﻿Absorption of the /3 Rays from XJ\ 



ramum. 



389 



Substance. 



Magnesium Oxide... 

 Aluminium Oxide... 



Ferric Oxide 



Zinc Oxide 



Cuprous Oxide 



Cupric Oxide 



Arsenious Oxide ... 

 Ferrous Sulphide ... 



Zinc Sulphide 



Arsenious Sulphide 

 Stannous Sulphide 

 Antimony Sulphide 



Cuprous Iodide , 



Lead Iodide 



Formula. 



MgO 



ALA 



Fe 2 3 



ZnO 



Cu.,0 



Cut) 



As.,0 3 



FeS 



ZnS 



As 2 S 8 



SnS 



Sb 2 S 3 



Cu.,I, 



Pbl ' 



Calculated. 



56 

 o-8 

 64 

 6-8 

 (>-8 

 07 

 7-7 

 6-7 

 6-8 

 7'5 

 89 

 8-6 

 94 

 10-8 



Observed. 



55 

 59 

 64 

 7 3 

 71 

 6-9 

 7-5 

 6-5 

 6-8 

 7-5 

 89 

 8-5 

 91 

 10-8 



calculated values is good ; in the majority of cases the differ- 

 ence does not exceed the probable experimental error. Hence 

 we may fairly assume that the absorption per molecule is 

 equal to the sum of the absorptions of the constituent atoms, 

 or in other words that the absorption per corpuscle of an 

 element is not altered when the element enters into a state of 

 combination. 



This result was used to calculate the values of the ratio \/p 

 for strontium, barium, and uranium. In these cases, where 

 the element to be determined forms a large proportion of the 

 compound, the proportional error introduced is not great. It 

 is clear from the formula, however, that if an element forms 

 only a small proportion by weight of the compound, its 

 absorption will appear as the small difference between two 

 large quantities, and the proportionate error will be greatly 

 increased. Hence it was not possible to use this method to 

 calculate values for the elements of small atomic weight. 



Secondary Radiati 



on. 



As mentioned before, McClelland* has shown that even 

 with homogeneous primary /3 rays there would be a departure 

 from the simple exponential law of absorption if any appre- 

 ciable amount of secondary radiation were produced in the 

 absorbing substance. He also showed that using radium 

 rays the secondary radiation was very considerable. How- 

 ever, as previously explained, in the case of the large majority 



McClelland, Scientific Trans. Roy. Soc. Dublin, 1906. 



