10 



Dr. A. S. Eve on the Coefficient of Absorption 



this case the framework was covered with aluminium leaf 

 '0003 cm. thick, weighing about '0008 gram to the square 

 centimetre, with a stopping power of about 7 mm. of air. 

 It is easily shown by experiment that when the walls are 

 covered with two or three times this thickness of aluminium 

 leaf, neither the /3 nor the 7 ray ionizations are altered to an 

 extent which can be detected. 



Thus the experimental conditions ensure the desired 

 result, that is, the investigation of an elementary volume of 

 air, practically removed from any important modification 

 due to reflected or scattered or secondary radiation from 

 surrounding bodies, excepting the air and radiating source. 



Long Range Experiments. 



It was found possible to detect the direct /3 radiation 

 from 14*6 mg. of pure radium bromide (8*5 mg. of radium) 

 placed within two test-tubes, at a distance of 7 metres, and 

 to make fairly accurate measurements at 6 metres. The 

 test-tubes reduced the /3 radiation from radium C to about 

 half value, and the radiation from radium B was negligible. 



A single experiment may be given in detail where the 

 ionization from yS and 7 rays together was measured at 

 ranges from 1 to 6 metres. The j3 rays were then cut off to 

 a great extent by two sheets of aluminium, each 1*7 mm. 

 thick, placed at the radium. These would permit about one 

 per cent, of j3 rays to pass through them, and w T ould reduce 

 the 7 radiation by about two per cent. 



After deducting the natural leak, the readings in divisions 

 per minute were taken and the results obtained are as 

 follows, where I is the ionization due to j3 rays, and I / to 

 7 rays : — 



Table I. 



Metres. 



i+r. 



I'. 



IV 2 . 



I. 



logelr' 2 . 



fl. 



1 

 2 



3 

 4 

 5 

 6 



69 

 143 



4-47 

 1-94 

 1-02 



•58 



125 



3-78 



1-57 



■87 



•57 



•39 



125 

 15-1 

 141 

 139 

 111 

 140 



56-5 



10-52 



2-90 



1-07 



•45 



•19 



4-03 



•0029 

 •0048 

 •0042 

 •0041 

 •0051 



3-74 



326 



2-84 



2-43 



1-92 



•0044 cm. 



