20 
Messrs Campbell and Wood, 
been described in the foregoing paragraphs. But under the 
action of light, caesium is more active than rubidium and 
rubidium than potassium : whereas the intensity of the ionising 
rays emitted is greater in the case of potassium than in that 
of rubidium, and greater for the latter metal than for caesium. 
However I do not think it is necessary to conclude that there 
is no connection between the two effects. For the intensity of 
the rays emitted by a thick layer of any substance depends on 
the absorption of the substance for the rays that it admits. If 
1 0 is a constant measuring the true activity of the substance 
and X is its coefficient of absorption for its own rays, the intensity 
of the rays emerging from a layer of infinite thickness is 
lo 
X ' 
Now the penetration of the rays from rubidium is only ^ of 
that of the rays from potassium. Hence if allowance is made 
for the difference of penetration it would appear that the ratio 
of the activity of rubidium to that of potassium is about 12. 
The well-known relations between the properties of potassium, 
rubidium and caesium would lead us to expect that the difference 
between the rays from caesium and the rays from rubidium is 
of the same kind as that between the rays from rubidium and 
those from potassium : it is to be expected that the rays from 
caesium would be even less penetrating than those from 
rubidium. Caesium may be pouring off negatively charged rays 
at a greater rate than rubidium, but they may possess so little 
energy that they are incapable of ionising a gas. Such rays 
might give a large photo-electric effect but they would give no 
ionising effect. 
(It is worth mentioning that the experiments described above 
cannot have been influenced by the photo-electric effect. The 
vessel containing the potassium is almost perfectly light-tight 
and no difference in the effect waS observed whether the 
laboratory was filled with bright sunlight or dimly lit by 
artificial light.) 
| 7. The intensity of the ionisation caused by the rays from 
potassium sulphate was compared roughly with that caused by 
the a rays from uranium : an accurate comparison is impossible 
without erecting new apparatus, for, since the penetration of the 
rays is different, the proportion absorbed in the air of the testing 
vessel will be different. It was found that an area of a layer of 
uranium oxide ‘785 cm. 2 in area gave *795 times the ionisation 
caused by a layer of potassium sulphate of the same thickness 
with an area of '961 cm. 2 (The uranium was covered by a sheet 
of tin foil to cut off the a rays.) Hence the activity of uranium 
as measured by the (3 rays from a layer T5 cm. thick is about 
1000 times that of potassium. 
