t masses instead of small blocks. This is
not so wasteful by any means; but even now there is room for new and
helpful inventions.

III

HOUSES OF SAND

If you wanted to build a house, of what should you build it? In a new
country, people generally use wood; but after a time wood grows
expensive. Moreover, wood catches fire easily; therefore, as a country
becomes more thickly settled and people live close together in cities,
stone and brick are used. Large cities do not allow the building of
wooden houses within a certain distance from the center, and sometimes
even the use of wooden shingles is forbidden. Of late years large
numbers of "concrete" or "cement" houses have been built. Our
grandfathers would have opened their eyes wide at the suggestion of a
house built of sand, and would have felt anxious at every rainfall
lest their homes should suddenly melt away. Even after thousands of
concrete buildings were in use, many people still feared that they
would not stand the cold winters and hot summers of the United States;
but it has been proved that concrete is a success provided it is
properly made.

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 grave