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 [Pg 116] 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 -particles velocities about as great as we can ever expect to find in nature. Since radio-activity always gives rise to a new element, and since the element is determined by its nucleus, the -particles as well as the -particles must come out of the nucleus. Since the -particles are electrons, this shows that the nucleus of a radio-active element must contain electrons. This is to be expected in all elements, because the atomic weight increases about twice as fast as the atomic number, so that the atomic number (which is the net charge in the nucleus) must be the result of a number of hydrogen nuclei about twice as great as the atomic number and a number of electrons about equal to the atomic number. This is not always exactly true, but at any rate it is likely to be a first approximation. There is therefore no reason to be surprised by the fact that electrons come out of the nuclei of radio-active elements.