678 Mr. A. 8. Eve on the Secondary Radiation 
penetrating secondary rays are given by granite, slate, brick, 
and cement. 
If three centimetres of water be placed in a thin tray 
on a brick or slate slab, the secondary radiation will be 
decreased by placing a thin sheet of lead at the bottom of the 
water. The lead radiates more than the slate or brick, but 
its rays are less penetrating ; and because it screens more 
than it causes there is a net loss. 
A similar experiment may be made with glass or wood 
raised above slate. A sheet of lead placed below the glass or 
wood will decrease the total radiations. 
The total radiation from a number of layers of different 
substances may therefore be dependent on the order in which 
they are arranged. 
The different values obtained for the coefficients of absorp- 
tion by aluminium, when various substances are used as 
radiators, point to the fact that the absorptions of these rays 
by the air in the electroscope will be different. For example, 
iron gives secondary radiation 75 and brick 49; but d/ for 
iron is 20 and for brick 13. Therefore the radiation from 
brick will pass through the electroscope with less absorption 
than that from iron. The energy absorbed by the air in the 
two cases will be in the ratio of 13 to 20. The corrected 
value from brick relative to iron will be 49 x a or 75°4. 
If similar corrections are made with ebonite as standard, 
and applied to Table I., we can arrange the results as 
follows :— 
Sec. Rad. 
Substance. Secondary Radiation. Density. 
OPPS cnsndls ee 103 it 
BPRS) gat ot bee 100 11-9 
PA a... bos eons 98 1365 
7 Ee Re ate ef 93°5 12°6 
Aluminium ...... 40 15°4 
eae? C2 Soh eee 198 i ie 
Casa Re oa 45 19°2 
Pam se 21 a3 
Bbonite.. 650% 32 29 
Water’ io t.weo! 30) 30 
Mahogany ...... 24 37 
Gramie sii: she 113 42 
DIMEA aw Sie a ee 114 A4 
Peper | pr 2 acre ee 98 47 
Millboard ........ 24°6 61 
es ee eee 25 re 
Bese wOOw 2... ies pa 75 
: lod 
Pie ee ee ay 1 
