by
Professor Giesel, then by M. Becquerel, Professor Rutherford, and by
many other experimenters after them. All the methods which have
already been mentioned in principle have been employed in order to
discover whether they were electrified, and, if so, by electricity of
what sign, to measure their speed, and to ascertain their degree of
penetration.

The general result has been to distinguish three sorts of radiations,
designated by the letters alpha, beta, gamma.

The alpha rays are positively charged, and are projected at a speed
which may attain the tenth of that of light; M.H. Becquerel has shown
by the aid of photography that they are deviated by a magnet, and
Professor Rutherford has, on his side, studied this deviation by the
electrical method. The relation of the charge to the mass is, in the
case of these rays, of the same order as in that of the ions of
electrolysis. They may therefore be considered as exactly analogous to
the canal rays of Goldstein, and we may attribute them to a material
transport of corpuscles of the magnitude of atoms. The relatively
considerable size of these corpuscles renders them very absorbable. A
flight of a few millimetres in a gas suffices to reduce their number
by one-half. They have great ionizing power.

The beta rays are on all points similar to the cathode rays; they are,
as M. and Madame Curie have shown, negatively charged, and the charge
they carry is always the same. Their size is that of the electrons,
and their velocity is generally greater than that of the cathode rays,
while it may become almost that of light. They have about a hundred
times less ionizing power than the alpha rays.

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.]

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