98 THE ROYAL SOCIETY OF CANADA 



For air Chadwick* obtains the value /ii/Di= .048, D^ being the 

 density of the air: this gives /ii = 6«0 X 10-\'m.-\ considerably higher 

 than the value previously obtained by Evef and Hess, J (/x^ =4'4X 10-'^ 

 cm.-* and 4*47 X 10-'''cm.-\ respectively). 



For rock Rutherford'' takes the value 1x2/^2"" '^'^, ^^^^ very 

 different from the value obtained by McClelland (/i2/D2= •034). 

 Taking as a rough value D2=3'0 we have fi^=-12 cm.-* 



We denote by n^ the number of ions produced per c.c. per second 

 in air at normal temperature and pressure at a distance of 1 cm. from 

 1 curie of radium supposed concentrated at a point: (absorption of 

 ^'-rays by air and by the substance of the radium salt neglected.) 



Eve"^ obtains for the value of n^ the number h^ =3 -74X10" 

 while a more recent determination by Moseley and Robinson^ gave the 

 value no =6X10". 



Q represents the mean radium content for sedimentary rocks. 

 A table of value for various rocks as determined by Strutt is quoted 

 by Rutherford:^ a value of the order of Q=3'0Xl0-*^grams per c.c. 

 of sedimentary rock may be taken to represent a rough value. 



If the radium content is distributed uniformly throughout a semi- 

 infinite solid bounded by a plane the value of n at a height z above the 

 surface is given by 



n= UfiZi), (2) 



f (/^i^i) being the function defined by equation (1). This function 

 has very simple properties which often enable us to write down without 

 calculation the intensity of radiation in more complicated cases of 

 distribution: thus if we draw a horizontal plane at a depth «2 below 

 the surface it is easily shown that the contribution to n arising form 

 the distribution of radium below this plane is given by 



Hence by subtraction we obtain the intensity due to a plate of 

 thickness Z2 ^t^ the form 



*Chaiiwick, Proc. Loud. Phijs. Soc, 24, p. 152, 1912. 

 tEve, Phil. Mag., July, 1911, p. 12. 

 jlk'ss, y>/.s. ZcU, 12, Nov. 1911, p. 998. 



* Rutherford, Radio-activity, 19113, p. 637. 

 » Eve, Phil. Mag., Oct., 1911, p. 551. 



• Rutherford, Radio-activity, 1913, p. 637. 

 ' Rutherford, Radio-activity, 1913, p. 645. 



