation in the atmosphere. This method depends upon 

 the discovery made by Rutherford that radium emanation 

 is readily absorbed by specially prepared cocoanut charcoal. 

 The maximum values obtained by the absorption method 

 were seven times the minimum, ranging from 18 XlO" 12 to 

 127 XlO -12 gram of radium for each cubic meter of air, the 

 probable average value being 80X10 -12 . The well-known 

 experiments of Rutherford and Soddy on the condensation of 

 radium emanation suggested the possibility of a quantitative 

 separation of the emanation from the atmosphere by means of 

 liquid air. The qualitative separation was indeed accomplished 

 in 1903 by Ebert.|| In my experiments, undertaken at the 

 suggestion of Professor H. JST. McCoy, air drawn from out of 

 doors at ground level was passed through a purification train 

 composed of KOH solution, H 2 S0 4 , CaCl 2 , and solid KOH, 

 and then through a coil of copper tubing immersed in liquid 

 air, and was finally collected in aspirators made of carboys of 

 known capacity. The coil was made of copper tubing with 

 an outside diameter of 3'2 inm and walls 0*5 mm in thickness. 

 This was wound concentrically about an axis in such a way as 

 to allow ample space between the turns so that every part of 

 the coil would be completely bathed in the cooling liquid. 



*Phvsik. Zeitschr., ii, 590, 1901. f Phil. Mag., x, 98, 1905. 



J Phil. Mag., xiv, 724, 1907. §Loc. cit. 



|| Sitzb. Akad. Wiss. Miinchen, xxxiii, 133, 1903. 



