152 
IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
the photographic method, but the relative intensities of the scat- 
tering at different angles measured by the ionization method is 
greatly different from that estimated from the photographic 
method. The liquids give only one maximum of scattering, which 
occurs a few degrees away from the incident beam. The scatter- 
ing then decreases to a minimum at 90° and then slowly increases 
again. The measurements have extended from 2° to 166°. In 
all cases the scattering approaches zero close in to the incident 
beam. For the solid substances mentioned the intensity of the 
scattering actually becomes zero within a narrow region accessible 
to the ionization chamber. 
The total amount of scattered radiation from the scattering sub- 
stances has been estimated in the following way: It has been 
assumed that a beam of X-rays in passing through a piece of 
matter is partly scattered in a series of coaxial cones, the intensity 
of the scattered energy being uniform between two cones of semi- 
apex angles 0 and #+d(9, Measurements of the intensity of the 
primary beam of X-rays were made, and the total scattered energy 
calculated from the scattering curves on the above assumption 
was expressed as a fraction of the primary beam. The total mass 
absorption coefficient of the primary’ beam was determined at the 
same time. The mass scattering coefficient, and the true mass ab- 
sorption coefficient of the substance can be determined* from these 
measurements and a knowledge of the linear dimensions and den- 
sity of the scattering material. Recently a method has been de- 
veloped for determining the fraction of the primary beam scattered, 
by continuously rotating the ionization chamber at a predetermined 
variable speed, which is so determined as to automatically perform 
the integration which was previously worked out from the scat- 
tering curve. Both methods give mass scattering and true mass 
absorption coefficients in good accord with J. J. Thomson’s theory. 
The hydrogen values were calculated from the results on benzene 
and mesitylene. 
State University oe Iowa. 
