Radiation from the Earth's Surface. 407 



Measurements were then made o£ the relation between the 

 thickness of the lead screen and the decrease in the ionization, 

 screens ranoino- in thickness from 1 mm. to about 40 cms. 

 beino- employed. The results of repeated experiments indi- 

 cated that this radiation was subject to the ordinary law of 

 absorption, and that it was cut down to half value by a 

 thickness of lead somewhere between "25 and '5 of an inch, 

 being almost completely absorbed by 2 inches of lead. The 

 greatest quantity of lead employed in the construction of a 

 cage was about 10,500 lbs., the value thus obtained being 

 practically the same as with a cage 2 inches thick. The in- 

 dividual observations were not as consistent as one could 

 wish, sometimes showing a variation of as much as 10 per 

 cent., but the averages obtained from a very extended series 

 of experiments clearly indicated the above results. As will 

 be shown further on, the current measured in these experi- 

 ments was almost inconceivably minute, and thus any slight 

 disturbing influence, which would be incapable of being de- 

 tected by ordinary methods, was apt to become a serious 

 factor of disturbance, and had to be guarded against with 

 the utmost care. Altering the distribution of the lead slabs 

 revealed the fact that the radiation was of practically uniform 

 intensity in all directions, coming equally from the floor^ 

 walls_, and ceiling of the laboratory. 



It was thought possible that this radiation might be due in 

 some way to the fact that radium and other radioactive sub- 

 stances had been used in the laboratories, and might in some 

 way have affected the walls of the room. To test this point 

 a series of experiments was carried out in the basement of 

 the University Library, where no radioactive materials had 

 been used. From observations extending over a period of 

 more than two weeks it was definitely shown that this effect 

 was present in the library as well as in the Physics building, 

 and therefore could not in any way be due to contamination 

 with radioactive substances. 



The next thing to be ascertained was whether this radiation 

 would be reduced on passing through other metals besides 

 lead. For this purpose small slabs of polished iron were 

 obtained which could be built up in the form of a cage as in 

 the case of the lead. In this way iron was found to also cut 

 down the radiation. The relation between the thickness of 

 iron and the absorption was not determined, the object being 

 merely to show that this screening effect was not a property 

 peculiar to lead. 



The effect of water was also tried. A brass cylinder was 

 obtained with a top which could be bolted down and rendered 



