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McC.iELitanp —On Secondary Radiation and Atomie Structure. 
Apparatus AND MrtHop oF OBSERVATION. 
The apparatus described in the previous paper is here somewhat modified to 
enable measurements to be made with substances in the form of powders or as 
liquids. 
The radium is placed at R, so that a stream of B and y rays passes through a 
hole in a thick lead screen, and falls on a plate P of the substance under examina- 
tion, which is supported on a stand S. When a liquid or powder is used, it is 
contained in a small tray with very thin mica walls, so that little radiation comes 
from the walls of the containing vessel. 
The secondary radiation is measured by the ionisation it produces in the.vessel 
7; this is a brass tube of 20 cms. length, and 7°5 ems. internal diameter, the end 
towards P being covered with a sheet of tinfoil about 013 mm. thick. An inner 
terminal, insulated by a paraffin cork, is fixed along the axis of 7, and joined to a 
Dolezalek electrometer; 7’ is kept at a high potential by joining it to storage 
cells, and the rate of charge of the inner terminal is measured by the electrometer 
in the usual way. 
The distance from FR to P is about 25 ems., and from P to the tube 7 about 
9:5 cms. The secondary radiation has therefore to pass through 9°5 cms. of air, 
and a sheet of tinfoil ‘(013 mm. thick, before it reaches the ionisation chamber ; 
any easily-absorbed radiation is therefore excluded, and, as shown in the previous 
paper, we are dealing with a radiation consisting only of 6 particles. 
The vessel containing the radium and the air-space between it and the plate P 
absorbs all the a rays and the more easily-absorbed 8 rays, so that the radiation 
acting on the plate P is of a penetrating character, consisting, as it does, of the 
more penetrating 8 rays and of y rays. 
A large number of elements have been compared by this apparatus as regards 
the intensity of the secondary radiation from them. Some of the substances were 
in the form of plates 5 cms. square; others were used as powders. In that case 
they were contained in small mica trays of the same size, and, to make the condi- 
tions as much alike as possible, substances which were only available as powders 
were tested against another substance in powder which was also available in the 
form of a plate. 
In every case a sufficient thickness was used to absorb all the incident 6 rays. 
Too small a value was assigned to aluminium in the previous paper, because a 
sufficient thickness was not used: the scale of the other numbers in that paper 
is also somewhat altered by making a small correction in the value of the secon- 
dary radiation from lead. Great care has been taken in comparing substances which 
give nearly the same secondary radiation, so as to place them in the proper order; 
and it is believed that the errors of observation will not be found to have in any 
