XI] AND OF ORDINARY MATERIALS 367 
observed the presence of a very penetrating radiation inside build- 
ings. 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 Cooke!. 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 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. 
This result is not unexpected, 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 y rays. It is then to be expected 
that the very similar excited radio-activity which is present in 
1 Phil. Mag. Oct. 1903. 
