858 Dr. A. F. Kovarik on the Absorption and 



activity, the logarithm of the ionization when plotted against 

 the thickness of the absorbing material gives a straight line. 

 The conclusion generally drawn from such a result is that 

 the absorption follows an exponential law, and that the rays 

 are homogeneous. This, however, should be done with due 

 caution, for the logarithm of the ionization changes slowly 

 with the ionization, and a straight line would be obtained 

 when the absorption follows the exponential law only ap- 

 proximately. This is clearly illustrated when we consider 

 the differential curve for the reflected rays, say from lead, in 

 the case of any of the above mentioned radioactive materials. 

 For Ra E rays, for example, after the soft rays are absorbed 

 we get a straight line with the value of /x = 42*4 when air is 

 underneath, a straight line with a value of //, = 44'8 when 

 lead is underneath, and a straight line for the differential 

 curve with a value of /jl = 51*0 cms. -1 Al. Now, the differ- 

 ential curve is obtained by taking the differences of two 

 exponentials, supposing they are such, and this difference 

 cannot, therefore, be an exponential ; but on account of the 

 comparatively small differences in the exponents the 

 differential curve approximates to an exponential. 



It must further be remembered that the rays after passing 

 through matter are scattered and their velocity is changed, 

 and while the /3-particles of one velocity predominate there 

 are, however, /3-particles of smaller and greater velocities* 

 present as well, and a distribution of velocities of this kind 

 may be the one which is required for such an approximation 

 to the exponential law as is generally obtained. 



Reflexion of the ^-particles of different velocities. 



It was shown above that the reflexion of the /5-particles is 

 important in the study of the absorption curves. McClelland f 

 and Schmidt % have shown that elements of higher atomic 

 weight reflect a larger percentage of the incident /3-particles 

 than the elements of lower atomic weight. In the course of 

 this investigation it was noticed that the /3-particles from 

 Ra E and ActC were not reflected equally readily by the same 

 substance. This suggested that the velocity of the /3-particles 

 plays an important role in the problem of reflected rays. 

 Systematic experiments were therefore carried out to study 

 the amount of reflexion of the /3-particles from Ra E and the 



* W. Wilson, he. cit. 



t J. McClelland, Sci. Trans. Roy. Dublin Soc. ix. pts. 1 & 2, pp. 1 & 

 (1906). 



t H. W. Schmidt, Jahrbuch der Radioaktivitat und Elektronik, iv. 

 p. 451 (1908). 



