M‘Crietitanp—The Energy of Secondary Radiation. 23 
The column headed « in Table I. has been calculated from this result by 
substituting the known values of p for the different substances. It will be 
noticed that as much as 884 per cent. of the incident energy absorbed by any 
element of a lead plate is set free again as secondary radiation, the corresponding 
number for uranium being 89 per cent., and for the element of lowest atomic 
weight tested, carbon, the percentage is 45. 
For values of p in the neighbourhood of 5 « varies very slowly as p varies, 
so that an error in determining p for lead (the substance used) would not produce 
much effect on the calculated values of x. When the secondary radiation from 
different elements was plotted against the atomic weight in the previous paper, 
the substances fell into well-defined divisions, corresponding to the chemical 
periods; this plotting was the same as plotting p against the atomic weight. 
Now « isa more fundamental constant than p, and might be expected to bring 
out more clearly the divisions referred to; but this advantage is counterbalanced 
by the fact that the total variation of « is less than that of p. When « is plotted 
against the atomic weight, the divisions corresponding to the chemical periods are 
clearly shown, but no better than in the previous paper, where p was used. 
Some DEDUCTIONS FROM THE ABOVE RESULTS. 
The fact that such a large percentage of the energy of the radiation absorbed 
by any element of volume is set free again as secondary radiation has very 
important bearings in many directions. 
Consider, in the first case, the rate of absorption of B rays by a thick plate of 
a substance. ‘The true coefficient of absorption of the actual f particles incident 
on the plate is the quantity » in the above equations; the coefficient of 
absorption of the total radiation passing through the plate, partly primary 
and partly secondary, is not p, but is 
VA or be WK. 
In the case of lead, for example, « is about ‘88, so that the actual coefficient 
of absorption for lead is only one-third of what the coefficient would be if 
there were no secondary radiation. In other words, the average distance to 
which the incident 8 particles penetrate into the lead is only one-third of the 
distance they appear to penetrate, due to the successive generation of other 
B particles. 
Further, in the usual method of determining the coefficient of absorption, 
the intensity of the radiation is measured in some way; then a plate of the 
