236 



Prof. J. A. McClelland 



on 



various parts of the primary pencil in producing secondary 

 radiation. The less penetrating /3 rays are more efficient 

 than the more penetrating, and the efficiency falls off more 

 and more as we get into that part of the curve A corresponding 

 to the 7 rays. But even when the curve A has become 

 practically horizontal — that is, when there is nothing left in 

 the primary pencil but 7 rays — there is still some secondary 

 radiation. 



The magnetic experiments described later show that this 

 part of the secondary radiation — the part produced by 7 rays 

 — also consists of negatively charged particles. 



Different Substances as sources of Secondary Rays. 



The experiments described in this paper have all been 

 carried out (unless when otherwise stated) with a lead plate 

 as the source of secondary rays. 



A large number of substances have been tested as sources 

 of secondary rays. The substances were placed in the form 

 of plates at the position P, fig. 1. The experiment described 

 above in which tinfoil was used, shows that it is important 

 when comparing substances to use in each case a thickness 

 sufficient to give the maximum effect ; this was done when 

 comparing different substances. 



The following list shows the results obtained, the radiation 

 from lead beino- taken as 100. 



Substance. 



Secondary 

 Radiation. 



Density. 



Atomic 

 weight. 



Secondary 



Radiation 



Atomic weight 



Lead 



100 

 92 

 80 

 75 

 65 

 64 

 62 

 31 

 33 

 27 

 23 

 19 

 18 



11-4 



21-5 



73 



8-4 

 72 

 8-9 



7-8 

 2-5 

 2-7 

 115 

 •80 

 •52 

 •90 



2064 

 194-3 



118-8 



65 



63-2 



56 



27 



•48 

 •47 

 •67 



1-00 

 1-01 

 110 



1-22 



Platinum 



Tin 



Brass 



Zinc 



Copper 



Iron 



Glass 



Aluminium .., 



Ebonite 



Cardboard 



Wood 



Paraffin 





The numbers show that, taken generally, the substances of 

 greater density produce the greater secondary radiation; but 

 there are many exceptions to the rule, and the secondary 

 radiation is by no means proportional to the density. Platinum, 



