darkened distinctly on those spots, where little holes or figures had 
been cut in the sereen of copper foil which had been placed between 
the salt and the plate. The action is strongest with the chloride and 
weakest with the sulphate. I attribute this to the greater absorption 
which the rays undergo in the sulphate itself, for this salt has a 
higher density than the chloride and, therefore may be expected to 
show a greater absorption. This explanation can also serve for a 
few deviating results. In two experiments, it appeared that Rb,SO, 
had produced no effect; now in these cases the salt had accidentally 
been used in the form of fairly large crystals and not in powder, as 
usual. The surface of the powder is, of course, larger and conse- 
quently more rays will reach the plate than in the case where 
crystals are employed. Perhaps, this reasoning may explain also the 
results of STRONG *) who, in the exposure of different potassium salts 
to photographie plates, observed effects of very varying intensily ; 
for instance strong action with potassiumeyanide and practically 
none with the urate. 
The rubidium salts investigated by me were obtained from diffe- 
rent dealers (Merck, KAHLBAUM, DE HAREN, SCHUCHARDT); the fact 
that they show no difference in action goes to prove that the phe- 
nomenon must be attributed really to rubidium and not to some 
impurity. 
2. Other investigators have already shown that the radiation of 
potassium and rubidium consists mainly, probably even exclusively, 
of p-rays. Now, e-rays may, however, elude observation sometimes, 
as they act but faintly on sensitive plates and consequently practically 
not at all with slightly active substances. Moreover when we are 
dealing with a-particles of very small velocity and corresponding small 
penetrating power, only an exceedingly small portion of the «-parti- 
eles will arrive in the surrounding gas and the ionisation current, 
generated by them, which is measured with the electroscope, will 
be very weak; it may even be of little importance in regard to the 
current caused by the g-rays. If now we may apply the results 
obtained with strongly active substances to feebly active compounds, 
the a-rays, if present here, may be expected to possess a slight 
velocity, since we may assume as a rule: the larger the activity of 
a substance, the greater the velocity of the «-particles. A possible 
occurrence of a-rays demands an investigation all the more, because 
the absorptior of the radiation in different substances, like tin foil 
1) Amer. Chem. Journ, 42, 127. 
