of Radium, and Secondary Rays icJiiclt they produce. 639 
of three groups of substances. Thus, i£ the three groups of 
rays be denoted by A, B, and C, then the group A is better 
absorbed by one of the three groups of substances than either 
of the groups B and C, and the group B better absorbed by 
one of the other groups of substances than either of the 
groups A and C, and lastly the group of rays C is better 
absorbed by the remaining group of substances than either 
of the groups of rays A and B. To give an illustration, the 
ratio of the secondary cathode radiation from lead to that 
from zinc is decreased if the 7 rays are passed through a 
thick screen of lead before falling on the radiators, showing 
that the rays producing the larger part of the radiation from 
zinc are decreased in a less proportion by the lead screen 
than the rays producing the larger part of the radiation from 
lead. 
It became of interest, therefore, to investigate the pro- 
perties of the secondary 7 rays from different substances 
exposed to the 7 rays of radium, and to compare their pro- 
perties with those of the primary rays. Moreover, it was 
thought that such an investigation might lead to an indirect 
confirmation of the above result. The experiments described 
in this paper were accordingly undertaken. 
It appeared from the experiments described in the paper 
mentioned, that the amount of secondary 7 radiation, as 
measured by its ionizing power, is small in comparison with 
that of the incident primary radiation. It was necessary, 
therefore, to design a sensitive apparatus for measuring the 
secondary radiation. 
teelect 
A vertical section of the apparatus used is shown in fig. 1. 
A is a quantity of radium (about 30 mgrs.) in a glass tube 
surrounded by lead sheeting 2 mm. thick to cut off the 
