pounded up and sifted, will make your mortar of a better composition to
use.

2. The reason why lime makes a solid structure on being combined with
water and sand seems to be this: that rocks, like all other bodies, are
composed of the four elements. Those which contain a larger proportion
of air, are soft; of water, are tough from the moisture; of earth, hard;
and of fire, more brittle. Therefore, if limestone, without being
burned, is merely pounded up small and then mixed with sand and so put
into the work, the mass does not solidify nor can it hold together. But
if the stone is first thrown into the kiln, it loses its former property
of solidity by exposure to the great heat of the fire, and so with its
strength burnt out and exhausted it is left with its pores open and
empty. Hence, the moisture and air in the body of the stone being burned
out and set free, and only a residuum of heat being left lying in it, if
the stone is then immersed in water, the moisture, before the water can
feel the influence of the fire, makes its way into the open pores; then
the stone begins to get hot, and finally, after it cools off, the heat
is rejected from the body of the lime.

3. Consequently, limestone when taken out of the kiln cannot be as heavy
as when it was thrown in, but on being weighed, though its bulk remains
the same as before, it is found to have lost about a third of its weight
owing to the boiling out of the water. Therefore, its pores being thus
opened and its texture rendered loose, it readily mixes with sand, and
hence the two materials cohere as they dry, unite with the rubble, and
make a solid structure.

CHAPTER VI

POZZOLANA

1. There is also a kind of powder which from natural causes produces
astonishing results. It is found in the neighbourhood of Baiae and in
the country belonging to the towns round about Mt. Vesuvius. This
substance, when mixed with lime and rubble, not only lends strength to
buildings of other kinds, but even when piers of it are constructed in
the sea, they set hard under water. The reason for this seems to be that
the soil on the slopes of the mountains in these neighbourhoods is hot
and full of hot springs. This would not be so unless the mountains had
beneath them huge fires of burning sulphur or alum or asphalt. So the
fire and the heat of the flames, coming up hot from far within through
the fissures, make the soil there light, and the tufa found there is
spongy and f