McCann anv Hacxerr—Secondary Radiation from Compounds. 29 
APPARATUS FoR THE EXPERIMENTAL DETERMINATION. 
The apparatus used was the same as in the later determinations of the 
radiations from elementary substances, but for convenience it may be briefly 
described here :— 
The radium is placed at #& (fig. 1) so that a stream of 8 (and y) rays passes 
through a hole in a thick lead screen and falls on P, the substance under 
examination, which is generally in the form of a fine powder contained in a 
small mica vessel resting on the support S. 
The secondary radiation which is emitted by the substance P in all directions, 
is measured by the ionisation it produces in the vessel 7’; this is a brass tube of 
20 cm. length and 7:5 em. internal diameter, the end towards P being covered 
with a sheet of tinfoil. An inner terminal, insulated by a paraffin cork, is fixed 
along the axis of 7’ and joined to a sensitive electrometer; 7’ is kept at a high 
potential and the ionisation is measured in the usual way by the rate of charging 
of the inner terminal. 
The distance from R to P is about 25 cm., and from P to the tube 7’ about 
9°5 cm. ‘The vessel containing the radium and the air space between it and the 
substance at P absorb all the a rays and the more easily absorbed 8 rays, so that 
the radiation acting on the substance consists chiefly of 6 rays with some y rays. 
In every case the upper surface of the substance under examination was placed 
in exactly the same position, and a sufficient thickness was used in each case to 
absorb all the incident B rays, so that the incident energy absorbed was the same 
for each substance tested. Of course corrections were always applied for what- 
ever small ionisation existed inside Z’ when the substance P was removed and 
the stand S lowered. 
MerHop or CancuLatring THEORELICALLY THE SECONDARY RADIATION FROM A 
ComMPouND. 
In one of the previous papers* referred to above, the secondary radiation from 
a large number of elements was determined, the intensity of the secondary radiation 
being expressed as a percentage of the primary radiation that produced it, the 
thickness of substance used in every case being great enough to allow of no 
transmission of Brays. This percentage we shall denote by the letter p, as in the 
previous paper. 
In that paper another constant for each element was deduced theoretically from 
the known value of p; this constant was denoted by x, and expressed the ratio 
* Trans. Roy. Dublin Soc., vol. ix., part 11., 1906. 
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