l. 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.

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 come