202 Hacxrert—The Secondary Radiation excited by y Rays. 
The intensity is practically the same for all elements whose atomic weight lies 
below 150, but shows a great increase for lead and the other elements which 
belong to the same period, and a second marked increase for uranium, the 
element of highest atomic weight which makes up a period with the radio-active 
elements, radium and thorium. 
When y rays pass through a plate, they may produce an emission of 
secondary radiations of various kinds; but it is only the penetrating kind, 
consisting of 8 rays, or corpuscles travelling with high velocities, which is 
measured in this paper. 
APPARATUS. 
In these experiments the source of the y rays was 50 milligrammes of 
(SL Sa ES a II ITE PS Lic 
3 Electrometer. 
radium bromide enclosed in a glass vessel, sealed so that no emanation escaped 
during the observations. The radium (RZ, in figure) was placed at the bottom of 
a hole in a rectangular block of lead, which was built up of smaller blocks whose 
dimensions were 9 cm. x 7cm.x 5cm. The aperture defining the pencil of y rays 
was made by two lower blocks, each with a cylindrical hole in the centre, and a 
third with a conical opening, so as to obtain a well-defined pencil combined 
