4 Mr. J. Satterly on tlie Amount of Radium 



measure another batch of emanation quickly afterwards, 

 owing to the smaller quantity of excited activity deposited on 

 the interior of the can, and consequently lesser time taken by 

 this to decay into insignificance. 



In all cases, therefore, my readings of the leak were made at 

 this first maximum. The leak due to the air from the second 

 aspirator added to the leak due to the air from the first 

 aspirator full, gives the total leak due to the emanation 

 collected from the charcoal. This is not, however, all col- 

 lected from the air, as the charcoal itself contains radium, 

 and if left to itself gradually accumulates radium-emanation. 

 From the total leak mentioned above must be subtracted the 

 leak due to the emanation produced from the charcoal itself, 

 and the remainder is a measure of the emanation absorbed 

 from the air by the charcoal during the exposure. 



The experiments in 1908, the results of which are given 

 in my former paper, were carried out with coconut-charcoal 

 contained in porcelain tubes. These tubes were 2 feet long, 

 and the central foot of the tube was filled with coarsely 

 powdered charcoal kept in place by asbestos wads. The cress 

 section of the tubes was 1*8 sq. cm., and a foot-length of 

 tube contained 39 grms. of charcoal. The amounts of ema- 

 nation absorbed were small, and to increase the accuracy of 

 the work it was decided to work with tubes of larger bore. 

 In the 1909 experiments opaque silica tubes were used of 

 cross section 8'0 sq. cm. ; a foot length of these tubes con- 

 tained 155 grms. of charcoal. The charcoal was kept in 

 place by spirals of wire gauze. The air-stream being kept 

 the same in the two series of experiments, a greater proportion 

 of the emanation in the air would be absorbed with the silica 

 tubes than with the porcelain tubes. 



In the 1908 experiments drying agents (calcium chloride 

 and strong sulphuric acid) were used to absorb the water 

 vapour contained in the atmospheric air drawn through the 

 tubes. The absorptive power of charcoal decreases as the 

 charcoal gets wet. Another reason for drying the air was 

 that if the air was not dried the charcoal absorbed the water 

 from the air ; and when the porcelain tube was heated the 

 water was given off and condensed on the cold portions of 

 the tube beyond the furnace, often causing the tube to crack. 

 There is, however, an objection to drying- agents in that they 

 themselves may give off or absorb radium emanation. In the 

 present series of experiments no drying-agents were used. 

 Silica tubes will not crack under the conditions mentioned 

 above. Also, with such a large quantity of charcoal the water 

 caught will be lodged in the first portions of the charcoal 

 column which the air meets, and there will be plenty of dry 



