408 Mr. H. Lester Cooke on Penetrating 



water-tight by means o£ a rubber washer. The electroscope 

 was placed inside this vessel and the rate of discharge ob- 

 tained with the apparatus resting exposed on the slate in the 

 laboratory. The run was then repeated with the cylinder 

 containing the electroscope immersed in a large tank, so as 

 to be surrounded on all sides by a thickness of water of at 

 least 2^ ft. Under these conditions it was found that the 

 rate of discharge was reduced as in the experiments where 

 lead and iron were used. The tank used in these experi- 

 ments was of iron, about an inch in thickness ; it is thus 

 doubtful how much of the radiation was cut off by the iron^ 

 and how much by the water. The onJy two conclusions 

 w^hich can be drawn with certainty from these experiments 

 are that the iron and water combined reduced the amount of 

 penetrating radiation, and that the water did not supply any 

 such radiation to an appreciable extent. 



Radioactivity of Ordinary MateAals. 



The walls of the laboratory in which the experiments w^ere 

 carried on were of brick. It was therefore thought advisable 

 to ascertain what would be the effect of surrounding the 

 electroscope with a brick cage. New^ bricks were obtained 

 and two cages built, the sides of one being one brick in 

 thickness and the sides of the other four bricks thick. Placing 

 the electroscope inside these cages caused the rate of collapse 

 of the leaves to increase 40 and 50 per cent, for the small 

 and large cage respectively. This increase, however, was 

 not due to a penetrating form of radiation, for when a casing 

 of lead a millimetre thick was interposed between the electro- 

 scope and the bricks the leakage was immediately reduced to 

 its normal value. This also precludes the idea of the leakage 

 being due to an emanation from the bricks, which was sug- 

 gested as an explanation of the effect. The rays from the 

 bricks were evidently of a comparatively low penetrating- 

 power, probably being very similar in character to the (3 rays 

 given off by radium. Experiments were tried to detect the 

 presence of very easily absorbed rays from bricks, the base 

 of the electroscope being removed, and the instrument placed 

 on a brick support. Bricks covered with gold leaf^ bare 

 bricks, and powdered bricks, were successively tried in this 

 way. The removal of the base of the electroscope slightly 

 increased the ionization of the inclosed air, but not to an 

 extent sufficient to indicate the presence of an appreciable 

 amount of easily absorbed rays. Since the ionization was 

 the same when the electroscope surrounded by lead one 

 millimetris in thickness was placed inside the brick cage, as 



