G36 Mr. R. D. Kleeman on the Secondary Cathode 



as can be expected, when it is remembered that the amount of 

 secondary radiation from a substance depends on the nature 

 of the screen placed in the path of the 7 rays (see next 

 section), and that the relative values of the radiating powers 

 are liable to greater uncertainty than the relative differences. 



§11. 



It was found, as has already been mentioned, that the 

 7 rays from radium are heterogeneous, and that the different 

 constituent rays give rise to different relative values for the 

 secondary radiation of a set of substances. 



The constituent rays are selectively absorbed, and a partial 

 separation of different sets of rays from the original bundle 

 could therefore be effected by means of metal screens. 



Some slight alterations were made in the apparatus to 

 make it more suitable for the investigation of this pheno- 

 menon. The gauze was removed, and the plate c and the 

 dishes a and b were discarded. The substances which were 

 used as secondary radiators were in the form of plates equal 

 in dimensions to the plate c. The plate under investigation 

 and the plate with which it was compared were placed one 

 on top of the other on the table d, which was then raised by 

 means of the screw e till the top plate or radiator made con- 

 tact with the edges of the aluminium box forming one of its 

 sides (see fig. 3, § I.). The difference in the amount of 

 secondary radiation from a substance and a standard 

 substance was, as in the foregoing experiments, compared 

 with that of two standard substances. 



The above modification of the apparatus increased its 

 sensitiveness, by reason of the larger radiating surface 

 employed, but it restricted the number of substances that 

 could be used as radiators. 



The substances used as radiators were : carbon (black 

 lead contained in a shallow dish), aluminium, sulphur (a plate 

 of sulphur, the surface of which was made a conductor of 

 electricity by covering it with a thin layer of black lead), 

 iron, copper, nickel, zinc, tin, and lead. 



Screens of various substances were placed in the path of 

 the 7 rays. A screen was placed at ST, and the differences 

 between the radiating powers of the above substances and 

 aluminium determined, the radiation from the substances 

 being produced by the 7 rays not absorbed by the screen. A 

 screen of some other substance was then placed at N, and 

 the process repeated, and so on. The screens used were of 

 the following substances : iron, copper, zinc, tin, mercury 



