280. Comparison of the radio-activity of the atmosphere 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 in the air of caves and cellars, in natural carbonic acid, and in deep well water, and these emanations produce excited radio-activity on all bodies in contact with them. The question now arises whether these effects are due entirely 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 rate of decay of the radio-active product in 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. A large number of observers have found that the radium emanation is present in the water of thermal springs and in the sediment deposited by them. Bumstead and Wheeler have shown that the emanation from the soil and surface water of New Haven is identical with that from radium. If the emanations from the earth and from radium are the same, the excited activities produced should have the same rate of decay. The emanation from well water in England approximately fulfils this condition (section 276), but an observation recorded by Ebert and Ewers (section 276) 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.
Bumstead has given undoubted evidence that the thorium as well as the radium emanation is also present in the atmosphere at New Haven,