or solid. If we were to judge merely from the
temperatures reasonably believed to exist at a depth of some twenty
miles, and if we might overlook the question of pressure, we should
certainly say that the earth's interior must be in a fluid state. It
seems at least certain that the temperatures to be found at depths of
two score miles, and still more at greater depths, must be so high that
the most refractory solids, whether metals or minerals, would at once
yield if we could subject them to such temperatures in our laboratories.
But none of our laboratory experiments can tell us whether, under the
pressure of thousands of tons on the square inch, the application of
any heat whatever would be adequate to transform solids into liquids.
It may, indeed, be reasonably doubted whether the terms solid and
liquid are applicable, in the sense in which we understand them, to the
materials forming the interior of the earth.

"A principle, already well known in the arts, is that many, if not all,
solids may be made to flow like liquids if only adequate pressure be
applied. The making of lead tubes is a well-known practical illustration
of this principle, for these tubes are formed simply by forcing solid
lead by the hydraulic press through a mould which imparts the desired
shape.

"If then a solid can be made to behave like a liquid, even with such
pressures as are within our control, how are we to suppose that the
solids would behave with such pressures as those to which they are
subjected in the interior of the earth? The fact is that the terms solid
and liquid, at least as we understand them, appear to have no physical
meaning with regard to bodies subjected to these stupendous pressures,
and this must be carefully borne in mind when we are discussing the
nature of the interior of the earth."

THE VOLCANO A SAFETY VALVE

Whatever be the state of affairs in the depths of the earth's crust, we
may look upon the volcano as a sort of safety-valve, opening a passage
for the pent-up forces to the surface, and thus relieving the earth from
the terrible effects of the earthquake, through which these imprisoned
powers so often make themselves felt. Without the volcanic vent there
might be no safety for man on the earth's unquiet face.

Professor J. C. Russell, of Michigan University, presents the following
views concerning the status and action of volcanoes:--

"When reduced to its simplest terms, a volcano may be defined as a