388 Absorption of the Radioactive Emanations by Charcoal. 



For wood charcoal three curves were taken, corresponding 

 to 170, 16-1, and — 74*2 degrees centigrade. (See rig. 10.) 

 The non-absorption curve is practically the same as that 

 for 170°. 



The curves show at once that the above law holds. Wood 

 charcoal, under ordinary conditions, is not a good absorber, 

 yet it is found to be affected by temperature according to the 

 law. The absorption by animal charcoal is affected in a 

 similar manner, but the experimental curves are omitted. 



In a special experiment it was determined that the cocoanut 

 charcoal begins to give off' carbon dioxide at 211° C, yet at 

 even higher temperatures than this it showed some power of 

 absorbing the emanation. By using a large quantity of charcoal > 

 and keeping the speedof the air-current constant at l*65c.c. per 

 second, it was found that the ionization-current in the testing- 

 vessel kept decreasing, showing a slight and gradually de- 

 creasing absorption up to about 300° C, when the ionizatiou 

 current began to increase. This increase was no doubt due 

 to the loss of absorbing charcoal by oxidization, and to the 

 increased conductivity of the gas in the testing-vessel owing 

 to the presence of carbon dioxide. (For a-ray ionization the 

 conductivity of carbon dioxide is 1*5 times that of air.) 



In connexion with the above experiments, it should be 

 noticed that it is hardly probable that the charcoal becomes 

 " saturated " with thorium emanation — that is, that it reaches 

 a state when it cannot occlude any more emanation — as it 

 would in the case of ordinary gases. It must be remembered 

 that the emanation which has been occluded by the charcoal 

 is itself decaying at the rate of half- value in 54 seconds. 



In the observations of ionization currents given above, the 

 numbers represent the current when it has reached a steady 

 state. At slow speeds the current keeps increasing for a few 

 minutes until it becomes steady, but at high speeds it reaches 

 a steady value almost immediately. 



In conclusion, it may be stated that the percentage 

 absorption of thorium emanation by means of charcoal 



(1) depends on the nature of the charcoal used, being 



greatest for coconut, intermediate for animal, and 

 least for wood charcoal ; 



(2) is decreased by increasing the velocity of the gas- 



current which conducts the emanation through the 

 charcoal ; 



