at, that the
water is forced up the fissure or pipe E B, and runs over the rim of the
basin. When the pressure is thus diminished, the steam in the upper part
of the cavity A expands, until all the water D is driven into the pipe;
and when this happens, the steam, being the lighter of the two fluids,
rushes up through the water with great velocity. If the pipe be choked
up artificially, even for a few minutes, a great increase of heat must
take place; for it is prevented from escaping in a latent form in steam;
so that the water is made to boil more violently, and this brings on an
eruption.

Professor Bunsen, before cited, adopts this theory to account for the
play of the "Little Geyser," but says it will not explain the phenomena
of the Great one. He considers this, like the others, to be a thermal
spring, having a narrow funnel-shaped tube in the upper part of its
course, where the walls of the channel have become coated over with
siliceous incrustations. At the mouth of this tube the water has a
temperature, corresponding to the pressure of the atmosphere, of about
212 degrees Fahr., but at a certain depth below it is much hotter. This
the professor succeeded in proving by experiment; a thermometer
suspended by a string in the pipe rising to 266 degrees Fahr., or no
less than 48 degrees above the boiling point. After the column of water
has been expelled, what remains in the basin and pipe is found to be
much cooled.

Previously to these experiments of Bunsen and Descloizeaux, made in
Iceland in 1846, it would scarcely have been supposed possible that the
lower part of a free and open column of water could be raised so much in
temperature without causing a circulation of ascending and descending
currents, followed by an almost immediate equalization of heat. Such
circulation is no doubt impeded greatly by the sides of the well not
being vertical, and by numerous contractions of its diameter, but the
phenomenon may be chiefly due to another cause. According to recent
experiments on the cohesion of liquids by Mr. Donny of Ghent, it appears
that when water is freed from all admixture of air, its temperature can
be raised, even under ordinary atmospheric pressure, to 275 degrees
Fahr., so much does the cohesion of its molecules increase[780] when
they are not separated by particles of air. As water long boiled becomes
more and more deprived of air, it is probably very free from such
intermixture at the bottom of the G