108 Mr, A. S. Eve on the Radioactive 



in the air escapes from the earth ; and i£ we suppose that 

 radium is uniformly distributed to the above amount, we can 

 readily calculate the depth from which the emanation must 

 freely come in order to keep up the supply in the atmosphere. 

 Let x be the average depth through which that radium is 

 distributed which gives rise to the emanation in the atmo- 

 sphere. Since 610 tons of radium suffice to produce the 

 amount of emanation over the land of the adobe, we have 



XX ITT 2 610 



1 o ~28xl0 7 ' 



.7 7TV 6 



6 



where r is the earth's radius, and since 



r = 6*4 x 10 cms. 



x = 18 metres. 



If we take the lower estimate of 170 tons of radium, the 

 corresponding depth is about 5 metres. 



Emanation arising from greater depths than these would 

 probably disintegrate before reaching the surface of the 

 earth. 



IV. On the Cause of the Natural Ionization observed in 

 the At) no sphere. 



The question arises whether the natural ionization of the 

 air at the surface of the earth is due to the radioactive matter 

 contained in it, and whether this cause will wholly account 

 for the effect, or whether there are other causes at work, 

 known or unknown in character. 



Take, for example, the discharge of the cylinder of wires 

 suspended in the large iron tank in the Engineering Building. 

 This wire lost its charge at the same rate whether it received 

 initially a positive or negative charge. The saturation current 

 from the wire to the sides of the vessel was 6"llx 10 2 E.S. 

 units, and the rate of production of ions was given by (7 = 9*6 

 per c.c. in a second. Is such ionization of the atmosphere 

 to any extent an inherent property, or would air entirely free 

 from radiating matter, or its influence, cease to produce ions, 

 and would it become a non-conductor? The only ionizing 

 agents under such conditions are (1) radiation due to radio- 

 active matter contained in the air, (2) radiations due to active 

 matter on the surface, or in the material of the sides of the 



