6There is a species of sand which, naturally, possesses extraordinary qualities. It is found about Baiæ and the territory in the neighbourhood of Mount Vesuvius; if mixed with lime and rubble, it hardens as well under water as in ordinary buildings. This seems to arise from the hotness of the earth under these mountains, and the abundance of springs under their bases, which are heated either with sulphur, bitumen, or alum, and indicate very intense fire. The inward fire and heat of the flame which escapes and burns through the chinks, makes this earth light; the sand-stone (tophus), therefore, which is gathered in the neighbourhood, is dry and free from moisture. Since, then, three circumstances of a similar nature, arising from the intensity of the fire, combine in one mixture, as soon as moisture supervenes, they cohere and quickly harden through dampness; so that neither the waves nor the force of the water can disunite them.
2That these lands are affected with heat, as surmised, is evident, because in the mountains of Cumæ and at Baiæ, sweating places are excavated, in which the hot vapour rising upwards from the intensity of the fire, strikes through the earth, and so escapes in these places that they are singularly beneficial for the purpose. It is moreover said that in former times fires under Vesuvius existed in abundance, and thence evolved flames about the fields. Thus that which we call sponge-stone, or Pompeian pumice-stone, burnt from another species of stone, appears to be acted on by fire so as to possess a quality of this sort.
3The species of sponge-stone, however, thence obtained, is not found except in the neighbourhood of Ætna and the hills of Mysia, which the Greeks call κατακεκαυμένοι, and places of such description. If, therefore, in these places hot springs and heated vapours are found in the cavities of the mountains, and the spots are recorded by the antients to have been subject to fires issuing out of the lands, it seems certain that the moisture is extracted from the sand-stone and earth in their neighbourhood, by the strength of the fire, as from lime-stone in a kiln.
4Dissimilar and unequal actions being thus concentrated towards the same end, the great want of moisture quickly supplied by water binds and strongly cements them, and also imparts a rapid solidity, by means of the heat common to both the bodies. It is needless to enquire why, as there are many hot springs in Tuscany, we do not there find a powder, which, for the same reason, would harden under water: should I be thereon questioned, I would thus explain the circumstance.
5All lands do not possess similar qualities; nor is stone universally found. Some lands are earthy, others gravelly, others gritty, others sandy: in short, the quality of land, in different parts of the earth, varies as much as even the climate itself. For instance; on the side of the Apennines towards Tuscany, sand-pits are found in abundance; whereas, on the other side of the Apennines, facing the Adriatic, none are discoverable: so also in Achaia, Asia, and universally on the other side of the sea, such things are not known. It does not therefore follow, that in all places abounding with hot springs all other circumstances should be similar. Nature has not made all things to suit the convenience of man, but differently and fortuitously.
6Hence, in places where the mountains are not earthy, but of stone, the force of the fire escaping through the chinks burns that which is soft and tender, whilst that which is hard is left. Thus the earth of Campania, when burnt, becomes a powder; that of Tuscany a coal. Both of these are of great use in building, one species being very serviceable in land works, the other in works under water. In Tuscany, however, the quality of the material is softer than sandstone, but harder than earth; and from its entire subjection to the action of the sub-existing fire, it becomes that sort of sand which is called carbunculus.