The Radioactivity of the Alkali Metals. 
21 
§ 8. The ratio between the activities of the elements as 
measured in this way showed that it might not be impossible to 
obtain a photographic impression from the potassium rays. The 
experiment was tried. A sensitive (Cadett Spectrum) plate was 
enclosed in a light-tight paper envelope and placed immediately 
under a thick brass plate pierced with several holes : on the brass 
plate a packet of paper containing potassium sulphate was placed. 
An exposure of 28 days was given : on development the outline 
of the plate and of the holes in it could be distinctly traced. 
Several other plates were exposed at the same time in such a 
way as to provide ‘ blank ’ experiments : in no other case was 
the same effect obtained. 
Experiments are now in progress with the object of using 
the photographic effect for measuring the deflection of the rays 
in a magnetic and electric field. 
Summary. 
1. It is found that potassium salts exhibit a radioactivity 
greater than that of any other substance tried hitherto, which 
does not contain one of the so-called radioactive elements. 
2 — 3. A searching investigation showed that this radio- 
activity is an intrinsic property of the potassium and is not due 
to any impurity contained in the salts. A comparison of the 
activities of different salts of potassium shows that the activity 
is an atomic property. 
4. The penetration of the rays from potassium salts has been 
measured. The rays are heterogeneous and vary in penetrating 
power from that of the /3 rays of uranium downwards. 
5. The activity of rubidium salts is less than that of potassium 
salts, and that of caesium, sodium and lithium salts is too small 
to be measured. The penetration of the rubidium rays is much 
smaller than that of the potassium rays. 
6. Considerations are urged tending to connect this activity 
with the photo-electric effect. 
7. The activity of potassium as measured by its ionising 
power is one-thousandth of that of uranium as measured by the 
ionisation caused by the ft rays from that substance. 
8. An attempt, which appears to be successful, has been 
made to obtain a photographic effect from potassium rays. 
