Messrs Campbell and Wood, The Radioactivity , etc. 
15 
The Radioactivity of the Alkali Metals. By Norman R. 
Campbell, M.A., Trinity College, and Alexander Wood, B.A., 
Emmanuel College. 
\Read 12 November, 1906.] 
§ 1. It bas been shown in a recent paper* that the natural 
radioactivity of the elements is an atomic property, and that 
the radioactivity of a compound can be calculated when that of 
its constituent elements is known. This discovery makes it 
possible to investigate the activity of elements which could not 
be examined previously with any convenience. 
It seemed of especial interest to examine the alkali metals : 
accordingly experiments were made on potassium sulphate. The 
apparatus and methods used were those described in the paper 
just mentioned. It was found that this salt was much more 
active than any substance previously tried which did not contain 
one of the ‘ radioactive elements.’ Thus the activity of lead as 
measured by the method adopted is 9'3 arbitrary units : that of 
potassium sulphate is 70. But it soon became apparent that 
the numbers for lead and for the potassium salt were not strictly 
comparable : for the rays from the potassium salt were far more 
penetrating than those from the lead and were not all absorbed 
by the air in the testing vessel. When a sheet of paper of a 
surface density equal to that of a layer of air 3 5 centimetres 
thick was placed over the potassium salt, so as to absorb the 
rays coming from it, no appreciable difference in the ionisation 
was observed. The same sheet of paper would cut out more 
than half of the ionisation due to the rays from lead or from any 
known radioactive element. 
§ 2. It seemed likely at first sight that the remarkable 
activity of the potassium salt was due to some radioactive 
impurity contained in it. But on consideration it appears that 
an impurity consisting of any of the known radioactive elements 
would not account for the phenomena. By far the larger part 
of the ionisation caused by such elements is due to the a rays 
emitted by them : but the most penetrating a rays known would 
’*e more than half absorbed by the sheet of paper which had 
nii effect on the ionisation due to the potassium. However a, 
°f fanatic search for an impurity was undertaken, 
the ry (l) The sample of potassium sulphate used was tested 
m tim emana {q on> A saturated solution of the salt in water was 
the livi n a closed vessel for four weeks. At the end of that 
The 
which Wt * Proc. Camb. Phil. Soc. xm, p. 282. 
