99 
Chemistry. — “The radioactivity of rubidium and potassium com= 
pounds.” Il. By Dr. E. H. Bicaner. (Communicated by Prof. 
A. F. HorLr.EMAN). 
(Communicated in the meeting of April 26, 1912). 
Some time ago I described a series of experiments undertaken with 
the object of demonstrating the radioactivity of rubidium and even- 
tually of other alkali metals by the photographic method *). I then 
only noticed an action on the sensitive plate with rubidium sulphate ; 
the salts of other alkalis produced no effect. 1 have repeated these 
experiments and, as announced previously, I have inquired more in 
particular, whether the phenomenon might be attributable to a 
previous exposure of the salt to the light; in that case there can 
be no question of a real radioactivity, but we should have here 
an analogism of the wellknown experiments with calcium sulphide. 
According to NirwENGLOWSKI, this substance acts on a photographic 
plate by means of rays which penetrate through aluminium, but 
only when it has been previously exposed to the light. In the 
present meaning of the word we cannot call calcium sulphide 
radioactive, because an external influence is at work; if the same 
happened with rubidium and potassium, these substances could neither 
be included among the radioactive ones. And because they differ in 
various respects from the other active substances, there is still some 
doubt left about this matter. It was, therefore, desirable to carry 
out some experiments in this direction. 
For this purpose | have exposed, simultaneously, in one box, some 
photographie plates to the action of RbCI, RbNO, and Rb,SO, in 
the manner described previously, but of each salt two specimens 
were taken; one of these had been kept in complete darkness from 
4 to 5 months, the other had been exposed to broad daylight for 
some days previous to the experiment. When developing after 90 
days, no difference was found between the action of the two 
specimens, both having affected the plates in the same manner. Hence, 
it again becomes more probable that we are dealing here indeed 
with true radioactivity. 
For the rest I have been able to confirm my previous results. Again, 
I have not succeeded in getting an action on the sensitive plate 
either with salts of potassium or with salts of caesium, sodium, and 
lithium, but on the other hand rubidium did affect the plate. With 
RbCl and RbNO,, also with Rb,SO, I found that the plate had 
1) These Proc. 1909, p. 154. 
