quantity of heat
than it would, did it commence at any ordinary temperature and had only
to lose the heat consequent on contraction. That is to say, in
estimating the past period during which solar emission of heat has been
going on at a high rate, much must depend on the initial temperature
assumed; and this may have been rendered intense by the proto-chemical
changes which took place in early stages.[21]

Respecting the future duration of the solar heat, there must also be
differences between the estimates made according as we do or do not take
into account the proto-chemical changes which possibly have still to
take place. True as it may be that the quantity of heat to be emitted
is measured by the quantity of motion to be lost, and that this must be
the same whether the approximation of the molecules is effected by
chemical unions, or by mutual gravitation, or by both; yet, evidently,
everything must turn on the degree of condensation supposed to be
eventually reached; and this must in large measure depend on the natures
of the substances eventually formed. Though, by spectrum-analysis,
platinum has recently been detected in the solar atmosphere, it seems
clear that the metals of low molecular weights greatly predominate; and
supposing the foregoing arguments to be valid, it may be inferred, as
not improbable, that the compoundings and recompoundings by which the
heavy-moleculed elements are produced, not hitherto possible in large
measure, will hereafter take place; and that, as a result, the Sun's
density will finally become very great in comparison with what it is
now. I say "not hitherto possible in large measure", because it is a
feasible supposition that they may be formed, and can continue to exist,
only in certain outer parts of the Solar mass, where the pressure is
sufficiently great while the heat is not too great. And if this be so,
the implication is that the interior body of the Sun, higher in
temperature than its peripheral layers, may consist wholly of the metals
of low atomic weights, and that this may be a part cause of his low
specific gravity; and a further implication is that when, in course of
time, the internal temperature falls, the heavy-moleculed elements, as
they severally become capable of existing in it, may arise: the
formation of each having an evolution of heat as its concomitant.[22] If
so, it would seem to follow that the amount of heat to be emitted by
the Sun, and the length of the