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



emanation in the air varies between rather wide limits from 

 time to time and notably with atmospheric conditions. The 

 high value obtained in my fourth experiment is without doubt 

 due to the fact that that determination was made immediately 

 after a heavy rain and general thaw, following several weeks 

 of freezing weather with an unusual covering of snow on the 

 ground. Probably the accumulated emanation was suddenly 

 liberated by the rapid melting of the snow and consequent 

 softening of the earth's crust. It. is of interest to note that 

 experiment (5) was made during a prevailing high barometric 

 pressure, and experiment (6) during a low pressure. The dif- 

 ference in the barometric readings on the two dates amounted to 

 20 mm of mercury. The other weather conditions were normal. 

 A complete study was made of the activity curve of the ema- 

 nation collected in experiment (4), which proved it to be iden- 

 tical with that of the radium emanation. The activity reached 

 the maximum value in about three hours after introduction 

 into the electroscope and decreased in the usual way to half 

 value in about 3*5 days. This showed that the active material 

 consisted only of radium emanation. The conditions of the 

 experiment eliminated the possibility of introducing into the 

 electroscope any thorium emanation which may have been con- 

 densed in the copper coil, since only a few minutes are required 

 for thorium emanation to decay to a very small fraction of its 

 initial value. The following table indicates the course of the 

 activity curve of the emanation in experiment (4). The differ- 

 ent times are placed in column (1), and in column (2) are the 

 corresponding activities in scale divisions of the graduated eye- 

 piece passed over by the gold-leaf per minute. The activities 

 are corrected for the natural leak of the electroscope. 



(1) (2) 



11 min. -761 



60 " _. '852 



126 " -963 



180 " 1-100 max. activity. 



19 hours -927 



2 days -703 



2-8 " -604 



3-3 " ._. _ _ '546 



3-82 " . ._ ;__ -490 



Conclusions. 

 The results of these experiments show that \>j cooling atmos- 

 pheric air to the temperature of liquid air the radium emana- 

 tion in it can be completely condensed and its amount accu- 

 rately determined. Six measurements made at Chicago showed 

 that the average amount of radium emanation per cubic meter 

 of air could be maintained by l'OXlO -10 gram of radium. 

 Kent Chemical Laboratory, University of Chicago, May 20, 1908. 



