aw any conclusions respecting the evolution of heat during nebular
condensation? And do such conclusions affect in any way the conclusions
now current?

In the first place, it seems inferable from physico-chemical facts at
large, that only through the instrumentality of those combinations which
formed the elements, did the concentration of diffused nebulous matter
into concrete masses become possible. If we remember that hydrogen and
oxygen in their uncombined states oppose, the one an insuperable and the
other an almost insuperable, resistance to liquefaction, while when
combined the compound assumes the liquid state with facility, we may
suspect that in like manner the simpler types of matter out of which the
elements were formed, could not have been reduced even to such degrees
of density as the known gases show us, without what we may call
proto-chemical unions: the implication being that after the heat
resulting from each of such proto-chemical unions had escaped, mutual
gravitation of the parts was able to produce further condensation of the
nebulous mass.

If we thus distinguish between the two sources of heat accompanying
nebular condensation--the heat due to proto-chemical combinations and
that due to the contraction caused by gravitation (both of them,
however, being interpretable as consequent on loss of motion), it may be
inferred that they take different shares during the earlier and during
the later stages of aggregation. It seems probable that while the
diffusion is great and the force of mutual gravitation small, the chief
source of heat is combination of units of matter, simpler than any known
to us, into such units of matter as those we know; while, conversely,
when there has been reached close aggregation, the chief source of heat
is gravitation, with consequent pressure and gradual contraction.
Supposing this to be so, let us ask what may be inferred. If at the time
when the nebulous spheroid from which the Solar System resulted, filled
the orbit of Neptune, it had reached such a degree of density as
enabled those units of matter which compose the sodium molecules to
enter into combination; and if, in conformity with the analogies above
indicated, the heat evolved by this proto-chemical combination was great
compared with the heats evolved by the chemical combinations known to
us; the implication is that the nebulous spheroid, in the course of its
contraction, would have to get rid of a much larger