ave
accumulated for the last six years, among which some people find
themselves a little lost. It may, perhaps, not be useless to indicate
the essential results actually obtained.

The researches on radioactive substances have their starting-point in
the discovery of the rays of uranium made by M. Becquerel in 1896. As
early as 1867 Niepce de St Victor proved that salts of uranium
impressed photographic plates in the dark; but at that time the
phenomenon could only pass for a singularity attributable to
phosphorescence, and the valuable remarks of Niepce fell into
oblivion. M. Becquerel established, after some hesitations natural in
the face of phenomena which seemed so contrary to accepted ideas, that
the radiating property was absolutely independent of phosphorescence,
that all the salts of uranium, even the uranous salts which are not
phosphorescent, give similar radiant effects, and that these phenomena
correspond to a continuous emission of energy, but do not seem to be
the result of a storage of energy under the influence of some external
radiation. Spontaneous and constant, the radiation is insensible to
variations of temperature and light.

The nature of these radiations was not immediately understood,[32] and
their properties seemed contradictory. This was because we were not
dealing with a single category of rays. But amongst all the effects
there is one which constitutes for the radiations taken as a whole, a
veritable process for the measurement of radioactivity. This is their
ionizing action on gases. A very complete study of the conductivity of
air under the influence of rays of uranium has been made by various
physicists, particularly by Professor Rutherford, and has shown that
the laws of the phenomenon are the same as those of the ionization due
to the action of the Roentgen rays.

[Footnote 32: In his work on _L'Evolution de la Matiere_, M. Gustave
Le Bon recalls that in 1897 he published several notes in the Academie
des Sciences, in which he asserted that the properties of uranium were
only a particular case of a very general law, and that the radiations
emitted did not polarize, and were akin by their properties to the X
rays.]

It was natural to ask one's self if the property discovered in salts
of uranium was peculiar to this body, or if it were not, to a more or
less degree, a general property of matter. Madame Curie and M.
Schmidt, independently of each other, made systematic researches in