When we speak of construction, we mean the material used in building and the way it is put together. The different historical periods of building are now classed into distinct dates, which have been arrived at by observing the material used, and the way it is used, in buildings of which there is no doubt as to the date of erection, and comparing it with others. The early Greek Period in Italy is marked by massive walls of masonry--walls built from the stone of the vicinity, the blocks being rough as hewn out of the quarry,--polygonal. The later Greek Period and the Etruscan are identical, being formed of square blocks of stone, headers, and stretchers. In the time of the kings of Rome the stones were squared, and were of tufa, lapis ruber, tophus. In the earliest walls they are close jointed; in the second period the edges are bevelled. During the Republic the stones were also squared, but the material was of peperino. Lapis Albanus and other forms of working up the material were introduced. Pieces of stone, fixed together with cement, gave a new kind of wall called _opus incertum_. This was improved upon by facing the outside of the small pieces of stone and making them of one uniform size--small polygonal. Then the stones were cut into wedge shapes: the point being inwards, and being laid in regular rows it has the appearance of network, and is called _opus reticulatum_. This work, introduced in the last years of the Republic, went out of fashion after the time of Tiberius, but was revived by Hadrian, who always set his reticulated work in bands of brick like a picture frame, thus distinguishing his from the earlier work, the inside of the walls in those cases being concrete. The earliest brick building which we have is the Pantheon. Thus it was under Augustus that brick was first used by the Romans. It was his boast that he found Rome of brick, and left it marble; which is only true in a certain sense, for he did not build of solid marble, but cased veneering marble on to the brickwork. One period of Roman brickwork can easily be distinguished from the others by measuring the number of bricks in a foot, and noticing their uniformity of size. This, of course, does not refer to ornamental brickwork. The brickwork of Nero is the best in the world--thin narrow bricks, tiles, with very little mortar between them. Before his time it was not quite so good; but after, it gradually declined till the cement is as thick as the bricks.