The water with which these three substances are to be mixed must be clean and must contain no acid and no strong alkali. As a general rule, there must be twice as much broken stone as sand. When people first make concrete, they often expect too much of their materials. A good rule for the strongest sort of cement, strong enough for floors on which heavy machines are to stand, is one fourth of a barrel of cement, half a barrel of sand, and one barrel of gravel or broken stone. Apparently this would make one and three fourths barrels; but in reality it makes only about one barrel, because the sand fills in the spaces between the gravel, and the cement fills in the spaces between the grains of sand. There are many sorts of machines on the market for mixing the materials; but small quantities can just as well be mixed by hand. The "mixing-bowl" is a platform, and on this the sand is laid. Then comes the cement; and these two must be shoveled together several times. While this is being done, the broken stone or gravel must be wet, and now it is put on top of the sand and cement and well shoveled together, with just enough water added so that the mass will almost bear the weight of a man. Concrete is impatient to be hardening, and if it is not put into the right place, it will begin promptly to harden in the wrong place, and nothing can be done with it afterwards. If it is to be made in blocks, the moulds must be ready and the concrete put into them at once and well tamped down. For such uses as beams and the sides of tanks where great strength is needed, the cement is often "reinforced," that is, rods of iron or steel are embedded in it. For floors, a sheet of woven wire is often stretched out and embedded. At first only solid blocks, made to imitate rough stone, were used for houses, but the hollow block soon took their place. This is cheaper; houses built this way are warmer in winter and cooler in summer; and it prevents moisture from working through the walls. Many cities have regulations about the use of hollow blocks, all the more strict because concrete is comparatively new as a building material. In Philadelphia the blocks must be composed of at least one barrel of Portland cement to five barrels of crushed rock or gravel. They must be three weeks old or more before being used; the lintels and sills of the doors must be reinforced; and every block must be marked, so that if the building should not prove to be of proper strength, the maker may be known. There would seem, however, to be little question of the quality of the blocks, for samples must pass the tests of the Bureau of Building Inspection.