4. The cement employed in laying marble in common or large edifices, is somewhat different from that just described, as it consists of about three fourths of lime and one of sand. The latter substance is obtained, in an unmixed state, on the bays in every part of the world; hence it has received the appellation of _bay sand_. 5. When it cannot be conveniently had in a pure state, particles of the same kind can be separated in sufficient quantities from their admixture with other substances. This is effected by sifting the compound through a sieve, into a small stream of water, which carries off the lighter particles that are unfit for use, whilst the sand, by its superior specific gravity, sinks to the bottom. The part which may be too coarse, remains in the sieve. This, however, except the rubbish, can be used in the coarser kinds of masonry. 6. The mortar, used in laying bricks and common stone, has a greater proportion of sand, which is generally of an inferior quality. Besides, the materials are incorporated with less care. Lime for the purposes of building is procured chiefly by calcining limestone in a kiln, with wood, coal, or some other combustible substance. It is also obtained by burning chalk, marble, and marine shells. Water poured upon newly-burnt or _quick_ lime, causes it to swell, and fall to pieces into a fine powder. In this state it is said to be _slacked_. 7. Masonry is often required in situations under water, especially in the construction of bridges and locks of canals. Common mortar resists the action of the water very well, when it has become perfectly dry; yet, if it is immersed before it has had time to harden, it dissolves, and crumbles away. 8. The ancient Romans, who practised building in the water to a great extent, discovered a material, which, when incorporated with lime, either with or without sand, possessed the property of hardening in a few minutes even under water. This was a kind of earth found at Puteoli, to which was given the name of _pulvis puteolanus_, and which is the same now called _puzzolana_. 9. A substance denominated _tarras_, _terras_, or _tras_, found near Andernach, in the vicinity of the Rhine, possesses the same quality with puzzolana. It is this material which has been principally employed by the Dutch, whose aquatic structures are superior to those of any other nation in Europe. Various other substances, such as baked clay and calcined greenstone, reduced to powder, afford a tolerable material for water-cements. Several quarries of water lime, which is similar in appearance to common limestone, has been lately discovered in the United States, which, being finely pulverized and mixed with sand, makes very good water-cement.