20 ROYAL SOCIETY OF CANADA 
through the powdered charcoal. He was kind enough to assign to me 
the experiment of using this method to determine in a direct manner the 
amount of radium emanation in the atmosphere. The method initially 
appeared so simple that results were expected in a few days, but a large 
amount of work extending over several months was actually needed for 
determination. 
When air containing emanation is drawn through a tube of cocoa- 
nut charcoal slowly, the emanation is completely absorbed, together with 
a considerable quantity of nitrogen, oxygen and carbon dioxide, and with 
small amounts of the rarer gases, such as helium, neon and argon. 
When the tube is heated in a bunsen flame these gases are driven off and 
may be collected over water. They- can then be introduced into a gold 
leaf electroscope, and the activity of the emanation measured by the 
movement of the gold leaf, observed with a microscope with a graduated 

eye-piece. The electroscope is calibrated in the usual manner by intro- 
ducing into it the emanation from a measured quantity of a standard 
solution of radium. Thus the amount of emanation absorbed by the 
charcoal is measured in terms of the amount of radium required to 
produce it. 
But if the air and emanation are passed through the charcoal tube 
rapidly, only a small fraction of the emanation is absorbed; and if 
emanation has been introduced into the charcoal, it can be removed, 
not only by heating, but to a large extent by a strong current of air. 
It is necessary, therefore, to pass the air through the charcoal 
slowly, certainly not faster than 1 c.c. per second; but at this rate it 
will take 11 to 12 days to pass one cubic metre through the tube, and 
it must be remembered that the emanation from radium decays to 
half value in about 3.8 days. Now, according to my previous experi- 
ments, by the active deposit method, a cubic metre of air should contain 
the emanation from 10—° grams of radium, and if there were no loss 
from decay, there would be no difficulty in measuring such a quantity 
with an electroscope. But a further trouble arose. It was found that 
the charcoal itself contained a small quantity of radium, and my earlier 
experiments were vitisted by this .unexpected fact. Fresh «ccoanuts 
