16 Absorption by Air of the Beta Rays from Radium C. 



It follows that the average range is the area below the 

 curve, divided by N , or 



j N<fo/N = le-^dx= 1/fi. 



Thus, the /3 particles reached an average distance from 

 the source in air= 1/-004, or 2' 5 metres. 



If it is correct to regard a 7 ray as an entity, with a 

 linear path, then its average range is the inverse of '00004, 

 or 250 metres. 



If the /3 rays were ejected all with the same velocity, the 

 above statement would have a conciseness which is lacking 

 in the case of the complex rays from radium C. 



The writer proposes to employ the method above described 

 to determine the value of fju for the /3 rays from radium E, 

 and for Rontgen rays. 



Summary. 



1. By varying the distance r between some radium and a 

 thin-walled electroscope, the law of absorption by air of the 

 complex rays from radium C has been established as approxi- 

 mately exponential. 



2. If I is the ionization due to /3 rays in the electroscope 

 at a distance r from the radium, then Ir 2 varies as e~^ r nearly, 

 where //, is the coefficient of absorption by air of the {3 ravs. 



3. The values of /j, have been found as '0033 cm. -1 at the 

 shorter range 60 to 160 cm., and '0045 at longer ranges 

 2 to 5 cm. 



4. If air absorbs /3 and 7 rays respectively in the same 

 proportion as does aluminium, the coefficient of absorption 

 of the 7 rays by air is between '000021 and "000031. 



5. With limitations, the average effective range of the 

 /3 rays from their source may be stated as 2*5 metres. In 

 the same sense, the average path of the 7 rays is 250 metres, 

 if the 7 rays consist of entities with linear paths. 



April, 1911. 



JSote added May, 1911. 



Measurements have recently been made to determine the 

 value of fi, when /3 rays, from the active deposit of radium 

 on thin aluminium foil, pass to a thin-walled electroscope 

 through air. The method employed was that given above, 

 and the range was from 40 to 100 cm. The values of p found 

 in a number of experiments were from '012 to *013, about 

 three or four times as large as when radium in a glass tube 



