Radium Emanation in Atmosphere near Earth's Surface. 725 



the maximum is 16 times the minimum, and the quantity 

 depends in a subtle manner on a variety of meteorological 

 conditions. This difficulty was to a great extent overcome 

 by taking the average of a number of determinations. 



In the autumn of 1906 Professor Rutherford found that 

 charcoal, prepared from coconuts, entirely absorbed the 

 radioactive emanations, provided they were passed very 

 slowly through the charcoal. He was good enough to assign 

 to me the work of using his method in the determination of 

 the amount of emanation in the atmosphere. 



Air was therefore drawn through glass tubes, containing 

 charcoal, for various times and at various speeds. The tubes 

 were heated by Bunsen flames, and the gases were collected 

 over water and introduced into an electroscope, which had 

 previously been exhausted by a water-pump. 



Difficulties at once arose. In order to absorb the emana- 

 tion completely, it is necessary that the velocity of the air- 

 current through the charcoal should be small, certainly not 

 exceeding 1 c.c. per second. At this rate, it would take a 

 little more than 11 days to pass one cubic metre through the 

 charcoal ; and it will be remembered that radium emanation 

 decays to half-value in a little less than four days. It was 

 soon found that the proportion of emanation absorbed was a 

 function of the speed. The greater the speed, the less per- 

 centage of a given quantity of emanation was absorbed. It 

 was found best to run the air through the charcoal at a fair 

 speed, thus absorbing only a fraction of the emanation passing 

 through the charcoal. Air was then bubbled through standard 

 solutions of radium, carrying the resulting emanation through 

 the tubes containing charcoal. These were heated, and the 

 gases driven off were collected, and introduced into the electro- 

 scope as before. It will be noted that the method is a com- 

 parative one, and that the conditions are identical, except that 

 in one case there is the unknown amount of emanation in the 

 atmosphere, and in the other case the known amount of 

 emanation extracted from the radium solution by bubbling 

 air through it. 



After some trials with various electroscopes, a very satis- 

 factory one was made as described in a previous paper *. A 

 filter flask was silvered inside ; a good earth- connexion was 

 made from the silver coating ; a rubber stopper sealed the 

 top, and held the wire, sulphur-bead insulation, and light 

 Dutch-metal leaf. Some of the silvering was scraped away, 

 and the leaf was read by a microscope with a graduated eye- 



Phil. Mag. August 1907. 



Phil. Mag. S. 6. Vol. 14. No. 84. Dec. 1907. 3 C 



