Owing to the non-conductivity of the air in its normal condition the leaf will, if the insulation of the stopper be good, remain projecting almost horizontally for some time until, as it loses its charge by a slow leakage, it gradually settles down close to the strip.
If, however, a piece of radium be brought near while it is sticking out, the leaf will fall almost instantly. X-rays have a similar effect even from several feet or yards away.
The intensity of the radio-activity of different substances can be compared by noting the difference
in the rate at which the leaf falls under the influence of each.
What is happening, then, to the atoms of radium, which causes them to show these curious effects and to give off these strange rays? To give any intelligent answer to that question we are bound to assume that which the older generation of scientists thought impossible, namely, that atoms can be broken up. Then we are forced to believe that the atoms of this particular substance radium are of a peculiarly flimsy unstable sort, so that they cannot permanently hold their parts together but are liable to break up, as far as we can see through their own inherent weakness and under the influence of disruptive forces at work within themselves.
We must remember, however, that the tiniest speck of matter which we can see contains a number of atoms of such a size as to be quite beyond the grasp of our minds. To give a rough idea of it in figures is useless as no one can comprehend the real value of a figure or two followed by probably from a dozen to twenty "noughts." It is best to content ourselves with the general statement that a speck of matter only just visible to the eye contains an exceedingly vast number of atoms. Of course a speck of radium is no exception to this and we must remember, too, that all of them do not break up at once. Indeed, the number breaking up at any time are actually countable by means of a very simple contrivance and a sensitive electrometer. Consequently, in view of the enormous number present and the comparatively small number breaking up
at any moment, it is not surprising to hear that, so it is estimated, the process can go on for an almost indefinite number of years, certainly for hundreds. There are, moreover, certain facts which we need not go into here from which the above fact can be clearly inferred, quite apart from what has been said about the vast numbers of the atoms.
It seems as if the uranium atoms break up first, giving off helium atoms and electrons and leaving an intermediate substance called Ionium which in its turn breaks up giving off the same things again and leaving radium. That in its turn goes through a complicated series of changes still giving off the same alpha particles or atoms of helium and electrons until, it is suggested, it finally settles down into the simple commonplace metal lead of which we make bullets and water pipes and such-like ordinary things.
We see then that all through its history—its radio-active history at any rate—this stuff is throwing off atoms of helium at a very high velocity (about 50,000 miles a second), and if it be enclosed in anything this enclosing vessel or substance will be subjected to a continual bombardment by the alpha particles. Now just as a piece of iron gets hot if we hammer it, so the enclosing matter is heated by the continual blows which it is receiving night and day, year in and year out, from the alpha particles.
Consequently the immediate surroundings of a speck of radium are always slightly raised in temperature.