42 THE ROYAL SOCIETY OF CANADA 
coming from known radioactive substances in the soil, it follows, 
therefore, that about 4-2 ions per c.c. per second must be accounted for 
by the presence of other types of radiation. 
From the observations made by a number of investigators, it is 
clear that one of these types is the secondary radiation produced at the 
walls of the zine receiver by the penetrating rays themselves. A second 
type which it was thought might possibly be present and contribute to 
the ionization in the receiver was a secondary radiation produced at 
the surface of the ground by the penetrating rays coming from the soil. 
Some observations were taken which seem to establish the existence of 
this type. 
The measurements which led to this conclusion were made by 
taking readings on the ionization of the air in the zine receiver as it 
was gradually brought up to a high and extensive brick wall forming 
part of a brick building which bounded the University lawn on one of 
its sides. These readings are recorded in Table III. and are represented 
by a curve in Fig. 1. 
TABLE III. 
Observation Station. ‘““q”’ No. of ions per c.c. 
per second. 
At the centre of the lawn............... RAR 15-1 
iS</;metres tron theiwalle NE EEE" 15-4 
12-2 if a a Fh sue AR rete La es RE 15-45 
10-7 ‘ i BWP UR Eee (BaP ALERT ase 16-4 
ré 34 bh COLONEL POL POY Sees BEI ps oy RARE 16-7 
4-6 et : Sh Wank CURA Lia od Sule RNR fu 16-7 
+3 5 N A TE ER Se TA CP VU 16-5 
From the numbers given in the table it will be seen that the 
ionization began to increase when the receiver was about 14 or 15 metres 
from the wall. At the centre of the lawn the reading was 15-1 while at 
13-7 metres from the wall it was 15-4. At a distance of 10-7 metres 
the reading was 16-4 ions per c.c. per second, and at 7-7 metres it was 
16-7. From there on to the wall the readings remained practically 
steady. As it had been shewn previously that at any one place readings 
could be taken with the apparatus which did not shew a variation greater 
than 3 per cent., it is clear from the numbers given above that the wall 
emitted a radiation whose range extended out in the air from the wall 
to a distance of between 14 and 15 metres. The absorbing power of this 
amount of air is roughly equivalent to between 6-5 and 7 millimetres of 
aluminium, and this thickness it has been pointed out is practically 
