638 Mr. R. D. Kleeman on tlte Secondary Cathode 



was obtained. The difference in the leaks is 5608 divisions 

 in 40 seconds, which is a measure of the difference o£ the 

 ionization produced by the secondary cathode rays from 

 aluminium and lead. And since an observation could be 

 repeated any desired number of times and the mean taken, a 

 considerable accuracy could be reached in the determination 

 of the difference in the amount of secondary radiation from 

 any two substances. As a rule, about 64 observations were 

 made in determining the difference in the radiating powers 

 of a given substance and aluminium, in comparison with that 

 of the two standard substances lead and aluminium. 



The principal results obtained in these experiments are 

 given in Table V. The nature and thickness of the screen 

 used in a set of determinations are given at the top o£ the 

 column containing these determinations. The difference 

 between the radiating powers of lead and aluminium has in 

 each case been reduced to 800. The results will now be 

 discussed and an endeavour made to draw some conclusions 

 from them. 



Since the difference in the amount o£ secondary radiation 

 from any two substances is independent o£ the intensity of 

 the primary y rays, i£ the difference o£ the two standard 

 substances is always reduced to the same figure, the secondary 

 radiation from a given substance with the different screens 

 in the above table should be, obviously, the same i£ the 

 screens produced a change in intensity only of the y rays. 

 But the figures distinctly show that the y rays that penetrated 

 the various screens differed in their power of producing 

 secondary radiation from a given substance. Therefore the 

 relative differences in each column of the table depend on 

 the nature and thickness of the screen used in their 

 determination. 



A beam of y rays will probably not change much in nature 

 when sifted through a screen of lead 2 mm. thick only, and 

 therefore the second column of the table gives approxi- 

 mately the differences in the radiating powers of the 

 substances under the influence of the full y radiation emitted 

 by radium in radioactive equilibrium. The screen of lead 

 placed in the path of the y rays served to cut off the /3 rays. 



In order to be able to draw any conclusions from the effect 

 of the various screens on the relative differences of the 

 radiating powers of the various substances, it is necessary to 

 compare these differences with those in the second column of 

 the table. 



Thus, it will be seen that the difference (C — Al) is de- 

 creased with a screen of lead, mercury, or bismuth, while the 



