[Eve] ABSORPTION BY AIR OF THE BETA RAYS 63 
The values of # found for distances from 2 to 5 metres under various 
conditions, were as follows :— 
4.4,4.5,3.9,5.0cm. x 10 
5 — 1 
and the mean value was {#—-+0045 em. 
3 
Eleven milligrams of radium bromide in solution in a thick-walled 
glass bottle emitted some f radiation, which could be screened off by 
aluminium and zine sheets placed near the bottle. The values of / for 
these 8 rays were ,— 
distance 1 to 2 metres #4 — _ .00385 
FL PARTONS DUR NCIS 
Mean #— -0040 em. 
It is concluded that for absorption at distances from r—2 to r—5 
metres the value of {4 is approximately -0045 cm. Re 
Short Ranges. 
Some weaker specimens of radium were next tested at shorter 
distances. The specimens chiefly used were (1) a capsule with a thin 
mica cover containing 0-56 mg. of radium bromide (0-33 mg. of radium) 
and (2) some radium bromide in a thin-walled small glass tube, con- 
taining -53 mg. of radium, the preparation being at about one-quarter 
of full strength. The latter specimen was kindly lent to me by Pro- 
fessor H. A. Wilson. 
The radium tube was hung by a thread at distances varying from 
40 to 160 cm. from the electroscope, and readings were taken both when 
advancing and retiring the radium. When measuring the joint # and 
y ionization, it was necessary to withdraw the large and massive electro- 
magnet to a distance, otherwise the effect of “reflected” radiation was 
strongly marked. The / rays were then cut off by screens and a strong 
field, and the ionization due to 7 rays alone was determined. It has 
been shown by Schmidt, and by Kovarik, that the value of is affected 
by the material behind the radioactive source. The experiment may 
be divided into two series; those in which there was air, or carbon, or 
solid paraffin behind the radium tube; and those in which lead plates 
weie placed behind. The values for / in em. ~’ x10 were— 
Air behind 33, 33, 31 Paraffin behind 35, 38 
Carbon NEO Lead 37:94, 40 
As r increases from 60 to 140 em. the value of # is about -0033 with 
air behind, and about -0038 with lead behind. 
In Figure 2 are shown graphically the results of two typical series 
of experiments with air and with lead behind the tube of radium. 
