round the electroscope decreased the rate of discharge about 30 per cent. Further increase of the thickness of the screen had no effect. When the apparatus was surrounded by 5 tons of pig-lead the rate of discharge was about the same as when it was surrounded by a plate about 3 cms. thick. An iron screen also diminished the rate of discharge to about the same extent as the lead. By suitably arranging lead screens it was found that the radiation came equally from all directions. It was of the same intensity by night as by day. In order to be sure that this penetrating radiation did not arise from the presence of radio-active substances in the laboratory, the experiments were repeated in buildings in which radio-active substances had never been introduced, and also on the open ground far removed from any building. In all cases a diminution of the rate of discharge of the electroscope, when surrounded by lead screens, was observed. These results show that a penetrating radiation is present at the surface of the earth, arising partly from the earth itself and partly from the atmosphere.
The result is not surprising when the radio-activity of the earth and atmosphere is taken into account. The writer has found that bodies made active by exposure to the emanations from thorium and radium give out γ rays. We may expect then that the very similar excited radio-activity which is present in the earth and atmosphere should also give rise to γ rays of a similar character. More recent work, however (section 286), indicates that this explanation is not sufficient to explain all the facts observed.
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-
