amed a number of these
at the beginning of this chapter, it will not be out of order to
repeat them here. The chief among these are the acid gases known as
hydrochloric acid, sulphurous acid, sulphuretted hydrogen, carbonic
acid, and boracic acid; and with these acid gases there issue hydrogen,
nitrogen ammonia, the volatile metals arsenic, antimony, and mercury,
and some other substances. These volatile substances react upon one
another, and many new compounds are thus formed. By the action of
sulphurous acid and sulphuretted hydrogen on each other, the sulphur
so common in volcanic districts is separated and deposited. The
hydrochloric acid acts very energetically on the rocks around the vents,
uniting with the iron in them to form the yellow ferric-chloride, which
often coats the rocks round the vent and is usually mistaken by casual
observers for sulphur.

Some of the substances emitted by volcanic vents, such as hydrogen and
sulphuretted hydrogen, are inflammable, and when they issue at a high
temperature these gases burst into flame the moment that they come
into contact with the air. Hence, when volcanic fissures are watched at
night, faint lambent flames are frequently seen playing over them, and
sometimes these flames are brilliantly colored, through the presence
of small quantities of certain metallic oxides. Such volcanic flames,
however, are scarcely ever strongly luminous, and the red, glowing light
which is observed over volcanic mountains in eruption is due to quite
another cause. What is usually taken for flame during a volcanic
eruption is simply, as we have before stated, the glowing light of the
surface of a mass of red-hot lava reflected from the cloud of vapor and
dust in the air, much as the lights of a city are reflected from the
water vapor of the atmosphere during a night of fog.

Besides the volatile substances which issue from volcanic vents,
mingling with the atmosphere or condensing upon their sides, there
are many solid materials ejected, and these may accumulate around the
orifice's till they build up mountains of vast dimensions, like Etna,
Teneriffe, and Chimborazo. Some of these solid materials are evidently
fragments of the rock-masses, through which the volcanic fissure has
been rent; these fragments have been carried upwards by the force of
the steam-blast and scattered over the sides of the volcano. But the
principal portion of the solid materials ejected from volcanic orifices
co