MeCietitanp— On Secondary Radiation. 177 
corresponding number for each substance is less than before, when the rays passed 
through tinfoil only ; the relative diminution is different for different substances, 
and is particularly great for ebonite, cardboard, and paraffin. In other words, of 
all the substances examined, the secondary rays from lead are the most penetrating ; 
those from paraffin, cardboard, and ebonite the least penetrating. ‘The difference 
in penetrating power of the rays from the different substances is not very 
marked. 
A rather rough experiment was tried to see if the temperature of the 
substance had any effect on its power of giving off this secondary radiation. A 
plate of copper was used, and its temperature increased by a strong bunsen-flame 
playing on the back of the plate: no change in the amount of secondary radiation 
was noticed. It should also be stated that no particular care was taken with the 
preparation of the surface of the different plates used; the surface was always 
well cleaned, but the degree of polish was not attended to, and did not seem to 
be of any importance. 
As all numbers in this paper are given in arbitrary units, it may be useful to 
state a result specifically to give an idea of the absolute magnitude of the radiation 
we are dealing with: 50 milligrams of radium bromide are placed 26 cms. from a 
lead plate as in fig. 1, the rays from the radium passing through a hole 1-2 em. in 
diameter in a lead screen; the lead plate is 9 cms. from the ionisation tube 7’; the 
electrometer gives a deflexion of 2000 scale-divisions per volt, and the total 
capacity of the system charged is ‘001 microfarads; the secondary radiation then 
produces a deflexion of 115 scale-divisions per minute. 
Relative Importance of Secondary 8 Rays. Penetrating Power of Secondary B Rays. 
In making some experiments on the penetrating power of the secondary 
B rays, and comparing it with that of the primary rays, some results were obtained 
WW ELECTROMETER 
> CELLS 
Fic. 4. 
which show that very different values may be deduced for the absorption of 6 rays 
according to the apparatus used, and which also show very forcibly the importance 
TRANS. ROY. DUB SOC., N.S., VOL. VIII., PART XIV. 2H 
