The gamma rays were discovered by M. Villard.[34] They may be compared to the X rays; like the latter, they are not deviated by the magnetic field, and are also extremely penetrating.  A strip of aluminium five millimetres thick will stop the other kinds, but will allow them to pass.  On the other hand, their ionizing power is 10,000 times less than that of the alpha rays.

[Footnote 34:  This is admitted by Professor Rutherford (Radio-Activity, Camb., 1904, p. 141) and Professor Soddy (Radio-Activity, London, 1904, p. 66).  Neither Mr Whetham, in his Recent Development of Physical Science (London, 1904) nor the Hon. R.J.  Strutt in The Becquerel Rays (London, same date), both of whom deal with the historical side of the subject, seem to have noticed the fact.—­ED.]

To these 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.