as been the average maintained
in the past, a simple division gives the number of years for which the
original supply is adequate. The supply will be exhausted, it will be
observed, when the mass comes into stable equilibrium as a solid body,
no longer subject to contraction, about the sun's centre--such a body,
in short, as our earth is at present.

This calculation was made by Lord Kelvin, Professor Tait, and others,
and the result was one of the most truly dynamitic surprises of the
century. For it transpired that, according to mathematics, the entire
limit of the sun's heat-giving life could not exceed something like
twenty-five millions of years. The publication of that estimate, with
the appearance of authority, brought a veritable storm about the heads
of the physicists. The entire geological and biological worlds were
up in arms in a trice. Two or three generations before, they hurled
brickbats at any one who even hinted that the solar system might be more
than six thousand years old; now they jeered in derision at the attempt
to limit the life-bearing period of our globe to a paltry fifteen or
twenty millions.

The controversy as to solar time thus raised proved one of the most
curious and interesting scientific disputations of the century. The
scene soon shifted from the sun to the earth; for a little reflection
made it clear that the data regarding the sun alone were not
sufficiently definite. Thus Dr. Croll contended that if the parent
bodies of the sun had chanced to be "flying stars" before collision,
a vastly greater supply of heat would have been engendered than if the
matter merely fell together. Again, it could not be overlooked that
a host of meteors are falling into the sun, and that this source of
energy, though not in itself sufficient to account for all the heat in
question, might be sufficient to vitiate utterly any exact calculations.
Yet again, Professor Lockyer called attention to another source of
variation, in the fact that the chemical combination of elements
hitherto existing separately must produce large quantities of heat, it
being even suggested that this source alone might possibly account for
all the present output. On the whole, then, it became clear that the
contraction theory of the sun's heat must itself await the demonstration
of observed shrinkage of the solar disk, as viewed by future generations
of observers, before taking rank as an incontestable theory, and that
computatio