28 Dr. A. S. Eve on the Ionization of the 



production of ions per cubic centimetre, for then an electro- 

 scope must be employed, nnd at once there is an uncertain 

 factor, namely the ionization due to the intrinsic radiation 

 from the sides of the employed vessel, probably attributable to 

 radioactive matter in the walls of the electroscope. Moreover 

 we are seeking for the value of q in free air, while in the 

 electroscope the ionization is in part due to the secondary 

 rays (ft rays) from the electroscope, and this secondary 

 radiation from the walls is in excess of that from free air. 

 Recollecting all these pitfalls, we will commence with a 

 careful estimate of the rate of production of ions due to the 

 radium products known to be present in the atmosphere. 



AlpJta Rays in the Atmosphere. 



The most simple and accurate method of measuring the 

 amount of radium emanation in the atmosphere is that of 

 collecting the emanation either by adsorption, using coconut 

 charcoal, or by condensation with liquid air. The results 

 obtained are in fairly good agreement. 



Eve*.... 60 



Satterley* 100 



Ashmanf 89 



Mean 83 



"We will select 80 as the mean value, and state that one 

 cubic metre of the atmosphere near the earth's surface con- 

 tains the amount of radium emanation in equilibrium with 

 80 x 10 ~ 12 gramme of radium J (1) 



There are of course considerable fluctuations with time 

 and position, due to barometric pressure, wind, rain, and to 

 the varying radium-contents of the underlying soils, so that 

 it is only an approximation to the mean value which is here 

 quoted. It is marvellous that the properties of this gas are 

 such as to enable us to detect its presence and measure its 

 amount, when it constitutes less than one thousand million 

 billionth (1/10 19 ) part of the atmosphere. 



From the results of Rutherford and Geiger § we can now 

 calculate the number of ions produced per cm. 3 , due to radium 



* Phil. Mag. Oct. 1908. 



t Amer Joum. Sci., Aug. 1908. 



% i.e. 80xl0-i 2 Curies. 



§ Proc. Roy. Soc. July 1909. 



