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and Various Styles of Construction.

3.

is traceable to the tomb of Cæcilia Metella, two miles from the city on the Via Appia, where quarries of this stone are still in use. This material was used for concrete and for pavements. The modern name is Selce.

VI. Pumex, or pumice-stone, was used for internal vaulting, for which its great lightness made it suitable. It is thrown out of the crater of Mount Vesuvius during its eruptions, in the form of white ashes, and, being deposited in thick layers, forms into beds, out of which it is quarried. The vaults of the Colosseum and Pantheon are made of this material.

For more than five hundred years cut stone was used, after which it gradually gave way to other substances: first, concrete; and later on, brick, both of which will be spoken of under the head of Modes of Construction.

The Roman mortar, Materia, was made of lime, mixed with sand, which was either Fossicia, river-sand, or Pulvis Puteolanus, rock-sand, Pozzolana. The latter is found in deposits of great depth over the Campagna, and its quarries on the eastern and southern sides of Rome extend in galleries sometimes miles long. These, when disused, were afterwards in some instances converted into Catacumbæ, catacombs, or places of burial. The best pozzolana sand is rough, crackles in the hand when rubbed, and, as Vitruvius mentions, leaves no stain on a white dress. This sand is of a deep red-and-brown colour, and used properly in the proportion of three to one of lime, and while the lime is quite fresh, becomes a mortar as hard and durable as the stone it cements. Large stones were often held together with clamps of metal, or with bolts of wood let in between two stones, as in the Claudian aqueduct. In the wall of Servius Tullius-where the stones were very large, seven to eight feet long-iron clamps were used, also in the Colosseum. In the Pantheon they were of bronze.

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:

Modes of Construction.

The methods of using the fore-mentioned materials a sufficiently distinct to allow of classification. Opus que ratum is the name given to the earliest kind of constructio in which the stones are rectangular blocks of Tufa, put gether without any cementing substance, and maintained their places by their own weight only. The earliest a best example is the wall known as that of Romulus, the Palatine hill, found at intervals at the foot of the cli Such rude masonry lasted, it is believed, for about a hu dred years after the foundation of the colony. It is usua called the Etruscan style, being represented in the walls Fiesole, Perugia, Cortona, Volterra, and other Etruri towns. The next division of this style differs from t first in the smaller size of the stones, and these are r exclusively of Tufa: the Peperino, from Albano, comes ir use. Examples of this second stage occur in the wall the kingly times on the Aventine, sometimes called 1 Wall of the Latins, after the first settlers on that hill; t seems next in age to the Palatine walls. Also the Pulchr Littus, on the bank of the Tiber, built as a protect against inundations; and a wall, with a tower deep in earth below the church of St. Anastasia, at the foot of Palatine, are specimens of the second style of the Kin and again, the wall of Servius Tullius, built against agger. Here the masonry shews great advance in sty Portions of the old walls of the Kings against the cl of the hills in various parts of Rome, and notably on Viminal, belong to this second style, the stones being { dually smaller as the buildings were later, and being with a tool, not split off the parent rock.

Besides the use of rectangular split or hewn blo

and Various Styles of Construction.

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there occurs another style of early construction, in the form of small stones in irregular polygons. In some districts this is made of the hard mountain limestone, in Rome it is of the Silex or Lava; and where found, it is proof that the material was not cleavable or readily shaped, and was therefore walled-in as the blocks came to hand. These are not cemented, but fitted together with small splinters of stones, wedged into the joints when necessary. But it is very rare in Rome, a small portion existing only on the Viminal; and there is something resembling it in the Emporium, near the port of Rome on the eastern bank of the Tiber, below the Aventine.

The next mode of construction is that called by Vitruvius Opus Incertum and Opus Antiquum, in which the stones are set in lime-mortar. That material was not brought into general use until between two and three centuries before the Christian era; and when the process of making concrete was understood, the advantages of it were seen at once. Walls could be built of rough stone without the trouble of cutting it, and could be as strong and durable as the best wall of cut stone. The earliest example remaining of this mode of construction in concrete is believed to be the lofty wall on the Palatine Hill, by the side of the wall of Romulus. It is extremely rude, and there is no attempt at ornamenting its surface. The earliest use of lime-mortar that can be fixed with a date is in the Emporium, B.C. 175; and here the surface of the wall is faced with the Opus Incertum, which preceded reticulated or network. The mass of the wall is concrete, of rough stone and mortar, but the rough surface is not allowed to appear. The same construction is admirably illustrated in the lofty wall called the Muro Torto, of the time of Sylla, against the face of the Pincian Hill, close to the Porta Flaminia, or northernmost gate of Rome. From that time forward the mass of a Roman wall is almost always of concr though generally faced with reticulated-work or brick.

The reason that the Roman walls are so durable is, they were made of concrete, the lime for which burnt on the spot, and used quite fresh. Every hour t lime is kept, it loses some of its strength: it abso moisture from the atmosphere, which causes it to expa and form crystals rapidly; and after it is once crystalli its binding power is gone: the more rough the mate that it has to combine with, the stronger is the concr The Opus reticulatum is often supposed by casual obse ers to be of brick, as the small diamond-shaped blo or wedges of Tufa look very like brick at first sight; th are also at the angles blocks of larger size, closely res bling modern English bricks, but rather larger; they a however, of stone, with a flat surface, and wedged or poin off at the back, and being driven into the concrete w wet, before the lime had set, the whole wall, including surface, is bound together in the firmest manner.

Opus lateritium succeeded to stonework about the ti of Augustus, and from that time was most generally us The Romans excelled in brick-making and in brick-walli as conspicuously as in stone. The earliest bricks, late. are said to have been made of the mud in the Tiber's b a natural compound of clay and sand. While the bri used in the walls of Babylon and Persepolis were sun-dr only, the Roman bricks were well burnt in kilns. T earliest examples are in the form of thin flat tiles, mix with Pozzolana sand, very well made and very hard. the first century, the Imperial brickwork attained its great excellence. No better specimen of it can be seen than the magnificent arcade of Nero's aqueduct his continuat of the Aqua Claudia, within the walls, near the Porta P nestina, now Porta Maggiore, and along the Cœlian H • See Note A, p. 1o.

and Various Styles of Construction.

7

After the early part of the second century the workmanship declines, and the very fine brickwork of the time of Hadrian is not maintained in the Colosseum, towards the end of the second century.

It may be here mentioned that the Colosseum, faced on the exterior with cut Travertine stone, while the main structure is concrete, has its inner walls faced with bricks, some of which are of the time of Nero. In the third and fourth centuries, although most important buildings were still erected, the brick walls in them were of very inferior quality to what had gone before. The Thermæ of Caracalla and of Diocletian, still standing in enormous masses, the later palaces of the Cæsars on the south-eastern part of the Palatine, and the grand villa of the Imperial Gordian family, splendid as they must have been, were not equal in construction to the Neronian aqueduct, or the Thermæ of Titus, or the Castra Prætoria of Tiberius's time, or the Porta Ardeatina, all in the circuit of the walls of Rome, and all shewing the same fine character of the early brickwork. One test of the age to which bricks belong, adopted upon the careful observation of Roman antiquaries, is to count the number of bricks occurring in a foot or yard of wall-work as it stands: the more bricks, the earlier and better the style. Those of Nero usually number nine or ten to the English foot; in later work the bricks become thicker, and more mortar is used between them; until in the time of Constantine, and in the fourth century generally, there are frequently only four bricks to the foot, as in modern building; and while the brick itself is coarser and more spongy in appearance, the mortar is an inch thick between the courses, or more, whereas in the early work it is scarcely shewn.

Another method of obtaining the dates of brick buildings is derived from the usage that prevailed of stamping the