the effect that the essence of the action consists in the outbreak of
water vapour at a high temperature, and therefore endowed with very
great expansive force. Along with this steam there are many other
gases, which always appear to be but a very small part of the whole
escape of a vaporous nature--in fact, the volcanic steam, so far as
its chemical composition has been ascertained, has the composition
which we should expect to find in rock water which had been forced out
from the rock by the tensions that high temperature creates.

Because of its conspicuous nature, the lava which flows from most
volcanoes, or is blown out from them in the form of finely divided
ash, is commonly regarded as the primary feature in a volcanic
outbreak. Such is not really the case. Volcanic explosions may occur
with very little output of fluid rock, and that which comes forth may
consist altogether of the finely divided bits of rock to which we give
the name of ash. In fact, in all very powerful explosions we may
expect to find no lava flow, but great quantities of this finely
divided rock, which when it started from the depths of the earth was
in a fluid state, but was blown to pieces by the contained vapour as
it approached the surface.

If the student is so fortunate as to behold a flood of lava coming
forth from the flanks of a volcano, he will observe that even at the
very points of issue, where the material is white-hot and appears to
be as fluid as water, the whole surface gives forth steam. On a still
day, viewed from a distance, the path of a lava flow is marked by a
dense cloud of this vapour which comes forth from it. Even after the
lava has cooled so that it is safe to walk upon it, every crevice
continues to pour forth steam. Years after the flowing has ceased, and
when the rock surface has become cool enough for the growth of