the
otherwise unbroken radiance of its spectrum. The inference was
irresistible, that the effect thus produced artificially was brought
about naturally in the same way, and that sodium formed an ingredient in
the glowing atmosphere of the sun.[386] This first discovery was quickly
followed up by the identification of numerous bright rays in the spectra
of other metallic bodies with others of the hitherto mysterious
Fraunhofer lines. Kirchhoff was thus led to the conclusion that (besides
sodium) iron, magnesium, calcium, and chromium, are certainly solar
constituents, and that copper, zinc, barium, and nickel are also
present, though in smaller quantities.[387] As to cobalt, he hesitated
to pronounce, but its existence in the sun has since been established.

These memorable results were founded upon a general principle first
enunciated by Kirchhoff in a communication to the Berlin Academy,
December 15, 1859, and afterwards more fully developed by him.[388] It
may be expressed as follows: Substances of every kind are opaque to the
precise rays which they emit at the same temperature; that is to say,
they stop the kinds of light or heat which they are then actually in a
condition to radiate. But it does not follow that _cool_ bodies absorb
the rays which they would give out if sufficiently heated. Hydrogen at
ordinary temperatures, for instance, is almost perfectly transparent,
but if raised to the glowing point--as by the passage of electricity--it
_then_ becomes capable of arresting, and at the same time of displaying
in its own spectrum light of four distinct colours.

This principle is fundamental to solar chemistry. It gives the key to
the hieroglyphics of the Fraunhofer lines. The identical characters
which are written _bright_ in terrestrial spectra are written _dark_ in
the unrolled sheaf of sun-rays; the meaning remains unchanged. It must,
however, be remembered that they are only _relatively_ dark. The
substances stopping those particular tints in the neighbourhood of the
sun are at the same time vividly glowing with the very same. Remove the
dazzling solar background, by contrast with which they show as obscure,
and they will be seen, and, at critical moments, actually have been
seen, in all their native splendour. It is because the atmosphere of the
sun is cooler than the globe it envelops that the different kinds of
vapour constituting that atmosphere take more than they give, absorb
more light than they are