The Radioactivity of the Alkali Metals. 
19 
uranium : for those rays be found that - = 9'5. The margin of 
error in the determination of - is about one unit. 
P 
It will be seen that the rays are very heterogeneous. They 
seem to vary in penetration from that of the very penetrating 
/3 rays of uranium downwards. But there is no sharp break 
in the curve representing the relation between the absorption 
and the thickness of absorbing material such as is found in the 
case of uranium and other elements which emit both a and 
(3 rays. 
§ 5. The other alkali metals have been examined to ascertain 
whether they show the same remarkable phenomena as potassium. 
Sodium, lithium and ammonium salts showed no more activity 
than the salts of the least active metals, such as zinc. Rubidium 
alum was tried and found to give effects of the same nature and 
about the same amount as the potassium salts. The activity of 
rubidium alum on the scale adopted throughout this paper is 
128 : but rubidium alum contains almost exactly one-sixth of 
its weight of rubidium metal. Hence the activity of the metal 
is 768. 
The rays from rubidium were found to be much less pene- 
trating than those from potassium. The absorption of the tin 
foil for the rays was so great that paper was used in place of 
the tin foil. It had been found that - for the same paper and 
the rays from potassium varied from 10'4 to 5’0. The value of 
the same quantity for the rays from rubidium is about 160. 
The smaller intensity of the rubidium rays and their greater 
absorption makes the accurate measurement of X impossible. 
Caesium alum was examined but showed no activity that 
could be measured. 
§ 6. The relation between the activity of potassium, rubidium 
and caesium deserves consideration. 
The penetration of the rays from potassium suggests that they 
are /3 rays : no measurements have yet been made to determine 
the sign of any charge that they may carry, but it seems probable 
that they will be found to be negatively charged. Now it is well 
known that the alkali metals give off copious streams of negative 
electricity under the action of light, and it has been shown by 
Prof. Thomson* that rubidium and the alloy of sodium and 
potassium give off’ negative electricity even in the dark. It 
seems very probable that the negatively charged rays which he 
detected consist, at least in part, of the ionising rays which have 
* J. .T. Thomson, Phil. Mag. Nov. 1905. 
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