The sand of Puzzoli was used by the Romans, as it is by the moderns, to form a mortar capable of hardening under water. Pliny gives us some idea of the Roman bricks, which are known to have been of an excellent quality. There were three sizes of bricks used by the Romans. 1. Lydian, which were 1½ foot long and 1 foot broad. 2. Tetradoron, which was a square of 16 inches each side. 3. Pentadoron, which was a square, each side of which was 20 inches long. Doron signifies the palm of the hand: of course it was equivalent to 4 inches. X.--PRECIOUS STONES AND MINERALS. Pliny has given a pretty detailed description of the precious stones of the ancients; but it is not very easy to determine the specific minerals to which he alludes. 1. The description of the diamond is tolerably precise. It was found in Ethiopia, India, Arabia, and Macedonia. But the Macedonian diamond, as well as the adamas cyprius and siderites, were obviously not diamonds, but soft stones. 2. The _emerald_ of the ancients (_smaragdus_) must have varied in its nature. It was a green, transparent, hard stone; and, as colour was the criterion by which the ancients distinguished minerals and divided them into species, it is obvious that very different minerals must have been confounded together, under the name of emerald. Sapphire, beryl, doubtless fluor spar when green, and probably even serpentine, nephrite, and some ores of copper, seem to have occasionally got the same name. There is no reason to believe that the _emerald_ of the moderns was known before the discovery of America. At least it has been only found in modern times in America. Some of the emeralds described by Pliny as losing their colour by exposure to the sun, must have been fluor spars. There is a remarkably deep and beautiful green fluor spar, met with some years ago in the county of Durham, in one of the Weredale mines that possesses this property. The emeralds of the ancients were of such a size (13½ feet, large enough to be cut into a pillar), that we can consider them in no other light than as a species of rock. 3. Topaz of the ancients had a green colour, which is never the case with the modern topaz. It was found in the island Topazios, in the Red Sea.[94] It is generally supposed to have been the _chrysolite_ of the moderns. But Pliny mentions a statue of it six feet long. Now chrysolite never occurs in such large masses. Bruce mentions a green substance in an emerald island in the Red Sea, not harder than glass. Might not this be the emerald of the ancients? [94] The word topazo is said by Pliny to signify, in the language of the Troglodytes, _to seek_. 4. _Calais_, from the locality and colour was probably the Persian turquoise, as it is generally supposed to be. 5. Whether the _prasius_ and _chrysoprasius_ of Pliny were the modern stones to which these names are given, we have no means of determining. It is generally supposed that they are, and we have no evidence to the contrary.