raids.

Had the war lasted a few months longer, however, the work of American
scientists would have made our counter-attack in the air a formidable
one. At the signing of the armistice hundreds of cylinders of compressed
helium lay at the docks ready for shipment abroad. Extracted from
the natural gas of Texas wells by new and ingenious processes,
this substitute for hydrogen, almost as light and absolutely
uninflammable, produced in quantities of millions of cubic feet,
would have made the dirigibles of the Allies masters of the air. The
special properties of this remarkable gas, previously obtainable only
in minute quantities, would have sufficed to reverse the situation.

SOLAR HELIUM

Helium, as its name implies, is of solar origin. In 1868, when
Lockyer first directed his spectroscope to the great flames or
prominences that rise thousands of miles, sometimes hundreds of
thousands, above the surface of the sun, he instantly identified
the characteristic red and blue radiations of hydrogen. In the
yellow, close to the position of the well-known double line of
sodium, but not quite coincident with it, he detected a new line,
of great brilliancy, extending to the highest levels. Its similarity
in this respect with the lines of hydrogen led him to recognize
the existence of a new and very light gas, unknown to terrestrial
chemistry.

Many years passed before any chemical laboratory on earth was able
to match this product of the great laboratory of the sun. In 1896
Ramsay at last succeeded in separating helium, recognized by the same
yellow line in its spectrum, in minute quantities from the mineral
uraninite. Once available for study under electrical excitation in
vacuum tubes, helium was found to have many other lines in its
spectrum, which have been identified in the spectra of solar
prominences, gaseous nebulae, and hot stars. Indeed, there is a
stellar class known as helium stars, because of the dominance of
this gas in their atmospheres.

[Illustration: Fig. 30. Solar prominences, photographed with the
spectroheliograph without an eclipse (Ellerman).

In these luminous gaseous clouds, which sometimes rise to elevations
exceeding half the sun's diameter, the new gas helium was discovered
by Lockyer in 1868. Helium was not found on the earth until 1896.
Since then it has been shown to be a prominent constituent of nebulae
and hot stars.]

The chief importance of helium lies in the clue it has afforded to