, [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