ned, so to speak, in a universal molecular
clang. Thus prismatic analysis has no power to identify individual kinds
of matter, except when they present themselves as glowing vapours.

A spectrum is said to be "reversed" when lines previously seen bright on
a dark background appear dark on a bright background. In this form it is
equally characteristic of chemical composition with the "direct"
spectrum, being due to _absorption_, as the latter is to _emission_. And
absorption and emission are, by Kirchhoff's law, strictly correlative.
This is easily understood by the analogy of sound. For just as a
tuning-fork responds to sound-waves of its own pitch, but remains
indifferent to those of any other, so those particles of matter whose
nature it is, when set swinging by heat, to vibrate a certain number of
times in a second, thus giving rise to light of a particular shade of
colour, appropriate those same vibrations, and those only, when
transmitted past them,--or, phrasing it otherwise, are opaque to them,
and transparent to all others.

It should further be explained that the _shape_ of the bright or dark
spaces in the spectrum has nothing whatever to do with the nature of the
phenomena. The "lines" and "bands" so frequently spoken of are seen as
such for no other reason than because the light forming them is admitted
through a narrow, straight opening. Change that opening into a fine
crescent or a sinuous curve, and the "lines" will at once appear as
crescents or curves.

Resuming in a sentence what has been already explained, we find that the
prismatic analysis of the heavenly bodies was founded upon three classes
of facts: First, the unmistakable character of the light given by each
different kind of glowing vapour; secondly, the identity of the light
absorbed with the light emitted by each; thirdly, the coincidence
observed between rays missing from the solar spectrum and rays absorbed
by various terrestrial substances. Thus, a realm of knowledge,
pronounced by Morinus[400] in the seventeenth century, and no less
dogmatically by Auguste Comte[401] in the nineteenth, hopelessly out of
reach of the human intellect, was thrown freely open, and the chemistry
of the sun and stars took at once a leading place among the experimental
sciences.

The immediate increase of knowledge was not the chief result of
Kirchhoff's labours; still more important was the change in the scope
and methods of astronomy, which, set on foot in