[eve] ABSORPTION BY AIR OF THE BETA RAYS 67 
Summary. 
1. By varying the distance r between some radium and a thin- 
walled electroscope, the law of absorption by air of the complex rays 
from radium C has been established as approximately exponential. 
2. If I is the ionization due to # rays in the electroscope at a 
distance r from the radium, then Ir? varies as e—” nearly, where pis 
the coefficient of absorption by air of the B rays. 
3. The values of # have been found as -0033 em.—! at the shorter 
range 60 to 160 cm., and -0045 at longer ranges 2 to 5 cm. 
4. If air absorbs # and y rays respectively in the same proportion 
as does aluminium, the coefficient of absorption of the vy rays by air is 
between -000021 and -000031. 
5. With limitations, the average effective range of the # rays from 
their source may be stated as 2-5 metres. In the same sense, the aver- 
age path of the y rays is 250 metres, if the y rays consist of entities with 
linear paths. 
Measurements have recently been made to determine the value of p, 
when f rays, from the active deposit of radium on thin aluminium foil, 
pass to a thin-walled electroscope through air. The method employed 
was that given above, and the range was from 40 to 100 cm. The 
values of # found in a number of experiments were from -012 to -013, 
about three or four times as large as when radium in a glass tube was 
employed at longer ranges. The explanation appeared at first to be 
simple, and the increased value of was attributed to the # rays from 
radium B. I found, however, that this large value of the coefficient of 
absorption persisted from 14 to 90 minutes after the thin foil was re- 
moved from the radium emanation, during which time the proportion 
of radium C to radium B was increasing. This point requires special 
investigation, but at present it appears that radium C emits a con- 
siderable proportion of slow § rays, comparable to those from radium B. 
The active deposit of thorium was also examined from 20 to 30 em. 
from the centre of the electroscope. In this case the 9 rays from Th A 
and Th D pass into and through the electroscope. The value of yw for 
air was in this case between -024 and -025, but the intensity of the 
radiation at my disposal was insufficient for a good determination. 
A. 8. Eve, 
May, 1911. 
