Matter present in the Atmosphere. 103 



corresponds to 78 scale-divisions ; and from 1 gramme of 

 radium bromide to 3" 9 x 10 8 scale-divisions. 



Hence we can determine the amount of radium bromide 

 required to maintain the supply of emanation contained in 

 1 cubic kilometre of air at Montreal. For the emanation 

 in the iron tank, whose volume is 18*7 cubic metres, caused 

 a fall of potential corresponding to 3*6 scale-divisions, and 

 1 gramme of radium in radioactive equilibrium would 

 supply emanation corresponding to 3*9 X 10 8 scale- divisions ; 

 therefore 1 cubic kilometre of the air contains emanation 

 supplied by '49 gramme of pure radium bromide. 



Thus, making certain assumptions, we can form a rough 

 estimate of the probable amount of emanation in the atmo- 

 sphere. We can first of all calculate the amount of emanation, 

 supposed equal in quantity per cubic metre to that in the air 

 at Montreal, distributed in a spherical shell around the earth 

 1 kilometre high. Since the surface of the earth is 5 x 10 8 

 square kilometres, the amount in this shell corresponds to the 

 emanation released from 2*5 x 10 8 grammes of radium bromide 

 in radioactive equilibrium. 



Several observers have shown that the excited activity at 

 high altitudes is equal to, if not greater than, that on the 

 plains ; and it thus seems probable that this distribution of 

 emanation might extend for at least 10 kilometres. In that 

 case, the emanation in a shell 10 kilometres high must be 

 released from 2'b x 10 9 grammes, or 2160 tons. 



Now, three-quarters of the surface of the earth is covered 

 with water, and if we further suppose that the emanation 

 arises from the land alone, we obtain one- quarter of the 

 above value, or 610 tons. 



This is an estimate of the total amount of radium required 

 to furnish the emanation in the atmosphere over the land of 

 the earth's surface. It is probable that a vastly larger 

 amount exists in the earth, since the greater part of the 

 emanation would be transformed without ever issuing from 

 the earth's surface. 



An objection may be raised to the above estimate, inasmuch 

 as the measurements were taken in the building, and not 

 out-of-doors. It was found that air driven from the room 

 through the tank did not affect the result, and the production 

 of ions was proceeding at a very slow rate, as will be seen in 

 Section III., so that there was no reason to suppose that the 

 conditions within the building were different from those 

 outside in respect to the amount of radioactive matter present 

 in the air. 



Nevertheless, it was decided to make some measurements 



