in the Atmosphere near the Eartlis Surface. 731 



near the earth's surface at Montreal has been determined 

 four times : — 



Grams of radium. 

 In 1906, by the active deposit method .... 82 x 10~ 12 

 In February and March 1907, with small 



tubes of charcoal (21 grams) 21 x 10~ 12 



In May 1907, with 660 grams of charcoal . 35 x 10 ~ 12 

 In July and August 1907, with 150 grams 



of charcoal 127 X 10~ 12 



These figures, whilst of the same order, are not in satis- 

 factory agreement. I have great confidence in the last large 

 result : every point has been carefully verified, and several 

 months of practice permit of a remarkable degree of accuracy 

 in the measurement of these small quantities — a degree of 

 accuracy which can only be realized by those who have done 

 practical work of this kind. It is, of course, possible that 

 the amount of emanation in the atmosphere may vary in 

 different seasons of the year. During July the weather was 

 moderately warm (65°-70° F.), with 8. and W. winds and 

 sun. Every day or two there were thunderstorms with rain. 

 The heavy rain may have forced emanation from the ground 

 in larger quantities than in winter and spring. The active 

 deposits on wires during July were not abnormal. The 

 ionization was not large ; for I found n-\- =125, n— = —830 

 on a hazy day, n+=100, n— =450 on a remarkably 

 clear day. It is my intention to continue to measure the 

 emanation in the atmosphere for some months to come, in 

 order to ascertain if there is a variation between summer and 

 winter values. 



A few notes are added which may save trouble to those 

 who are making experiments similar to these. The emanation 

 is given off from the charcoal in the latter part of the 

 heating ; so that it is necessary to heat thoroughly. With 

 emanation from 10~ 9 or 10 -10 gram of radium about 97 per 

 cent, of the total emanation absorbed by the charcoal may be 

 driven off by careful heating with two Bunsen burners. With 

 stronger solutions, of the order 10~ 7 gram, only #0 to 85 per 

 cent, can be driven off in this manner by the first heating. 

 Charcoal containing emanation can be partially de-emanated 

 by passing a strong current of air through it. A general 

 principle will be found to hold good in this case, as in most 

 others, that whatever is easily absorbed is easily extracted, 

 and conversely. 



I venture to recommend the last-described method for the 

 measurement of emanation in the atmosphere. It is well to 



