hard to resist the fire, and should therefore be tested before using. It
is an unnecessary expense to use either second or third quality
fire-brick. As the pyroligneous acid which results from the distillation
of the wood attacks lime mortar, it is best to lay up the brick with
fire-clay mortar, to which a little salt has been added; sometimes loam
mixed with coal-tar, to which a little salt is also added, is used. As
the principal office of this mortar is to fill the joints, special care
must be taken in laying the bricks that every joint is broken, and
frequent headers put in to tie the bricks together. It is especially
necessary that all the joints should be carefully filled, as any small
open spaces would admit air, and would materially decrease the yield of
the kiln. The floor of the kiln was formerly made of two rows of brick
set edgewise and carefully laid, but latterly it is found to be best
made of clay. Any material, however, that will pack hard may be used. It
must be well beaten down with paving mauls. The center must be about six
inches higher than the sides, which are brought up to the bottom of the
lower vents. Most kilns are carefully pointed, and are then painted on
the outside with a wash of clay suspended in water, and covered with a
coating of coal-tar, which makes them waterproof, and does not require
to be renewed for several years.

[Illustration: RECTANGULAR KILNS FOR THE MANUFACTURE OF CHARCOAL.]

The kilns were formerly roofed over with rough boards to protect the
masonry from the weather, but as no special advantage was found to
result from so doing, since of late years they have been made
water-proof, the practice has been discontinued.

The wood used is cut about one and a fifth meters long. The diameter is
not considered of much importance, except in so far as it is desirable