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.
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