."
Ambrosio said, "We remember it well, we saw it this very spring; we were
carried there to examine some ancient Roman baths, and we were struck by
the blue milkiness of the water, by the magnitude of the source, and by
the disagreeable smell of sulphuretted hydrogen which everywhere
surrounded the lake." The stranger said, "When you return to Latium I
advise you to pay another visit to a spot which is interesting from a
number of causes, some of which I will take the liberty of mentioning to
you. You have only seen one lake, that where the ancient Romans erected
their baths, but there is another a few yards above it, surrounded by
very high rushes, and almost hidden by them from the sight. This lake
sends down a considerable stream of tepid water to the larger lake, but
this water is less strongly impregnated with carbonic acid; the largest
lake is actually a saturated solution of this 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
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, to the presence of which, I conclude, its ancient
use in curing cutaneous disorders may be referred. Its temperature, I
ascertained, was in the winter in the warmest parts above 80 degrees of
Fahrenheit, and it appears to be pretty constant, for I have found it
differ a few degrees only, in the ascending source, in January, March,
May, and the beginning of June; it is therefore supplied with heat from a
subterraneous source, being nearly twenty degrees above the mean
temperature of the atmosphere. Kircher has detailed in his "Mundus
Subterraneus" various wonders respecting this lake, most of which are
unfounded, such as that it is unfathomable, that it has at the bottom the
heat of boiling water, and that floating islands rise from the gulf which
emits it. It must certainly be very difficult, or even impossible, to
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