Uranium is a rather rare element, though known for over a hundred years, and has an atomic weight of 238.5. In decomposing it gives off first a helium atom, weight 4; and after this action has been repeated three times the substance left is radium, atomic weight about 226.4. Thus radium is simply uranium after it has lost three helium atoms. Radium in its disintegration gives off three kinds of particles, namely, helium atoms (positively electrified), [Greek: b]-rays or electrons, and [Greek: g]-rays, the latter being identical with the X-rays, and having penetrating power sufficient to carry them through six inches of lead or a foot of solid iron. The final stage in this process of disintegration is the ordinary element lead, in which condition the atoms seem to have reached relative stability. Whether or not our stock of lead, with our other common elements that are not radioactive, was originally produced by the disintegration of these other elements, is merely a matter of conjecture. We know nothing at all about it.
The length of time it takes for half the atoms of an element to change is called its “life” or period. The periods of most of the radioactive substances have been calculated, that of uranium being very long. The calculated period of radium is 2,500 years, while that of polonium is only 202 days, and that of niton 5.6 days. These unquestioned facts, together with the enormous amount of heat evolved by the disintegration of these substances (that from radium being about 250,000 times the heat evolved by the combustion of carbon), have thrown a great deal of doubt upon the older estimates of the age of the earth.
The discussion of the details of these theories would be unprofitable. But through the mists of all these conflicting theories and probabilities two facts of tremendous importance for our modern world emerge in clear relief, namely, that the grand law of the conservation of matter still holds true, and hence that the matter of our world must have had an origin at some time in the past wholly different in degree and different in kind from any process going on around us that we call a natural process. These elements of high atomic weight that break down into others of lower atomic weight may be so rare because they have been about all used up in this process. At any rate, so far from revealing the origin of matter as a process now going on, these phenomena are an objective demonstration that all matter is more or less unstable and liable under some unknown but ever-acting force to lose some portion of that fund of energy with which it seems to have been primarily endowed. Not the evolution of matter but the degeneration of matter is the plain and unescapable lesson to be drawn from these facts. The varieties of matter may change greatly, and one variety or one chemical element may be transformed into another. But this transformation is by loss and not by gain. It is degeneration and not upward evolution that is