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



The entire coil occupied 75 cc of space and could be easily 

 kept submerged in liquid air in a small vessel. Two hundred 

 liters of dry air, free from carbon dioxide, could be drawn 

 through the coil at a moderate rate in six hours. At the end 

 of that time the coil was allowed to heat up and the volatil- 

 ized emanation was transferred to a standardized gas electro- 

 scope. This was made of an air-tight brass cylinder of about 

 one liter capacity, supporting a gold-leaf system. This elec- 

 troscope has been in use 'by Professor McCoy for more than 

 two years and has a very small, almost constant natural leak of 

 about 0*148 divisions per minute (whole scale 100 divisions), 

 when exhausted and refilled with fresh dry air. The electro- 

 scope was standardized by the method described by McCoy 

 and Ross,* which consisted in observing the rate of discharge 

 caused by the emanation from a portion of a mineral contain- 

 ing a known amount of uranium. This activity observation 

 was made just 3J hours after the separation of the emana- 

 tion from the mineral and 3 hours after its introduction into 

 the electroscope The amount of radium associated with one 

 gram of uranium in a mineral was taken as 3'4XlO~ 7 .t 



The results of four experiments calculated on the basis of 

 the amount of radium necessary to maintain the emanation 

 constant in one cubic meter of air are as follows : 



(1) 86 X ICr 12 gram radium. 



(2) 58X10- 12 " 



(3) 45X10- 12 " 



(4) 200 xicr 12 " " 



These results seem to show a considerable variation in the 

 amount of emanation in the air at different times ; similar 

 variations were found by Eve both by the excited activity 

 method as well as by that in which the emanation was sepa- 

 rated by means of charcoal. It was possible, however, that the 

 variations in my results were due to incomplete condensation 

 of the emanation. To decide this question, a second coil of 

 tubing exactly like the one described above was joined to the 

 first ; both coils were immersed in liquid air, and purified out- 

 side air was run through the two coils in series at the same 

 rate as in the four experiments above described. The emana- 

 tion in the first coil corresponded to 51X10" 12 gram of radium 

 per cubic meter. The second coil did not contain a trace of 

 emanation. This experiment clearly proved that the first coil 

 condensed all the emanation in the air that passed through it, 

 and showed conclusively that the observed variations were really 

 due to variations in the amount of emanation in the air at dif- 

 ferent times. Such being the case, simultaneous duplicate 

 determinations of the amount of radium emanation in the air 



* J. Am. Ch. Soc.,xxix, 1700, 1907. f Boltwood, this Jour. . xxv, 296,1908. 



