‹‹‹ Vitr. 8.2.3 | Table of Contents | Vitr. 8.2.5 ›››
4A corroboration of this may be seen in a hot bath; for it is absurd to suppose that there can be a spring above its ceiling; and yet that, when warmed by the heated air from the furnace, attracts the moisture from the pavement, whence it is carried up to the vaulting of the ceiling, where it hangs. For hot vapours always ascend, and at first, from their lightness, do not fall down, but as soon as condensed, their gravity prevents buoyancy, and they drop on the heads of the bathers. In the same manner the atmospheric air, when warmed by the sun, raises the moisture from all places, and gathers it to the clouds: for the earth acted upon by heat, drives out its moisture, as heat drives out perspiration from the human body.
4Of course there can be no springs above the vaultings of hot bathrooms, but the atmosphere in such rooms, becoming well warmed by the hot air from the furnaces, seizes upon the water on the floors, and takes it up to the curved vaultings and holds it up there, for the reason that hot vapour always pushes upwards. At first it does not let the moisture go, for the quantity is small; but as soon as it has collected a considerable amount, it cannot hold it up, on account of the weight, but sprinkles it upon the heads of the bathers. In the same way, when the atmospheric air feels the heat of the sun, it draws the moisture from all about, causes it to rise, and gathers it into clouds. For the earth gives out moisture under the influence of heat just as a man’s heated body emits sweat.