It is not necessary to give here a history of the discovery of radium, for every one knows the admirable researches of M. and Madame Curie.  But subsequent to these first studies, a great number of facts have 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 order to solve the question; various compounds of nearly all the simple bodies at present known were thus passed in review, and it was established that radioactivity was particularly perceptible in the compounds of uranium and thorium, and that it was an atomic property linked to the matter endowed