of Radium, and Secondary Rays which they produce. 651 
absorption of the secondary rays from different substances 
were carried out in the following manner : — Readings were 
taken, using no radiator, with a thin screen of the substance 
whose coefficient of absorption was to be found placed at C 
(see fig. 1), and then Avith a thicker screen of the same sub- 
stance in place of the thin one. The radiator, which was 
28 cm. long and 12 cm. broad in each case, was then placed 
at B, and the readings repeated. Each of the readings 
taken without the radiator was then subtracted from the 
corresponding reading taken with the radiator, the result 
giving the relative radiations corresponding to the two 
different thicknesses of the screen. The radiating plate was 
made as small as was possible without reducing the leak to 
an inconveniently small quantity. This was done in order 
that the maximum angle which the secondary rays made 
with the normal to the screen might be as small as possible, 
in the actual case the angle was about 45°. If \ denote the 
coefficient of absorption, we have then Ii = Io <?~ XlA , and 
I 2 = I e~ x i K , where \ is the leak in the chamber when the 
thickness of the screen is # ]5 and I 2 the leak when the 
thickness of the screen is x 2 , and I the leak that would be 
obtained without a screen. The elimination of I gives 
an equation from which X, is obtained. The value of the 
coefficient of absorption is somewhat influenced by the fact 
that the secondary rays traversed the screen at various angles. 
But it was found on calculation that this did not introduce 
any very appreciable error. Since the nature of the radiation 
from a substance depends to some extent on its thickness, 
the coefficient of absorption depends to some extent on the 
thickness of the radiator. Thus, in the case of a very thin 
radiator, the radiation would consist of the three groups of 
rays in almost equal amount; but as the thickness of the 
radiator is increased, the proportion of one of the groups is 
much decreased on account of being more easily absorbed 
by the plate than the other groups. It w T as on account of 
these considerations that the coefficient of absorption was not 
determined with greater accuracy than that given by two 
different thicknesses of the screen. 
Table IV. gives the results obtained w T ith the secondary 
rays from the radiators lead, zinc, and carbon, each of which 
belongs to one of the three groups of substances defined in 
this paper. The first column contains the substances whose 
coefficients of absorption were found. The coefficients of 
absorption of a substance corresponding to the different 
secondary rays radiated by the substances given in the first 
horizontal row, are given in the same horizontal row which 
