range-yellow ray identified with it in the haste of the
eclipse. From its vicinity to the D-pair (than which it is slightly more
refrangible), the prominence-line was, however, designated D_3, and the
unknown substance emitting it was named by Lockyer "helium." Its
terrestrial discovery ensued after twenty-six years. In March, 1895,
Professor Ramsay obtained from the rare mineral clevite a volatile gas,
the spectrum of which was found to include the yellow prominence-ray.
Helium was actually at hand, and available for examination. The
identification cleared up many obscurities in chromospheric chemistry.
Several bright lines, persistently seen at the edge of the sun, and
early suspected by Young[596] to emanate from the same source as D_3,
were now derived from helium in the laboratory; and all the complex
emissions of that exotic substance ranged themselves into six sets or
series, the members of which are mutually connected by numerical
relations of a definite and simple kind. Helium is of rather more than
twice the density of hydrogen, and has no chemical affinities. In almost
evanescent quantities it lurks in the earth's crust, and is diffused
through the earth's atmosphere.

The importance of the part played in the prominence-spectrum by the
violet line of calcium was noticed by Professor Young in 1872, but since
H and K lie near the limit of the visible spectrum, photography was
needed for a thorough investigation of their appearances. Aided by its
resources, Professor George E. Hale, then at the beginning of his
career, detected in 1889 their unfailing and conspicuous presence.[597]
The substance emitting them not only constitutes a fundamental
ingredient of the chromosphere, but rises, in the fantastic jets thence
issuing, to greater heights than hydrogen itself. The isolation of H and
K in solar prominences from any other of the lines usually distinctive
of calcium was experimentally proved by Sir William and Lady Huggins in
1897 to be due to the extreme tenuity of the emitting vapour.[598]

Hydrogen, helium, and calcium form, then, the chief and unvarying
materials of the solar sierra and its peaks; but a number of metallic
elements make their appearance spasmodically under the influence of
disturbances in the layers beneath. In September, 1871, Young[599] drew
up at Dartmouth College a list of 103 lines significant of injections
into the chromosphere of iron, titanium, chromium, magnesium, and many
other sub