The spectroscopic reaction which we may
use in the course of this separation is a thousand times less
sensitive than observation of the activity by means of the
electrometer.
Though the principle on which the operation of the concentration of
the radium rests is admirable in its simplicity, its application is
nevertheless very laborious. Tons of uranium residues have to be
treated in order to obtain a few decigrammes of pure salts of radium.
Radium is characterised by a special spectrum, and its atomic weight,
as determined by Madame Curie, is 225; it is consequently the higher
homologue of barium in one of the groups of Mendeleef. Salts of radium
have in general the same chemical properties as the corresponding
salts of barium, but are distinguished from them by the differences of
solubility which allow of their separation, and by their enormous
activity, which is about a hundred thousand times greater than that of
uranium.
Radium produces various chemical and some very intense physiological
reactions. Its salts are luminous in the dark, but this luminosity, at
first very bright, gradually diminishes as the salts get older.
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