ree from moisture. Hence, when the three substances, all
formed on a similar principle by the force of fire, are mixed together,
the water suddenly taken in makes them cohere, and the moisture quickly
hardens them so that they set into a mass which neither the waves nor
the force of the water can dissolve.

2. That there is burning heat in these regions may be proved by the
further fact that in the mountains near Baiae, which belongs to the
Cumaeans, there are places excavated to serve as sweating-baths, where
the intense heat that comes from far below bores its way through the
earth, owing to the force of the fire, and passing up appears in these
regions, thus making remarkably good sweating-baths. Likewise also it is
related that in ancient times the tides of heat, swelling and
overflowing from under Mt. Vesuvius, vomited forth fire from the
mountain upon the neighbouring country. Hence, what is called
"sponge-stone" or "Pompeian pumice" appears to have been reduced by
burning from another kind of stone to the condition of the kind which we
see.

3. The kind of sponge-stone taken from this region is not produced
everywhere else, but only about Aetna and among the hills of Mysia which
the Greeks call the "Burnt District," and in other places of the same
peculiar nature. Seeing that in such places there are found hot springs
and warm vapour in excavations on the mountains, and that the ancients
tell us that there were once fires spreading over the fields in those
very regions, it seems to be certain that moisture has been extracted
from the tufa and earth, by the force of fire, just as it is from
limestone in kilns.

4. Therefore, when different and unlike things have been subjected to
the action of fire and thus reduced to the same condition, if after
this, while in a warm, dry state, they are suddenly saturated with
water, there is an effervescence of the heat latent in the bodies of
them all, and this makes them firmly unite and quickly assume the
property of one solid mass.

There will still be the question why Tuscany, although it abounds in hot
springs, does not furnish a powder out of which, on the same principle,
a wall can be made which will set fast under water. I have therefore
thought best to explain how this seems to be, before the question should
be raised.

5. The same kinds of soil are not found in all places and countries
alike, nor is stone found everywhere. Some soils are earthy; others
gravell