
368 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
the earth and atmosphere should also give rise to y rays of 
a similar character. 
216. Comparison of the radio-activity of the atmo- 
sphere with that produced by the radio-elements. The 
radio-active phenomena observed in the earth and atmosphere are 
very similar in character to those produced by thorium and radium. 
Radio-active emanations are present 1n the air of caves and cellars, 
in natural carbonic acid, and in deep well water, and these emana- 
tions produce excited radio-activity on all bodies in contact with 
them. The question now arises whether these effects are due to 
known radio-elements present in the earth or to unknown kinds 
of radio-active matter? The simplest method of testing this point 
is to compare the rates of decay of the radio-active products m 
the atmosphere with those of the known radio-active products of 
thorium and radium. A cursory examination of the facts at once 
shows that the radio-activity of the atmosphere is much more 
closely allied to effects produced by radium than to those due to 
thorium. The activity of the emanation released from well water, 
and also that sucked up from the earth, decays to half value in 
about 3°3 days, while the activity of the radium emanation decays 
to half value in an interval of 3:7 to 4 days. Considering the 
difficulty of making accurate determinations of these quantities, 
the rates of decay of the activity of the emanations from the earth 
and from radium agree within the limits of experimental error. 
Bumstead and Wheeler have shown that the emanation from the. 
soil and surface water of New Haven is identical with the radium 
emanation. If the emanation from the earth is the same as that 
from-radium, the excited activity produced should have the same 
rate of decay as that from radium. The emanation from well 
water in England approximately fulfils this condition (section 212), 
but an observation recorded by Ebert and Ewers (section 212) 
seems to show that the excited activity due to the emanation 
sucked up from the earth decays at a very slow rate compared 
with that due to radium. 
On comparing the rates of decay of the excited activity derived 
from the atmosphere and of that produced by radium, the evidence 
is to some extent conflictmg. The activity of a negatively charged © 

