The black silex[2785] is in general the best; but in some localities, it is the red, and occasionally the white; as in the Anician quarries at Tarquinii, near Lake Volsinius,[2786] for example, and those at Statonia,[2787] the stone of which is proof against fire even.[2788] These stones, sculptured for monumental purposes, are subject to no deterioration by lapse of time: moulds, too, are made from them, for the purpose of fusing copper. There is a green silex, also, which offers a most powerful resistance to the action of fire, but is never found in any large quantities, and, in all cases, in an isolated form, and not as a constituent part of solid rock. Of the other kinds, the pale silex is but rarely used for erections: being of globular form, it is not liable to injury, but at the same time it is insecure for building purposes, unless it is well braced and tightly held together. Nor yet does river silex offer any greater security, for it always has the appearance of being wet. CHAP. 50.—OTHER STONES USED FOR BUILDING. When the nature of stone is doubtful, the proper precaution is, to quarry it in summer, and not to use it for building before the end of a couple of years, leaving it in the meantime to be well seasoned by the weather. The slabs which have been damaged will be found to be better suited for the foundations under ground: while those, on the other hand, which have remained uninjured, may be employed with safety, and exposed to the open air even. CHAP. 51.—THE VARIOUS METHODS OF BUILDING. The Greeks construct party-walls, resembling those of brickwork, of hard stone or of silex, squared. This kind of stonework is what they call “isodomon,”[2789] it being ” pseudisodomon”[2790] when the wall is built of materials of unequal dimensions. A third kind of stonework is called “emplecton,”[2791] the two exteriors only being made with regularity, the rest of the material being thrown in at random. It is necessary that the stones should lie over one another alternately, in such a way that the middle of one stone meets the point of junction of the two below it; and this, too, in the middle of the wall, if possible; but if not, at all events, at the sides. When the middle of the wall is filled up with broken stones, the work is known as “diatoichon.”[2792] The reticulated[2793] kind of building, which is mostly in use at Rome, is very liable to crack.[2794] All building should be done by line and rule, and ought to be strictly on the perpendicular. CHAP. 52. (23.)—CISTERNS. Cisterns should be made of five parts of pure, gravelly, sand, two of the very strongest quicklime, and fragments of silex not exceeding a pound each in weight; when thus incorporated, the bottom and sides should be well beaten with iron rammers. The best plan, too, is to have the cisterns double; so that all superfluities may settle in the inner cistern, and the water filter through, as pure as possible, into the outer one. CHAP. 53.—QUICK-LIME.