hy Air of the Beta Rays from Radium C. 15 



point on the curve should have its abscissa r multiplied by 

 some function o£ r,f(r), which increases with r ; whilst the 

 corresponding ordinate should have added to it 2 log e f (r). 



Without entering further into these obscure points, it 

 may be stated that the curves and values obtained represent 

 the facts and results usually most required, and that the 

 fi rays are absorbed by a law almost exponential, but with 

 fi increasing, probably gradually from '0033 cm. -1 to 

 •0045 cm. -1 , as r increases from 60 to COO cm. 



If a mean "001 is selected, it is possible to estimate 

 the value of the coefficient of absorption /j,' by air of the 

 7 rays from radium 0. 



Thus for aluminium, with /3 rays from radium C, 

 Kovank * finds fi= 13 cm. -1 , and with y rays McClelland 

 finds ///='103. If it is assumed that the same proportion 

 holds for ft and for 7 rays in passing through air, it follows 

 that 



//•001 = '103/13, 



so // = -000032. 



This is less than the result obtained from the value of 

 fjJ/D found by McClelland f, namely, *034, which leads to a 

 value 



^' = •000044, 



Soddy and RusselFs J value for yu//D gives a still larger 

 result, so that until further light is obtained, the intermediate 

 value "000044 may be adopted. 



Just as when the radioactive constant is X, the average life 

 is 1/A,, so we may say that when the coefficient of absorption 

 is //,, the average distance reached is l//i. But it w r ill be 

 remembered that the distance is measured " as the crow 

 flies," whilst the actual path is zigzag. 



If in a parallel pencil of /3 rays, N electrons start their 

 flight, then at a distance «r, only N remain effective for 

 ionization, so that — dN have a range between x and x + dx. 



Hence f° JAT 



•> No 



is the average distance attained from the source. 



* Phil. Mag. Nov. 1910. 

 t Phil. Mag-. Aug. 1904. 

 X Phil. Mag. Oct. 1909. 



