Electric Field of the a, ft, and Secondary Rays. 12?> 
The values of X are move difficult to determine, but using an 
aluminium screen 0'02l cm. thick, the values were about — 
Primary 24 
Secondary 29 
Tertiary 60? 
It is strange that whilst secondary rays from lead are so like 
the primary rays which cause them, yet the tertiary should be 
different in absorbability from both. Judging from the curves 
published by Allen, the tertiary rays from lead are absorbed 
at the same rate as the secondary rays from paper, and my 
results seem to agree; For the tertiary from lead and the 
secondary from carbon have values for X not far apart. 
Penetrating Secondary Rays. 
If lead and brick are respectively examined as radiators, 
and their radiations are in turn cut off from the electroscope 
by a thick book, or by a sufficient thickness of aluminium, it 
will then be found that brick becomes a more efficient radiator 
than lead. If curves are plotted with the thickness of screens 
as abscissae and the ionizations in the electroscope as ordinates, 
then the curves cross one another, The ionization values 
obtained in one experiment w T ere : — 
Table II. 
Thickness of Aluminium 
Screens. 
Lead. 
Brick. 
cm. 

106 
48 
•04 
37 
15 
•08 
20 
10 
•16 
8-3 
7-9 
•25 
57 
7*7 
•50 
4-3 
6-9 
The lead and brick radiators were both a little more than 
5 cms. thick, and of nearly the same area. Again, using 
two stout books as screens (Drude's 'Optics' 7 and Wood's 
' Optics '), protected in all cases from the radium by 15 cms. 
of lead, the figures obtained were — 
No radiator 4*1 divisions a minute, 
Lead radiator 4'8 ,, „ 
Brick radiator 6'8 
