-particles and becomes niton, with atomic number 86.
A substance may, however, be radio-active by sending out
-rays instead of
-rays. Some substances send out one kind, some the other; a very few can send out either, and can thus give rise to two different products of disintegration. The
-rays are simply very swiftly moving electrons, the most swiftly moving matter known to us; they attain velocities which reach very nearly the velocity of light. They have been known to travel at the rate of about 297,000 kilometres a second, while light covers 300,000 kilometres a second. (A kilometre is about five-eighths of a mile.) The velocity of light is a theoretical limit, which cannot be attained by anything material, so that we have in