
370 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
in the atmosphere. The observation of Elster and Geitel that the 
radio-activity of the air is much less near the sea than inland is 
at once explained, if the radio-activity of the atmosphere is due 
mainly to the diffusion of emanations from the soil into the air 
above it. 
The rare gases helium and xenon which exist in the atmosphere 
have been tested and found to be non-radio-active. The radio- 
activity of the air cannot be ascribed to a slight radio-activity 
possessed by either of these gases. 
In order to account for the effect observed, it 1s only necessary 
to suppose that the radio-active substance is present in minute 
quantity mixed with the soil. Suppose, for the purpose of illustra- 
tion, that the radio-activity of the atmosphere is due to the radium 
emanation escaping from the earth’s surface. The air sucked from 
the soil in many cases shows 20 times the conductivity of ordinary 
air. Now it will be shown (section 218) that the natural 
conductivity of air observed in closed vessels corresponds to a 
production of about 30 ions per c.c. per second. The active air of 
20 times the normal conductivity thus gives rise to about 600 ions 
per c.c. per second. In 100 litres of this active air the number of 
ions produced per second is therefore 6 x 10’. Now it has been 
found that the saturation current in a sealed vessel, due to the 
emanation from one gram of radium chloride, corresponds to a 
current of 2°5 x 10~ electro-magnetic units. Taking the charge of 
an ion as 11 x 10~ electro-magnetic units, this corresponds to a 
production of 2°3 x 10” ions in the gas per second. The emanation . 
present in 100 litres of air, of activity 20 times the normal activity, 
would thus correspond to the amount released by solution of 
3 x 10-8 of a gram of radium chloride. A very minute amount of 
radium per cubic foot of soil would account for the radio-active 
effects observed. 
217. Radio-activity of ordinary materials. It has been 
shown that radio-active matter seems to be distributed fairly 
uniformly over the surface of the earth and in the atmosphere. 
The very important question arises whether the small radio-activity 
observed is due to known or unknown radio-elements present in 
the earth and atmosphere, or to a feeble radio-activity of matter 

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