r points; but stone possessing
one or more of the above qualities, according to the purpose for which
it is required, may be regarded as good for that purpose.

Strength of Stone.--Stone should only be subjected to a compressive
strain. It is occasionally subject to a cross strain, as in lintels
over doors and windows; these are, however, contrary to the true
principles of construction, and should not be allowed except a
strong relieving arch is turned over them. The strength of stone in
compression is about 120 tons per square foot for the weakest stones,
and about 750 tons per square foot for the strongest. No stones are,
however, subjected to anything like this amount of compressive force;
in the largest buildings it does not amount to more than twelve or
fourteen tons per square foot.

Hardness of Stone.--This is of more importance than its strength,
especially in pavements or steps, where it is subject to great wear;
also in plinths and quoins of buildings where it is desired to preserve
a good face and sharp arris. The order of strength and hardness of stone
is--(1) Basalt, (2) granite, (3) limestone, (4) sandstone. Granite,
seinite, and gneiss take the first, place for strength, hardness and
durability, but they will not stand a high temperature. "Stones which
are of a fine, uniform grain, compact texture and deep color are the
strongest; and when the grain, color, and texture are the same, those
are the stongest which are the heaviest; but otherwise the strength does
not increase with the specific gravity." Great hardness is objectionable
when the stone has to be worked with a chisel, owing to the labor
required to work it. Hard stones, also, generally wear smooth, and
become polished, which makes them unsuitable for some purposes.
Brittleness is a defect which frequently accompanies hardness,
particularly in coarse-grained stones; it prevents them from being
worked to a true surface, and from receiving a smooth edge at the
angles. Workmen call those hard stones which can only be sawn into slabs
by the grit saw, and those soft which can be separated by a common saw.

Expansion of Stone by Heat.--Rocks are expanded by heat and contracted
by cooling. Variation in temperature thus causes some building stones
to alternately expand and contract, and this prevents the joints of
masonry from remaining close and tight. In the United States with an
annual thermometric range of more than 90 deg. Fah., this difficulty
l