. It may, indeed, be
that the unknown elements which spectrum analysis shows to exist in the
Sun, are metals of very low specific gravities, and that, existing in
large proportion with other of the lighter metals, they may form a
molten shell not denser than is implied by the facts. But this can be
regarded as nothing more than a possibility.

No need, however, has arisen for either relinquishing or holding but
loosely the associated conclusions respecting the constitution of the
photosphere and its envelope. Widely speculative as seemed these
suggested corollaries from the Nebular Hypothesis when set forth in
1858, and quite at variance with the beliefs then current, they proved
to be not ill-founded. At the close of 1859, there came the discoveries
of Kirchhoff, proving the existence of various metallic vapours in the
Sun's atmosphere.]

ADDENDA.

Speculative as is much of the foregoing essay, it appears undesirable to
include in it anything still more speculative. For this reason I have
decided to set forth separately some views concerning the genesis of the
so-called elements during nebular condensation, and concerning the
accompanying physical effects. At the same time it has seemed best to
detach from the essay some of the more debatable conclusions originally
contained in it; so that its general argument may not be needlessly
implicated with them. These new portions, together with the old portions
which re-appear more or less modified, I here append in a series of
notes.

NOTE I. For the belief that the so-called elements are compound there
are both special reasons and general reasons. Among the special may be
named the parallelism between allotropy and isomerism; the numerous
lines in the spectrum of each element; and the cyclical law of Newlands
and Mendeljeff. Of the more general reasons, which, as distinguished
from these chemical or chemico-physical ones, may fitly be called
cosmical, the following are the chief.

The general law of evolution, if it does not actually involve the
conclusion that the so-called elements are compounds, yet affords _a
priori_ ground for suspecting that they are such. The implication is
that, while the matter composing the Solar System has progressed
physically from that relatively-homogeneous state which it had as a
nebula to that relatively-heterogeneous state presented by Sun, planets,
and satellites, it has also progressed chemically, from the
relatively-homog