ly, mortar or cement.

All mortars and, in fact, all the cementing materials used (except
bituminous ones) in bricklaying have lime as their base, and depend upon
the setting quality of quicklime, which has to be mixed with sand or
some suitable substitute for it, to make mortars. Limes and cements are
far too wide a subject to be dealt with as part of an evening's lecture
on another topic, and no doubt they will hereafter form the subject of a
lecture or lectures. To-night I propose only to remind you that there
are such substances as these, and that they possess certain qualities
and are obtainable and available for the bricklayer's purposes, without
attempting an investigation into the chemistry of cements, or their
manufacture, etc. Ordinarily, brickwork may be divided into brickwork in
mortar and in cement; but there are many qualities of mortar and several
sorts of cement. Mortar made with what are called fat or rich
limes--that is to say, nearly pure lime, such as is got by calcining
marble or pure chalk--sets slowly, with difficulty, and is rarely
tenacious. Burnt clay or brick reduced to powder improves the setting of
such lime, especially if the two materials be calcined together; so will
an admixture of cement. Mortar made with what is known as slightly
hydraulic lime, that is to say, lime containing a small proportion of
clay, such as the gray stone lime of Dorking, Merstham, and that
neighborhood, sets well, and is tenacious and strong. Mortar made with
hydraulic lime, that is to say, lime with a considerable admixture of
clay, such as the lias lime, sets under water or in contact with wet
earth. It is best to use this lime ground to powder, and not to mix so
much sand with it as is used with stone lime. A sort of mortar called
selenitic mortar, the invention of the late General Scott, has been made
use of in many of the buildings of the School Board for London, and was
first employed on a large scale in the erection of the Albert Hall. The
peculiarity consists in the addition of a small dose of plaster of Paris
(sulphate of lime) very carefully introduced and intimately mixed. The
result is that the mortar so made sets rapidly, and is very hard.

It is claimed that a larger proportion of sand can be used with
selenitic lime than with ordinary, thus counterbalancing the extra
expense occasioned by royalty under the patent and special care in
mixing. When a limestone contains 20 to 40 per cent, of clay, it b