Radium,
in addition to the three groups of rays alpha, beta, and gamma,
disengages continuously an extremely subtle emanation, seemingly
almost imponderable, but which may be, for many reasons, looked upon
as a vapour of which the elastic force is extremely feeble.
[Footnote 35: It has now been shown that polonium when freshly
separated emits beta rays also; see Dr Logeman's paper in _Proceedings
of the Royal Society_, A., 6th September 1906.--ED.]
M. and Madame Curie discovered as early as 1899 that every substance
placed in the neighbourhood of radium, itself acquired a radioactivity
which persisted for several hours after the removal of the radium.
This induced radioactivity seems to be carried to other bodies by the
intermediary of a gas. It goes round obstacles, but there must exist
between the radium and the substance a free and continuous space for
the activation to take place; it cannot, for instance, do so through a
wall of glass.
In the case of compounds of thorium Professor Rutherford discovered a
similar phenomenon; since then, various physicists, Professor Soddy,
Miss Brooks, Miss Gates, M.
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