346 RADIO-ACTIVE PROCESSES [cH. 
scope to determine the conductivity of the enclosed gas. After 
standing for several days, he found that the conductivity of the air 
reached a constant maximum value, corresponding to three times 
the normal. It will be shown later (section 218) that the normal 
conductivity observed in sealed vessels corresponds to the produc- 
tion of about 30 ions per c¢.c. per second. The number of ions 
produced per second in the vessel by the radio-active earth was 
thus about 2 x 10° This would give a saturation current through 
the gas of 2°2 x 10 electro-magnetic units. Now the emanation 
from 1 gram of radium stored in a metal cylinder gives a satura- 
tion current of about 3:2 x 10~ electro-magnetic units. Elster and 
Geitel considered that most of the conductivity observed in the 
gas was due to a radio-active emanation, which gradually diffused 
from the clay into the air in the vessel. The increased conduc- 
tivity in the gas observed by Elster and Geitel would thus be 
produced by the emanation from 7 x 107° gram of radium. 
Taking the density of clay as 2, this corresponds to about 10~™ 
gram of radium per gram of clay. But it has been shown that if 
4-6 x 10“ gram of radium was present in each gram of earth, the 
heat emitted would compensate for the loss of heat of the earth by 
conduction and radiation. The amount of activity observed in the 
earth is thus about the right order of magnitude to account for the 
heat emission required. In the above estimate, the presence of 
uranium and thorium minerals in the earth has not been con- 
sidered. In addition, it is probable that the total amount of radio- 
activity in clay was considerably greater than that calculated, for it 
is likely that other radio-active matter was present which did not 
give off an emanation. 
I think we may conclude that the present rate of loss of heat 
of the earth might have continued unchanged for long periods of 
time in consequence of the supply of heat from radio-active matter 
in the earth. It thus seems probable that the earth may have 
remained for very long intervals of time at a temperature not very 
different from that observed to-day, and that im consequence the 
time during which the earth has been at a temperature capable of 
supporting the presence of animal and vegetable life may be very 
much longer than the estimate made by Lord Kelvin from other 
data. 
