with special reference to solutions of its salts. 559 
In the first paper by Mr Wood and myself on the activity of 
potassium experiments were described on samples of salts derived 
from different sources. These experiments have been repeated 
with greater accuracy. Three samples of the sulphate (Nos. 1, 2, 3 
of Table I) were procured from different chemical manufacturers : 
their activity was identical within the limit of error. A fourth 
sample (No. 4) was prepared from the ashes of burnt wood: it was 
known to be slightly impure, but its activity does not show any 
marked difference from that of the commercial samples derived 
from the Stassfurt deposits. Similar agreement was found 
between two samples of the chloride (1 and 2) and between 
two samples of the nitrate. 
It is indeed inherently improbable that any difference in 
commercial samples should be found, whatever the nature of 
the active principle : for nearly all commercial samples come 
from the same source. But the agreement of the sample derived 
from wood ashes seems to show that the association of the active 
principle with potassium is very close and that separation, even if 
it is possible, is likely to be very difficult. 
§ 4. When different salts of potassium were compared, it 
became apparent that the activity of a thick layer is not 
simply proportional to the percentage of potassium contained in 
the material of which it is composed. The variations from pro- 
portionality are shown in Table I, where the last column gives 
the activity of a thick layer of K.,0 calculated from the content 
and the activity of the material named in the first column. It 
must be remembered that the activity of a thick layer of an active 
substance is proportional not only to the content of the active 
element, but also to the ratio of the density to the absorption 
coefficient of the material for the rays which it emits. If Adm is 
the intensity of the total radiation emitted by a small particle of 
the substance of mass dm, X the absorption coefficient of the 
substance for its own rays and p the density, it is easy to show 
that the intensity (/) of the radiation (the absorption of which is 
supposed to follow the exponential law) emerging from a layer of 
thickness x is given by 
1= Ap/X (1 - e~ Kx ) (1), 
and the intensity /„ of the radiation from an infinitely thick 
layer is 
I 0 = A .p/X (2). 
If the proportion of the active substance by weight present is a, 
we must substitute ap for p. For all substances the quantity 
A should be constant, where 
A = — . X/p 
i 
( 3 ). 
