fathom a source which rises with so much violence from a subterraneous
excavation, and, at a time when chemistry had made small progress, it was
easy to mistake the disengagement of carbonic acid for an actual
ebullition. The floating islands are real, but neither the Jesuit nor
any of the writers who have since described this lake had a correct idea
of their origin, which is exceedingly curious. The high temperature of
this water, and the quantity of carbonic acid that it contains, render it
peculiarly fitted to afford a pabulum or nourishment to vegetable life.
The banks of travertine are everywhere covered with reeds, lichens,
confervae, and various kinds of aquatic vegetables, and, at the same time
that the process of vegetable life is going on, the crystallisations of
the calcareous matter, which is everywhere deposited in consequence of
the escape of carbonic acid, likewise proceed, giving a constant
milkiness to what, from its tint, would otherwise be a blue fluid. So
rapid is the vegetation, owing to the decomposition of the carbonic acid,
that, even in winter, masses of confervae and lichens, mixed with
deposited travertine, are constantly detached by the currents of water
from the bank and float down the stream, which being a considerable river
is never without many of these small islands on its surface; they are
sometimes only a few inches in size, and composed merely of dark-green
confervae or purple or yellow lichens, but they are sometimes even of
some feet in diameter, and contain seeds and various species of common
water-plants, which are usually more or less encrusted with marble. 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. Vegetables in such a temperature, and everywhere
surrounded by food, are produced with a wonderful rapidity, but the
crystallisations are formed with equal quickness, and they are no sooner
produced than they are destroyed together. Notwithstanding the