102 Recent Advances in Stellar Astronomy
evidence regarding the age of the stars, or the Sun; but
we have information about the age of the Earth that has
magnified our conception of the duration of the universe in
time, in as startling a fashion as the study of the globular
clusters has enlarged our idea of its extension in space.
The new method of measuring times is really very sim-
ple. Uranium is radioactive, and slowly “decays.” One by
one its atoms eject a part of their nuclei, and change into
atoms of a different element. These again break up, and
so on through a long and wonderful series of transforma-
tions, in which radium is one step. The particle ejected
from the nucleus is sometimes an electron, but oftener an
alpha particle, identical with the nucleus of a helium atom.
Finally, at the end of the list, there remains a stable atom
of lead—but not of ordinary lead, for its atomic weight
is 206, instead of 207 as usual. In the course of ages, this
radio-lead must accumulate in all uranium minerals. The
rate of accumulation is accurately known, from a study
of ra’dio-active phenomena and we can be sure that the
weight of lead produced in a milion years is 1/8000 of
that of the uranium which is present. By determining the
percentages of uranium and lead now present in a mineral,
and applying this principle, we can find out how old the
mineral is—provided, of course, that it contained no lead,
when it was originally formed by crystallization in the
molten rock mass. Such primitive lead would, however,
be ordinary lead, of higher atomic weight, and a de-
termination of the atomic weight of the lead derived from
one specimen will enable us to tell how much of it was
there when the mineral formed, and how much has been
produced by radio-activity since this time.
In this way reliable values can be obtained for the ages
of various minerals, and the dates of the eruption of the