and with those over-limed cements which expand
(both of which are not true Portland cements), the filling in is done in
equal sections, with a vertical space equal to each section left between
them until the first sections have become thoroughly hard, and these are
then filled in at a second operation. In order to provide for flues,
air-passages, and ways for electric installations, and for gas and
water, pipes (made of an insulating material if required) or cores of
the required shape are inserted in the plastic beton, and where
necessary suitable openings are provided on the face of the work.
Provision is also made for fixing joinery by inserting, where required,
slabs made or partly made of a material into which nails may be driven,
such as concrete made with an aggregate of burnt clay, coke, and such
like. Hollow lintels are also made of the slabs keyed together at their
vertical joints, and when in position these are filled in with beton.
This system, however, is only recommended for fire-place openings
instead of arches.

In Fig. 25, circular construction is exhibited as applied to the apsidal
end of a church, slabs similar to those shown in Fig. 21 being employed
for that purpose, while Figs. 22, 23, and 24 show forms of slabs
suitable for constructing cylinders with horizontal axes and domes. In
Fig. 19, which is the upper part of Fig. 20, is shown a system of
constructing floors of these slabs. It is only necessary to explain that
the slabs are first keyed to the lower flange of the iron joist by means
of a cement (bituminous preferred), and the combination is then fixed in
position, the edges of the slabs adhering to, or rather supported by,
the iron joist being rebated so as to receive and support intervening
slabs, the heading joints of which are laid to break with those of the
slabs supported by the joists. For double floors the iron joists are
made with a double flange on their lower edge, and are fitted to iron
girders, which cross in the opposite direction. This provision secures
the covering of the cross girders on their undersides by the ceiling
slabs. The concrete having been deposited upon the slabs, its upper
surface may be finished off in any of the usual ways, while the ceiling
may be treated in any of the ways described for the walls. This system
does not exclude the ordinary methods of constructing floors and roofs,
although it supplies a fireproof system. Where required, bricks, stone,
and, in