cient quantity to raise its own weight of water to the
boiling-point in an hour. The demonstrated wide distribution of
radio-active matter--making it at least an open question whether all
matter does not possess this property in some degree--has led to the
suggestion that the total heat of the sun may be due to radio-active
matter in its substance. Obviously, then, all estimates of the sun's age
based on the heat-supply must for the present be held quite in abeyance.
What is more to the point, however, is the fact, which these varying
estimates have made patent, that computations of the age of the earth
based on any data at hand are little better than rough guesses. Long
before the definite estimates were undertaken, geologists had proved
that the earth is very, very old, and it can hardly be said that
the attempted computations have added much of definiteness to that
proposition. They have, indeed, proved that the period of time to be
drawn upon is not infinite; but the nebular hypothesis, to say nothing
of common-sense, carried us as far as that long ago.

If the computations in question have failed of their direct purpose,
however, they have been by no means lacking in important collateral
results. To mention but one of these, Lord Kelvin was led by this
controversy over the earth's age to make his famous computation in which
he proved that the telluric structure, as a whole, must have at least
the rigidity of steel in order to resist the moon's tidal pull as it
does. Hopkins had, indeed, made a somewhat similar estimate as early as
1839, proving that the earth's crust must be at least eight hundred or
a thousand miles in thickness; but geologists had utterly ignored
this computation, and the idea of a thin crust on a fluid interior had
continued to be the orthodox geological doctrine. Since Lord Kelvin's
estimate was made, his claim that the final crust of the earth could
not have formed until the mass was solid throughout, or at least until
a honeycomb of solid matter had been bridged up from centre to
circumference, has gained pretty general acceptance. It still remains
an open question, however, as to what proportion the lacunas of molten
matter bear at the present day to the solidified portions, and therefore
to what extent the earth will be subject to further shrinkage and
attendant surface contortions. That some such lacunae do exist is
demonstrated daily by the phenomena of volcanoes. So, after all, the
crust