Chap. IV.

HOMAN.

95

they contained were driven out, whilst a portion of heat was acquired and retained,which was dissipated by immersion into water. For this reason limestone was said to beheavier before than after it was burnt, and that it lost one-third of its weight, although inbulk it remained the same. The pores of limestone being rendered open by the expulsionof air and water, enabled the sand more readily to mix with and adhere to it.

Neither pure earth nor sand without lime could form a cement or mortar, or unitetogether polished bodies ; and the Homans evidently well understood the method of pre-paring their lime, as well as mixing it with other materials for the composition of theirmortars and cements : all the works left us in which these are employed, time hashardened into a mass equal in strength to the stone or tile which is imbedded.

Pozzolatia, called a species of sand, or arenarium, is found abundantly in the neighbour-hood of Home, was used mixed with a proportionate quantity of lime to form a cement.The colour of Pozzolana varies, and the catacombs were probably formed by the extraction ofthis material. The ancient Pulvis Puteolana, mentioned by Vitruvius , was drawn fromthe neighbourhood of Pozzuoli, and its application may be seen in the ruins of Caligula ’sbridge in the port of Anzio, and in the mole of Pozzuoli . That found about Baia*,when mixed with lime and rubble, would harden as well under water as in ordinarybuildings, and this Vitruvius attributes to the heat of the earth, and the sulphur, bitumen,or alum, which the water holds in solution. Inward and subterraneous fires render thisearth light and dry, but when moisture supervenes, the particles cohere in such a manner,that neither the waves nor the force of water can disunite them.

Spong or Pompeian pumice-stone, burnt from another species of stone, is acted upon bythe fire in a similar way. Hot springs and heated vapours in the bowels of the earth weresupposed to do what was effected in the lime-kiln, and that the moisture driven out was,when quickly supplied by water, able again to unite the particles in a more solid state thanbefore, by means of the heat common to both bodies.

Some lands afford sand-pits in abundance, as the Apennines towards Tuscany , whileon the other side none are to be met with; and some mountains are not earthy, but ofstone. The force of the subterraneous fires, escaping through the chinks, burns that whichis soft and tender: thus the earth of Campania , so burnt, becomes a powder, and that ofTuscany , which is of a harder quality, is converted into coal. Both of these materials areof great use, one being serviceable for constructions on land, the other for works under water.In Tuscany the quality of the material is softer than sandstone, but harder than earth, andconstitutes that sort of sand called carbunculous.

Stones used by the Homans. — 'Hie practice adopted by the engineers of Italy of thepresent day, in the selection of their building materials, has not at all changed since the timeof the republic : the territory of Bolsena and Stratone is still renowned for stone, whichneither fire nor weather will affect; and a beautiful white stone, easily cut with the saw, andbearing a fine polish, is found throughout Lombardy , and applied in situations where frostcannot affect it. The limestones of Istria are used in Venice and elsewhere at the present day.

The ancients do not seem to have thoroughly understood the strength of the severalqualities of stone; but were satisfied that no weight they could expose it to under ordinarycircumstances would be too much for it to bear, or occasion it to crush ; if in a mountainmiles high it could carry the superincumbent weight, they had no fear of the result whenused in a structure of ordinary height.

In the Campagna of Home, a stone is found of a dark colour, which is easily worked, andresists both the action of fire and the atmosphere, but it has the peculiar property ofabsorbing all the water from the mortar or cement in which it is bedded, and therefore isonly applicable to walls or constructions in a dry situation.

Most of the building stones readily obtained are those of recent formation, occasioned bydeposits from water holding lime in considerable quantities dissolved by carbonic acid ;these deposits gave the ancients the notion that stone grew : the banks of the river Neva soincreased, that the valley became closed up and formed it into a lake, and in other situationsmasses of stone were seen to grow almost daily, from the deposit and evaporation of thewater. Such stone is always soft when first cut from the bed, and hardens as the water itcontains is evaporated from it. From the quality of the water many of the aqueductsbecame encrusted in a similar manner, and their channel considerably diminished.

The travertine, the tufa, the pepperino, and the gabina were used in foundations, for ex-ternal walls, and for the filling in of walls and vaults. The selce was only used for pavingstreets, and the internal parts of walls; the pumice stone, from its lightness, for the constructionof arches and domes. Tufa abounds in the neighbourhood of Rome , and particularly wherethe ancient excavations were made, beyond the Porta Maggiore , five miles from the ViaCollatina, on the left. Strabo , lib. v. p. 164., describes this material as a volcanic productof a reddish hue, not very compact, and easily decomposed by the action of the air alone.In foundations we find it abundantly used, as on the Palatine hill , in the temple of FortunaVirilis, and in the aqueduct of Claudius . When used for construction above ground, theexterior was covered with a coat of plaster : it was quarried in large masses.