Iv] NATURE OF THE RADIATIONS 93 
of the deviation of the a rays compared with that of the @ rays is 
much exaggerated in the figure. 
67. Ionizing and penetrating power of the rays. Of 
the three kinds of rays, the a rays produce most of the ionization 
in the gas and the y rays the least. With a thin layer of un- 
screened active material spread on the lower of two parallel plates 
5 ems. apart, the amount of ionization due to the a, 8, and y rays 
is of the relative order 10,000, 100,and 1. These numbers are only 
rough approximations, and the differences become less marked 
as the thickness of the radio-active layer increases. 
The average penetrating power of the rays is shown below. In 
the first column is given the thickness of the aluminium, which 
cuts each radiation down to half its value, and in the second the 
relative power of penetration of the rays. 

| Thickness of 
Aluminium in cms.| Relative power 
which cuts off half | of penetration 
the radiation 
Radiation 

| a rays 0:0005 cms. 1 
B 0°05 cms. 100 
MW By 8 cms. 10000 





The relative power of penetration is thus approximately inversely 
proportional to the relative ionization. ‘These numbers, however, 
only indicate the order of relative penetrating power. This power 
varies considerably for the different active bodies. 
The « rays from uranium and polonium are the least pene- 
trating, and those from thorium the most. The @ radiations from 
thorium and radium are very complex, and consist of rays widely 
different in penetrating power. Some of the @ rays from these 
substances are much less and others much more penetrating than 
those from uranium, which gives out fairly homogeneous rays. 
68. Difficulties of comparative measurements. It is 
difficult to make quantitative or even qualitative measurements of 
the relative intensity of the three types of rays from active sub- 
stances. The three general methods employed depend upon the 
action of the rays in ionizing the gas, im acting on a photographic 
