reflected in its varying modes of radiating light. Dr.
Schuster enumerated in 1879[689] four distinct oxygen spectra,
corresponding to various stages of temperature, or phases of electrical
excitement; and a fifth has been added by M. Egoroff's discovery in
1883[690] that certain well-known groups of dark lines in the red end of
the solar spectrum (Fraunhofer's A and B) are due to absorption by the
cool oxygen of our air. These persist down to the lowest temperatures,
and even survive a change of state. They are produced essentially the
same by liquid, as by aerial oxygen.[691]

It seemed, however, possible to M. Janssen that these bands owned a
joint solar and terrestrial origin. Oxygen in a fit condition to produce
them might, he considered, exist in the outer atmosphere of the sun; and
he resolved to decide the point. No one could bring more skill and
experience to bear upon it than he.[692] By observations on the summit
of the Faulhorn, as well as by direct experiment, he demonstrated,
nearly thirty years ago, the leading part played by water-vapour in
generating the atmospheric spectrum; and he had recourse to similar
means for appraising the share in it assignable to oxygen. An electric
beam, transmitted from the Eiffel Tower to Meudon in the summer of 1888,
having passed through a weight of oxygen about equal to that piled above
the surface of the earth, showed the groups A and B just as they appear
in the high-sun spectrum.[693] Atmospheric action is then adequate to
produce them. But M. Janssen desired to prove, in addition, that they
diminish proportionately to its amount. His ascent of Mont Blanc[694] in
1890 was undertaken with this object. It was perfectly successful. In
the solar spectrum, examined from that eminence, oxygen-absorption was
so much enfeebled as to leave no possible doubt of its purely telluric
origin. Under another form, nevertheless, it has been detected as
indubitably solar. A triplet of dark lines low down in the red,
photographed from the sun by Higgs and McClean, was clearly identified
by Runge and Paschen in 1896[695] with the fundamental group of an
oxygen series, first seen by Piazzi Smyth in the spectrum of a
vacuum-tube in 1883.[696] The _pabulum vitae_ of our earth is then to
some slight extent effective in arresting transmitted sunlight, and
oxygen must be classed as a solar element.

The rays of the sun, besides being stopped selectively in our
atmosphere, suffer also a marke