( 156 ) 
0.001 mm. thick. On this aluminium was now placed very carefully 
a quantity of the various salts so that the action must proceed 
through the aluminium. After the end of the experiment the foil 
was carefully examined for any holes formed but none were noticed; 
for the rest the arrangemen was the same as before. After an ex¬ 
posure of 97 days the same result was obtained as in the former 
experiments; the rubidium sulphate had caused a black spot on the 
plate but the other salts had not affected the same. The effect of 
the rubidium sulphate was, however, weaker than in the previous 
experiments. 
The general result, therefore, is that rubidium exhibits a stronger 
action than potassium or sodium, both of which had during the time 
of the experiment no marked influence on the plates. (In one ex¬ 
periment with potassium plates there probably was an action but so 
faint that it must be considered doubtful). As regards sodium, this 
agrees with the results of previous investigators; as regards potassium, 
however, it does not. This difference may, perhaps, be attributed 
to the fact that they made use of rubidium-alum, which only contains 
about 16 °/ 0 of rubidium whereas I worked with the anhydrous 
sulphate which contains about 64 °/ 0 of the metal. But it is also 
possible that the difference is caused by a different method of working, 
for it is known that when comparing radioactive substances quite 
different results are obtained when one works first with the electroscope 
and then with the photographic plate. In view of this, experiments 
according to the electroscopic method are in course of preparation, 
finally, a third possibility should be considered. It might be possible 
that Rb 3 S0 4 only then acts on the plate when it has been exposed 
previously to the light, in which case there would be no question 
of radioactivity, but the remarkable result of the experiments woul 
consist in the fact that the action should be due to the rubidium 
salt only. In order to come to some decision the following expen 
ment was made. Rb, S0 4 was dissolved in water, in the dark some 
drops were placed on a glass plate and after evaporation to dryness 
the plate was heated for some time at 150°, this leaving a resi ue 
of completely dry Rb s S0 4 . This was now covered with aluminiuin 
foil and a photographic plate was placed on it at a distance o 
0.5 m.m. After 100 days, the plate was developed just like a 
which had been placed in the box in exactly the same manner 
without being exposed to the salt. The same result was again 
tained; a black spot was visible only on the plate exposed to 
rubidium. . 
This result seems to show that rubidium emits indeed a radiation 
