, between Rome and Tivoli, is
the Lake of the Solfatara, called also Lago di Zolfo (lacus albula),
into which flows continually a stream of tepid water from a smaller
lake, situated a few yards above it. The water is a saturated solution
of carbonic acid gas, which escapes from it in such quantities in some
parts of its surface, that it has the appearance of being actually in
ebullition. "I have found by experiment," says Sir Humphry Davy, "that
the water taken from the most tranquil part of the lake, even after
being agitated and exposed to the air, contained in solution more than
its own volume of carbonic acid gas, with a very small quantity of
sulphuretted hydrogen. Its high temperature, which is pretty constant at
80 degrees of Fahr., and the quantity of carbonic acid that it contains,
render it peculiarly fitted to afford nourishment to vegetable life. The
banks of travertin are everywhere covered with reeds, lichen, confervae,
and various kinds of aquatic vegetables; and at the same time that the
process of vegetable life is going on, the crystallizations of the
calcareous matter, which is everywhere deposited, in consequence of the
escape of carbonic acid, likewise proceed. There is, I believe, no place
in the world where there is a more striking example of the opposition or
contrast of the laws of animate and inanimate nature, of the forces of
inorganic chemical affinity, and those of the powers of life."[316]

The same observer informs us that he fixed a stick in a mass of
travertin covered by the water in the month of May, and in April
following he had some difficulty in breaking, with a sharp-pointed
hammer, the mass which adhered to the stick, and which was several
inches in thickness. The upper part was a mixture of light tufa and the
leaves of confervae; below this was a darker and more solid travertin,
containing black and decomposed masses of confervae; in the inferior part
the travertin was more solid, and of a gray color, but with cavities
probably produced by the decomposition of vegetable matter.[317]

The stream which flows out of this lake fills a canal about nine feet
broad and four deep, and is conspicuous in the landscape by a line of
vapor which rises from it. It deposits calcareous tufa in this channel,
and the Tiber probably receives from it, as well as from numerous other
streams, much carbonate of lime in solution, which may contribute to the
rapid growth of its delta. A large proportion o