radiations there sometimes are added in the course of
experiments secondary radiations analogous to those of M. Sagnac, and
produced when the alpha, beta, or gamma rays meet various substances.
This complication has often led to some errors of observation.

Phosphorescence and fluorescence seem especially to result from the
alpha and beta rays, particularly from the alpha rays, to which
belongs the most important part of the total energy of the radiation.
Sir W. Crookes has invented a curious little apparatus, the
spinthariscope, which enables us to examine the phosphorescence of the
blende excited by these rays. By means of a magnifying glass, a screen
covered with sulphide of zinc is kept under observation, and in front
of it is disposed, at a distance of about half a millimetre, a
fragment of some salt of radium. We then perceive multitudes of
brilliant points on the screen, which appear and at once disappear,
producing a scintillating effect. It seems probable that every
particle falling on the screen produces by its impact a disturbance in
the neighbouring region, and it is this disturbance which the eye
perceives as a luminous point. Thus, says Sir W. Crookes, each drop of
rain falling on the surface of still water is not perceived as a drop
of rain, but by reason of the slight splash which it causes at the
moment of impact, and which is manifested by ridges and waves
spreading themselves in circles.

The various radioactive substances do not all give radiations of
identical constitution. Radium and thorium possess in somewhat large
proportions the three kinds of rays, and it is the same with actinium.
Polonium contains especially alpha rays and a few gamma rays.[35] In
the case of uranium, the alpha rays have extremely slight penetrating
power, and cannot even impress photographic plates. But the widest
difference between the substances proceeds from the emanation. 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