( 155 ) 
first place worth while to ascertain, whether other alkali salts have 
the same property as the potassium compounds. Particular attention 
had to be paid to the salts of rubidium and caesium, as these are 
isomorphous with potassium salts, and if there should really exist 
an unknown alkali metal, it was to be expected that this would 
also separate together with Rb- and Cs-salts. 
Experiments in . this direction have been made, partly before and 
partly during my investigation, by Campbell 1 ) and McLennan 8 ). By 
means of the electroscopic method they proved that rubidium had 
an activity of the same order as potassium, whereas compounds of 
Cs, Li, and Na did not appear to be active. This last result was 
confirmed by my first experiment; from a series of photographic 
plates which were exposed to the action of K s S0 4 , Rb,S0 4 , Cs s S0 4 , 
NaCl, and Li 2 CO s the only one which was found to be altered 
after development was that exposed to the rubidium. 
In the following experiments, I have provisionally left out the 
Cs and Li and have only worked with Rubidium salts, and for the 
sake of comparison, with the potassium and sodium salts. The expe¬ 
riments were conducted as follows: LuMiERE-plates were wrapped up 
in black paper into which was cut a circular hole in such a manner 
that the opening came on the sensitive side of the plate. The plates, 
emulsion downwards were then put on strips of cork, between 
which was placed an object glass with the salt to be examined. As 
the salt was just under the opening in the black paper an eventual 
radiation from the salt had to penetrate only 3—4 mM. of air 
before reaching the photographic plate. Three plates at a time were 
now placed in a large box into which was also put a dish with 
calcium chloride, and the whole after being wrapped up in black 
paper was placed under a bell-jar over calcium chloride and kept 
in a dark room. The various salts were used in the anhydrous 
state and on opening the boxes they appeared to be quite unaltered. 
All the plates were developed and fixed in exactly the same manner. 
The result of the experiments was that after exposures of 81 and 
117 days, the plates exposed to K s S0 4 and Na 2 S0 4 were quite unal¬ 
tered whereas a black spot was noticed on the plate exposed to the 
Rb, S0 4 . The phenomenon although weak is not in the least doubtful. 
Another series of experiments were then made in a somewhat 
different manner; the plates were again wrapped up in black paper 
with an opening, but this was now covered with aluminium foil 
*) Proc. Gambr. Phil. Son. 15, 11 (from an abstract). 
a ) Le Radium V, 142. 
11 * 
