ime mortar may have its strength considerably enhanced by the
addition of a small proportion of Portland cement. Roman cement is rarely
used for mortar, but is useful in some cases on account of the rapidity
with which it sets, usually becoming hard about fifteen minutes after
mixing. It is useful in tidal work and embankments, and constructions under
water. It has about one-third of the strength of Portland cement, by which
it is now almost entirely supplanted. Selenitic cement or lime, invented by
Major-General H. Y. D. Scott (1822-1883), is lias lime, to which a small
proportion of plaster of Paris has been added with the object of
suppressing the action of slaking and inducing quicker setting. If
carefully mixed in accordance with the instructions issued by the
manufacturers, it will take a much larger proportion of sand than ordinary
lime.

Lime should be slaked before being made into mortar. The lime is measured
out, deposited in a heap on a wooden "bank" or platform, and after being
well watered is covered with the correct proportion of sand. This retains
the heat and moisture necessary to thorough slaking; the time required for
this operation depends on the variety of the lime, but usually it is from a
few hours to one and a half days. If the mixing is to be done by hand the
materials must be screened to remove any unslaked lumps of lime. The
occurrence of these may be prevented by grinding the lime shortly before
use. The mass should then be well "larried," _i.e._ mixed together with the
aid of a long-handled rake called the "larry." Lime mortar should be
tempered for at least two days, roughly covered up with sacks or other
material. Before being used it must be again turned over and well mixed
together. Portland and Roman cement mortars must be mixed as required on
account of their quick-setting properties. In the case of Portland cement
mortar, a quantity sufficient only for the day's use should be "knocked
up," but with Roman cement fresh mixtures must be made several times a day,
as near as possible to the place of using. Cement mortars should never be
worked up after setting has taken place. Care should be taken to obtain the
proper consistency, which is a stiff paste. If the mortar be too thick,
extra labour is involved in its use, and much time wasted. If it be so thin
as to run easily from the trowel, a longer time is taken in setting, and
the wall is liable to settle; also there is danger that the lime