d and 1 of cement will give about 225 lb. per
sq. in. at twenty-eight days. Formerly the soundness of cement was
determined by keeping thin pats of the cement in cold water for
twenty-eight days, or in warm water (110 Deg.-120 Deg. F.) for
twenty-four hours, and examining for cracks or other signs of
expansion. Modern practice is to measure the expansion of a test piece
of cement kept in water at a temperature of 212 Deg. F. The simplest
and most generally used method is due to H.L. le Chatelier, and
consists in measuring the increase in circumference of a cylinder of
cement 30 mm. in diameter by means of a split ring encircling the
cylinder, the motion of which is magnified by two light rods extending
radially. Another quantitative test for soundness is that formulated
by L. Deval, who has shown that briquettes of 3 of sand and 1 of
cement kept in water for two days at 80 Deg. C. = 176 Deg. F. attain
approximately the same strength as similar briquettes attain at seven
days in water at the ordinary temperature. In like manner briquettes
kept at 176 Deg. F. for seven days are approximately equal in strength
to those kept at the ordinary temperature for twenty-eight days. A
cement not perfectly sound will give low results in the hot test, and
a cement of indifferent soundness will crack and go to pieces. The
test is admittedly severe, but can be passed without difficulty by
cement made with proper care and skill. There are many modifications
and elaborations of all the tests which have been mentioned. Cement
for all important work is submitted to a rigorous system of testing
and analysis before it is accepted and used.

_Hydraulic Lime_ is a cement of the Portland as distinct from the
pozzuolanic class. The most typical hydraulic lime is that known as
Chaux du Theil, made from a limestone found at Ardeche in France. This
limestone consists of calcium carbonate most intimately intermixed with
very finely divided silica. It contains but little alumina and oxide of
iron, which are the constituents generally necessary to bring about the
union of silica and lime to form a cement, but in spite of this the
silica is so finely divided and so well distributed that it unites
readily with the lime when the limestone is burned at a sufficiently
high temperature. English hydraulic limes are of a different class; they
contain a good deal of alumina and ferric oxide, and in composition
re