M‘Cievtanp— The Energy of Secondary Radiation. 11 
To test how the method worked, the radium was removed, and a little 
uranium oxide placed on the lower plate. We know that the radiation 
passing between the plate in this case does contain easily-absorbed a rays. 
Tt was found that with the plates less than 10 mm. apart, the current was 
approximately proportional to the pressure up to atmospheric pressure; but at 
greater distances apart, the curve of current against pressure was at first a 
line through the origin becoming nearly parallel to the axis of pressure as the 
pressure was increased. 
TOeeoeoe Zo 
Boca ee za aE | 
HOS OOS 0002s ea Oceaorace 
EEE} tpt tet DOGO SEbos 4 
(| a a eZ topeneenen 
| ane OOO 
Ree EE AEE pepe d 
9 Ye a OEE 
tt eA EE a a 
OUD Ee Coe DEC ESa Se SSS leak 
Ee EE dp naan 
(a | | | || | FZ 
, GOCUDOCAO OOOO DOSE eo eoeehee 
eR At tt eb bd 
el A a a | 
6 LUO CL ee See eS eae 
a 
A 8 || ec 
JOO ODL 4s oe esas 
COUDY EEO aco SSS eSsbeaheees 
eee 
a a a | a 
EPL ERE EEE 
a 
Bees eee oo deer ee eee ooo 
pote be 
Eee EEE EEE EEE 
sissies elnisimiaioicls BE SESSeSee eee) 
S 4 5 
Fic. 1.—PreEssurr IN cm. 
The result of one experiment is givenas an example. Curve A, fig. 1, 
was obtained with the plates 4°5 cm. apart, the ionisation being produced by 
the direct 8 rays and the secondary radiation; there is no evidence whatever 
of the presence of any easily-absorbed radiation. Curve B on the same figure 
refers to an experiment with the plates the same distance apart, but the ionisation 
was produced by uranium oxide on the lower plate. The curve shows at once 
the presence of the easily-absorbed a rays which have been almost totally absorbed 
at about 35 cm. pressure. 
C2 
