Hacxertr—TZhe Secondary Radiation excited by y Rays. 205 
the other. The intensity, then, after transmission is unaltered if the plates are 
interchanged ; and their radiating powers under the same intensity of y rays is 
immediately obtained. Ifthe rays were homogeneous, this assumption would not 
require verification. But, owing to the heterogeneity of the y rays, it was 
confirmed by direct experiment that the intensity of the rays after transmission 
through two or even three of the plates used in these observations was 
independent of their order of arrangement. 
In actual practice it was difficult to obtain numbers with the imcomplete 
apparatus just described. The amount of secondary radiation due to y rays is 
small. The numbers for the radiation from lead obtained with this apparatus 
were of the same order of magnitude as the natural 6 radiation from a tray of 
uranium powder of the same size. ‘To obtain numbers it was necessary to join 
up the inner terminal direct to the quadrant of the electrometer without any 
additional capacity, and to encase the connexions everywhere in glass tubes filled 
with paraffin and covered with tinfoil. The wire from the terminal was thus 
completely shielded and protected from disturbing influences of ionised air. 
There was no difficulty in obtaining steady readings. Under these conditions 
the readings for lead were 90 and 60 respectively, giving thirty divisions per 
minute for the secondary radiation from lead. The correction here is too large 
to obtain the difference accurately, and there was, from its character, no direct 
means of further reducing it. An additional capacity reduces both numbers 
at the same rate, which is no gain. This difficulty was surmounted by the 
introduction of the compensating device shown in the figure. The wire from the 
inner terminal ran through a short brass tube (C) along its axis, insulated and 
supported by the paraffin plugs which closed each end of the tube. The length of 
the tube was 20 cm., and its diameter 6 cm. The outside of the tube could be 
connected to a high potential of the opposite sign to that of the large cylinder. 
A small quantity of radium (S’) brought near the tube produced a current in it to 
the inner wire, which partially or even completely neutralised the current to the 
terminal in the large cylinder. By this means the correction was reduced to a 
convenient magnitude. It was comparatively easy to obtain a null point, such 
that the spot of light of the electrometer would not move more than one or two 
divisions in three minutes; but it was found simpler to have a slow rate of 
deflection for the correction. There is no trouble in arranging this; and the 
manner of taking the observations remains unchanged. 
RESULTS. 
The results of the investigation are given in Table I. In this table the 
elements are separated into their chemical periods. It will be noticed that the 
first two chemical periods, in so far as they are represented, stand out from the 
