Radiation from Substances of Low Atomic Weiglit. 147 



Other test cases were taken, and the formula found to be 

 reliable to within about 10 per cent. 



To ensure accuracy it was necessary to secure that no one 

 constituent had been completely absorbed by the two 

 absorbing layers. It was important also that the difference 

 between the two apparent coefficients should be as great as 

 possible. Having regard to these two points a piece of 

 carbon was taken and subjected to homogeneous beams of: 

 varying hardness. Assuming that the tertiary radiation in 

 each case consisted of secondary radiation scattered as a 

 homogeneous beam of unaltered hardness, together with 

 a radiation characteristic of carbon, the absorption coefficient 

 of the latter in aluminium was calculated from the observed 

 absorptions. 



The results of these experiments are given in the following 

 table : — 



Table TIT. 



Source of 

 Secondary radiation. 



Xfor 

 Secondary radiation. 



Calculated value of X 

 for the soft Tertiary 

 radiation. 



Cu 



128-9 



60-7 



24-6 



12-7 



6-75 



4-33 



319 

 315 

 184 

 165 

 125 

 70 



As 



Sr 



Mo 



Aff 



Sn 





Note. — The values of X in column 2 are for radiations 

 filtered through appropriate thicknesses of aluminium. 



In the case of those secondary beams known to contain a 

 soft constituent, this was removed by first passing the 

 radiation through absorbing sheets of aluminium before it 

 fell upon the tertiary radiator. 



A glance at column 3 shows at once that after making due 

 allowance for experimental error there is no semblance of 

 agreement between the values of X. 



We must then abandon our hypothesis as to the com- 

 position of the tertiary beam. There is no precedent for 

 supposing that if a characteristic radiation were emitted by- 

 carbon it would be other than homogeneous. Moreover, 

 experience justifies us in concluding that in snch a case its 

 degree of hardness would be constant and independent of the 

 hardness of the exciting radiation. 

 ° L2 



