eysers.

Among other results of the experiments of Bunsen and his companion, they
convinced themselves that the column of fluid filling the tube is
constantly receiving accessions of hot water from below, while it
becomes cooler above by evaporation on the broad surface of the basin.
They also came to a conclusion of no small interest, as bearing on the
probable mechanism of ordinary volcanic eruptions, namely that the tube
itself is the main seat or focus of mechanical force. This was proved by
letting down stones suspended by strings to various depths. Those which
were sunk to considerable distances from the surface were not cast up
again, whereas those nearer the mouth of the tube were ejected to great
heights. Other experiments also were made tending to demonstrate the
singular fact, that there is often scarce any motion below, when a
violent rush of steam and water is taking place above. It seems that
when a lofty column of water possesses a temperature increasing with the
depth, any slight ebullition or disturbance of equilibrium in the upper
portion may first force up water into the basin, and then cause it to
flow over the edge. A lower portion, thus suddenly relieved of part of
its pressure, expands and is converted into vapor more rapidly than the
first, owing to its greater heat. This allows the next subjacent
stratum, which is much hotter, to rise and flash into a gaseous form;
and this process goes on till the ebullition has descended from the
middle to near the bottom of the funnel.[781]

In speculating, therefore, on the mechanism of an ordinary volcanic
eruption, we may suppose that large subterranean cavities exist at the
depth of some miles below the surface of the earth, in which melted lava
accumulates; and when water containing the usual mixture of air
penetrates into these, the steam thus generated may press upon the lava
and force it up the duct of a volcano, in the same manner as a column of
water is driven up the pipe of a Geyser. In other cases we may suppose a
continuous column of liquid lava mixed with _red-hot water_ (for water
may exist in that state, as Professor Bunsen reminds us, under
pressure), and this column may have a temperature regularly increasing
downwards. A disturbance of equilibrium may first bring on an eruption
near the surface, by the expansion and conversion into gas of entangled
water and other constituents of what we call lava, so as to occasion a
diminution of pressur