McCiettanp— On Secondary Radiation. 175 
ducing secondary radiation. The less penetrating 8 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 y rays. But even when the curve 
A has become practically horizontal—that is, when there is nothing left in the 
primary pencil but y rays—there is still some secondary radiation. 
The magnetic experiments described later show that this part of the secondary 
radiation—the part produced by y 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 being 
taken at 100 :— 
Secondary 
Selina, Radiation Dea Wout. aan 
Atomic Weight.| 
Lead, . : 2 : 100 11:4 | 206-4 0:48 
Digi, 5 5 < 92 a5 | 194-3 0-47 
TiN: per De eae 80 73 | 1188 0:67 
Brass, . ; : ‘ 75 8°4 
Zine, . : : a: 65 UR | 65:0 1.00 
Copper, . ° : : 64 8:9 | 63:2 1:01 
Ibo, : : : 62 7:8 56:0 | 1:10 
Glass, . ; : : 31 Del | | 
JAN orrnnirvii 7a 33 2°7 My | Loy 
Ebonite, ; : : 27 1:15 
Cardboard, . es : 23 0°80 
Wood, . : 3 : 19 0°52 
Paraffin, ; : 3 18 0:90 
The numbers show that, taken generally, the substances of greater density pro- 
duce the greater secondary radiation, but there are many exceptions to the rule, 
