Some interesting experiments have been made by McLennan42 on the amount of excited radio-activity to be derived from the air when filled with fine spray. The experiments were made at the foot of the American Fall at Niagara. An insulated wire was suspended near the foot of the Fall, and the amount of excited activity on the wire compared with the amount to be obtained on the same wire for the same exposure in Toronto. The amount of activity obtained from the air at Toronto was generally five or six times that obtained from the air at the Falls. In these experiments it was not necessary to use an electric machine to charge the wire negatively, for the falling spray kept the insulated wire permanently charged to a potential of about -7500 volts. These results indicate that the falling spray had a negative charge and electrified the wire. The small amount of the excited radio-activity at the Falls was probably due to the fact that the negatively charged drops abstracted the positively charged radio-active carriers from the atmosphere, and in falling carried them to the river below. On collecting the spray and evaporating it, no active residue was obtained. Such a result is, however, to be expected on account of the minute proportion of the spray tested compared with that present in the air.
279. A very penetrating radiation from the earth’s surface. McLennan43, and Rutherford and Cooke44 independently, observed the presence of a very penetrating radiation inside buildings. McLennan measured the natural conductivity of the air in a large closed metal cylinder by means of a sensitive electrometer. The cylinder was then placed inside another and the space between filled with water. For a thickness of water between the cylinders of 25 cms. the conductivity of the air in the inner cylinder fell to about 63 per cent. of its initial value. This result shows that part of the ionization in the inner cylinder was due to a penetrating radiation from an external source, which radiation was partially or wholly absorbed in water.
Rutherford and Cooke observed that the rate of discharge of a sealed brass electroscope was diminished by placing a lead screen around the electroscope. A detailed investigation of the decrease of the rate of discharge in the electroscope, when surrounded by metal screens, was made later by Cooke45. A thickness of 5 cms. of lead 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.