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.