No one can succeed in any work unless he understands how it should be done. Concrete is made of Portland cement, mixed with sand and water and either broken stone, gravel, cinders, or slag; but if any one thinks that he can mix these together without knowing how and produce good concrete, he will make a bad mistake rather than a good building material. First, he must buy Portland cement of the best quality. This cement is made of limestone and clay, or marl, chalk, and slag. These are crushed and ground and put into a kiln which is heated up to 2500 deg. or 3000 deg.F.; that is, from twelve to fourteen times as hot as boiling water. The stone fuses sufficiently to form a sort of clinker. After this has cooled, it is ground so fine that the greater part of it will pass through a sieve having 40,000 meshes to the square inch. To every hundred pounds of this powder, about three pounds of gypsum is added. The mixture is then put into the bags in which we see it for sale in the stores. This powder is so greedy for water that it will absorb the moisture from the air around it. Even in the bags, it begins to harden as soon as it gets some moisture; and as soon as it hardens, it is of no use. The moral of that is to keep your cement in a dry place. The second substance needed in concrete is broken stone or gravel. Of course a hard rock must be selected, such as granite or trap rock. Limestone calcines in a heat exceeding 1000 deg. F., and therefore it cannot be used in fireproof construction. Soft rock, like slate or shale or soft sandstone, will not answer because it is not strong enough. Gravel is always hard. If you look at a cut in a gravel bank, you will usually see strata of sand and then strata of rounded pebbles of different sizes. The sand was once an ancient sea beach; the pebbles were dashed up on it by waves or storms or some change of currents. They were at first only broken bits of rock, but after being rolled about for a few thousand years in the ocean and on the shore, the corners were all rounded. Soft rock would have been ground to powder by such treatment. Sometimes, if there is to be no great strain on the concrete, cinders or pieces of brick may be used instead of stone; and for some purposes they answer very well. The third substance used in concrete is sand; but it must be the right kind of sand, having both fine and coarse grains. These grains need to be sharp, or the cement will not stick to them well. They must also be clean, that is, free from dirt. If you rub sand between your hands, and it soils them, then there is clay or loam with it, and it must not be used in making concrete unless it is thoroughly washed. Another way of testing it is to put it into a glass jar partly full of water and shake it. Then let it settle. If there is soil in the sand, it will appear as a stratum of mud on top of the sand.