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.
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