24 
for instance, cannot be represented by a simple exponential formula; 
on the contrary, it seems as if the radiation is composed of a part 
decidedly penetrating and of another one less so; the latter is then 
only of slight importance. | 
In two ways, I have attacked the problem of the presence of 
a-rays; firstly by observing whether zine sulphide became luminous 
under the influence of the salts. This method has the advantage that 
we can bring together the sait and the zine sulphide as closely as 
we like, and reduce as far as possible the absorption which the a- 
particles undergo in the air; consequently we may, perhaps, find in 
this manner a-rays of very slight penetrating power which would 
not be detected by other means. 
We know that light emitted under the influence of a-particles 
possesses a peculiar character and that, when examined under the 
microscope, it breaks up into numerous points which are formed 
at the spots, where the a-particles meet the zine sulphide; each 
scintillation, therefore, indicates an a-particle. In order to show the g- 
particles eventually present, an object-slide with a little KCl was put 
under the microscope; above it at a distance of about 2 m.m. was 
placed another slide which was coated at its lower surface, by means 
of Canada balsam, with a layer of zine sulphide. The whole arrange- 
ment is placed in the dark; it is, however, advisable, in imitation 
of ReGexNeR, to faintly illuminate a portion of the field of vision (for 
which purpose a “VERKADE waxine” light is very serviceable) in order 
to facilitate the adjustment. In this manner, we can readily show 
the a-particles of pitchblende, uranium oxide, and thorium oxide ; we 
shall be able to observe also all e-particles which can traverse a 
distance of at least 2 m.m. in the air. Neither with KCl, nor with 
RbCl, however, any scintillation was noticed in different experiments, 
though the vubservation lasted each time ten minutes. I then made 
the experiment in another way: to render the distance between the 
salt and the zine sulphide as small as possible, I mixed the two 
compounds. But even then I did not sueceed in observing a single 
flash of light. These experiments thus confirm the results communi- 
cated by HeEnrior*) in a paper which appeared after my experiments 
were closed, namely that rubidium and potassium do not emit e-rays. 
3. There is yet another way to demonstrate the emission of «- 
particles. It is well known that the heat generated by radium and 
other radioactive substances originates in the kinetic energy of the 
a-particles, which are stopped in the surrounding matter. A large 
1) Comptes Rendus, 152, 1384 (1911). 
