caused by the B and y Rays of Radium. 679 
There seems a tendency to form groups which may be more 
than fanciful. In any case the chief effect of the correction 
is to accentuate the high radiation from lead, granite, 
and slate. 
The error caused by the thin aluminium face of the electro- 
scope has a balancing effect in the opposite direction. 
Although this screen may reduce the recorded radiations 
below the actual values, the relative error will not exceed 2 
per cent., except in the case of the more penetrating rays, 
such as those from granite or slate. 
Theory.—The 8 rays are known to consist of negatively 
charged particles moving with different velocities. With 
these rays are present the y rays of greater penetration, 
probably of the type of Réntgen rays. The large range of 
velocities of the @ rays causes a wide variation in their degr ee 
of penetration. It is therefore impossible to assign any 
definite values for the initial coefficients of absorption of most 
substances exposed to the B rays. This difficulty has pre- 
vented the reduction of the preceding results to any definite 
mathematical theory.” For example, with aluminium, using 
the primary 8 rays we have :— 
Thickness in mm. r. 
OF to. “0 6°D 
“OO LOr2' aa 
pgs Mime Roe aU 16 
The theory which agrees most nearly with the results is 
that the secondary radiation set up ina thin layer is pro- 
portional to the density and to the rate of absorption of the 
primary rays. Ifthe amount absorbed in thickness dx be 
dl, then the secondary radiation is kpdI, where p is the density 
and kis a constant. The amount which reaches the surface 
will be kpdle~*" , where 2’ is the coefficient of absorption of 
secondary radiation. Now I=I,e~*, where ) is the coefti- 
cient of absor ption for primary radiation, and I, is the surface 
intensity of the primary rays. Therefore the total secondary 
radiation is :— 
fi ; kpr 
Or —(A+A))zr ar ee 
li kpre dx Aen 
; Ber r 
The ratio of the secondary radiation to eat was found 
_to be ey constant for lead, glass, wood, and paper, but 
brass and aluminium showed large deviation. The attempt 
to prove this theory was therefore abandoned, ‘as it was 
impossible to assign definite values to A. 
