our_--is a layer of gases lying immediately upon the
preceding one. Its thickness is, however, plainly much the greater of
the two; for whereas the reversing layer is only revealed to us
_indirectly_ by the spectroscope, a portion of the chromosphere may
clearly be _seen_ in a total eclipse in the form of a strip of scarlet
light. The time which the moon's edge takes to traverse it tells us that
it must be about ten times as deep as the reversing layer, namely, from
5000 to 10,000 miles in depth. Its spectrum shows that it is composed
chiefly of hydrogen, calcium and helium, in the state of vapour. Its red
colour is mainly due to glowing hydrogen. The element helium, which it
also contains, has received its appellation from [helios] (_helios_),
the Greek name for the sun; because, at the time when it first attracted
attention, there appeared to be no element corresponding to it upon our
earth, and it was consequently imagined to be confined to the sun alone.
Sir William Ramsay, however, discovered it to be also a terrestrial
element in 1895, and since then it has come into much prominence as one
of the products given off by radium.

Taking into consideration the excessive force of gravity on the sun, one
would expect to find the chromosphere and reversing layer growing
gradually thicker in the direction of the photosphere. This, however, is
not the case. Both these layers are strangely enough of the same
densities all through; which makes it suspected that, in these regions,
the force of gravity may be counteracted by some other force or forces,
exerting a powerful pressure outwards from the sun.

IV. THE PROMINENCES.

We have already seen, in dealing with total eclipses, that the exterior
surface of the chromosphere is agitated like a stormy sea, and from it
billows of flame are tossed up to gigantic heights. These flaming jets
are known under the name of prominences, because they were first noticed
in the form of brilliant points projecting from behind the rim of the
moon when the sun was totally eclipsed. Prominences are of two kinds,
_eruptive_ and _quiescent_. The eruptive prominences spurt up directly
from the chromosphere with immense speeds, and change their shape with
great rapidity. Quiescent prominences, on the other hand, have a form
somewhat like trees, and alter their shape but slowly. In the eruptive
prominences glowing masses of gas are shot up to altitudes sometimes as
high as 300,000 miles,[10] wit