A complete study of these details could not be made in Babylon as we were cramped for space and could not spread out the pieces. The chemical preservation of them was carried out in Berlin with great care under the able direction of Professor Rathgen. The antiquities from the ruined sites, more especially the pottery, were completely permeated with salts, saltpetre, and the like. These materials, owing to long exposure to air, had formed hard crystals on the surface, which had to be removed by long-continued soaking. Here in Babylon also we numbered each piece so that we could be certain at what part of the Processional Street each fragment had been found. The transverse cut in the wall _u_ 13 of the plan of Kasr (Fig. 13) gives an excellent insight into the method of construction. Over every course of brick is a thin layer of asphalt, and above this an equally thin layer of mud and then another course of bricks. The joints of the course, which are from 1 to 1½ centimetres thick, are also formed of asphalt and mud. In every fifth course a matting made of reeds, the stalks of which have been split and rendered flexible by beating, is substituted for the mud. The matting itself has rotted, but the impression left on the asphalt is still perfectly fresh and recognisable. In appearance it corresponds exactly with the ordinary matting in use in the neighbourhood to-day. A determined and very remarkable effort was obviously made to separate the courses, to prevent their adhering to each other, overlaid as they were with asphalt. This separation occurs in other parts of the city effected by reed straw instead of mud. Only in some few detached instances were the bricks laid immediately on the bitumen, where they fitted together as firmly as a rock, as in the wall 17 metres thick which in _k_ 13 runs through the great Principal Citadel, in the southern strongest part of the Ishtar Gateway, and also in the postament of the cella in the temple of Borsippa. We may add that asphalt and mud, or asphalt and reed straw are regularly used for joints throughout the period of the Babylonian kings. Only in his latest buildings, the Kasr, the Principal Citadel, and Babil, did Nebuchadnezzar change to lime mortar, while Nabonidus for his Euphrates wall turned once more to asphalt. The later builders, Persians, Greeks, and Parthians, employed mud for mortar. The asphalt mortar in the great defensive walls of Babylon and the inserted mats are mentioned by Herodotus (i. 179): he records that after every 30 courses of bricks a plaited mat was inserted. So large a number has not yet been observed by us. The lowest number is 5, the highest 13. In the Babylonian, inscriptions on buildings, especially on those of Nebuchadnezzar, asphalt is very often mentioned in connection with burnt brick, but never mud, lime, or reeds. VI THE ISHTAR GATE