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 Wolfenbüttel. Experiments on the radio-activity of the air in mid-ocean would be of great importance in order to settle whether the radio-activity observed in the air is due to the emanations from the soil alone. It is probable that the radio-activity of the air at different points of the earth may vary widely, and may largely depend on the nature of the soil.

Saake has found that the amount of emanation present in the air at high altitudes in the valley of Arosa in Switzerland is much greater than the normal amount at lower levels. Elster and Geitel have observed that there is also a larger number of ions in the air at high altitudes, and suggest that the curative effect of thermal springs and the physiological actions of the air at high levels may be connected with the presence of an unusual amount of radio-active matter in the atmosphere. Simpson made experiments on the amount of excited activity at Karasjoh, Norway, at a height of about 150 feet above sea level. The sun did not rise above the level of the horizon during the time the observations were taken. The average amount of excited activity obtained from the air was considerably greater than the normal amount observed by Elster and Geitel in Germany. This was the more surprising as the ground was frozen hard and covered with deep snow. Allan, working in Montreal, Canada, early observed that the amount of activity to be obtained from the air was about the same in summer as in winter, although, in the latter case, the whole earth was deeply frozen and covered with snow, and the winds blew from the north over snow-covered lands. Under such conditions, a diminution of the amount of activity is to be expected since the diffusion of the emanation must be retarded, if not altogether stopped, by the freezing of the soil. On the other hand, it appears difficult to escape from the conclusion of Elster and Geitel that the emanation present in the atmosphere is evolved from the earth itself.

Some interesting experiments have been made by McLennan on the amount of excited radio-activity to be derived from the air when filled with fine spray. The experiments were made at the