Emanation in the Lower Regions of the Atmosphere. 35 



Hence the average volume of radium emanation in one 

 cubic metre of the atmosphere at Cambridge 



= (105xlO-^)xC585xlO- 3 ) 

 = 6'lxl0- 14 c.c. 



or there is 6'1 xlO -20 c.c. of radium emanation in 1 c.c. of 

 air. 



Now there are 2*76 x 10 19 molecules of gas in 1 c.c. of gas 

 at 0° C. and 760 mm. Therefore the average number of 

 emanation molecules in 1 c.c. of the atmosphere near the 

 earth's surface 



= (6-1 xl0- 20 )x (2-76 xlO 19 ) 



= 1'7. 



Surely it is a triumph to be able to detect the existence of 

 a gas in the atmosphere when there are less than 2 molecules 

 of it present in a cubic centimetre. 



Volume of Helium produced from the Emanation in the Air. 



Rutherford and Boltwood* have shown that 1 gm. of 

 radium produces helium at the rate of 163 cub. mm. per year. 

 Therefore the amount of helium produced per year from the 

 radium emanation and its products in 1 c.c. of the atmosphere 



= Jx-16x(105xlO- 12 xlO- 6 ) 

 =:l-3xl0- 17 c.c. 



Allowing ;m equal amount to be produced from the thorium 

 products in the atmosphere, and realizing that most of the 

 helium produced by the disintegration of the earlier members 

 of the series in the earth's crust is kept there, it follows that 

 3xlO~ 17 c.c. of helium is produced by radioactive processes 

 per c.c. of the atmosphere per year. 



The amount of helium actually present in 1 c.c. of the 

 atmosphere is about 5 x 10 -6 c.c, so that these figures hardly 

 afford a method of calculating the age of the earth. 



Summary. 



(1) The amount of radium emanation in the atmosphere 

 near the earth's surface at Cambridge has been measured at 

 intervals during a year by the coconut charcoal method. 



(2) The average radium equivalent per cubic metre is 

 105 x 10" 12 gm. The lowest value is 35 x 10" 12 gm., and the 

 highest 350 x 10- 12 gm., a ratio of 1 : 10. 



(3) The amount of emanation is usually lowest during 



* Manchester Lit. and Phil. Soe. Mem, 54, 1909-1910. 

 D2 





