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



distance and taking logarithms for ordinates, a curve is 

 obtained indicating the difference between two exponentials. 

 The straighter portion gives a value of ^ = '0058 cm. -1 In 

 some cases the lead reflexion increased the ionization 

 S8 per cent., and copper 64 per cent. We may compare 

 the results of Kovarik, when aluminium absorbed the ft rays 

 from radium E, namely : — 



air behind yu, = 42 4 cm. -1 



lead „ yu, = 44-8 „ 



lead minus air. /a=51*0 „ 



Discussion. 



So far as I am aware, no previous measurements have 

 been made of the value of the coefficient of absorption of 

 ft rays in air. 



Rutherford * has given an estimate for the value for the 

 ft rays from uranium, namely, *006b\ This is based on the 

 value of yLt/density, for various metal sheets, which is about 

 5. It is not surprising that my value for air is less than 

 this, since fi/D decreases with 1) ; thus, for lead 10, silver 7, 

 aluminium 5. My value for /u/D for air is between 2 6 and 

 3'4, according to the experimental conditions. 



It is remarkable and contrary to expectation, that the 

 above described experiments give //- = '0033 for short ranges 

 and fi = *0048 for long ranges. With metal screens the 

 reverse order is usually observed, the less penetrating rajs are 

 first absorbed, and then fi decreases for the more penetrating 

 rays. Moreover, in the present case it is possible that some of 

 the ft rays of radium D at the shorter distance may reach the 

 electroscope. Some large values of /jl found with the capsule, 

 not recorded here, may find their true explanation in this 

 fact, 



Two different causes may be suggested to account for the 

 paradox. 



Either the ft rays ionize more efficiently towards the end 

 of their path when their velocities have been somewhat 

 reduced ; or at the longer ranges the scattering of the 

 particles by air has caused the distances, which were 

 measured on the straight, to be largely in defect of the 

 actual total distances traversed by the electrons in their 

 zigzag flights. 



k The first explanation suggests a relative increase of I 



at the longer ranges, due to increased ionization of lower 



speed ft particles. The second explanation suggests that any 



* ' Radioactivity/ p. 197. 



