Matter present in the Atmosphere. i07 



the wires to a condenser of known capacity consisting of two 

 concentric cylinders, and by observing the potential, both 

 before and after the connexion. 



The subsequent calculations are similar to those given by 

 Rutherford in ' Radioactivity/ page 72. 



Since S, the volume of the tank, equals 18' 7 x 10 6 c.c, and 

 C, the capacity, was 140 E.S. units, the current, i, is equal 

 to CV, when V is the fall of potential per second ; q is the 

 number of ions produced per second in a cubic centimetre, 

 and e, the charge on an ion, is taken as 3*4 x 10 "" 10 . But the 

 gold-leaf fell at a rate measured by 2*3 scale-divisions of 

 the microscope in one minute, and a scale-division represented 



3*6 



3*6 volts or ^T^r E.S. units. 

 oU(J 



Hence i = 6'44 x 10~ 2 , 



and i 



g= We 



= 10-1. 



A large number of similar experiments were made, and 

 the values ranged from 9*1 to 10*2, with a mean value 



q = 9*6 ions per c.c. per second. 



This is the smallest value yet obtained, and 1 think that the 

 error is not more than five per cent. H. L. Cooke, using a 

 well-cleaned brass vessel, found q = 10 in the Physics 

 Building at McGrill University about two years ago. Professor 

 Schuster, in the laboratory at Manchester, found q == 12. 

 These small values for q are only obtainable when the instru- 

 ments employed are well-cleaned, and removed from the 

 neighbourhood of radioactive substance. 



IV. An Estimate of the Total Depth in the Earth from which 

 the Radium Emanation passes to the Atmosphere. 



Professor Rutherford has given in ' Radioactivity ; an 

 estimate of a higher limit to the amount of radium which can 

 be present in the earth. If this amount were exceeded, the 

 temperature of the earth would have a value in excess of that 

 observed. He calculates that, on an average, not more than 

 4*6 x 10 ~ u grammes of radium can be present per one gramme 

 of the earth's constitutents. But the mass of the earth is 

 6*1 x 10 27 grms. Hence the total amount of radium in the 

 earth cannot exceed 28 x 10 13 kilos, or 28 x 10 7 British tons. 



The consensus of observers seems to show that the emanation 



