of heated
air, and from a fireclay surface, the employment of which renders it
absolutely impossible to overheat the air, or to contaminate it by
deleterious particles resulting from the decomposition of metal.
Moreover, the stoking of this class of furnace requires less arduous
attention than an iron stove. Its disadvantage is that, should the
temperature of the bath be allowed to fall markedly, it requires some
time for the extra heat to be made up again. Inasmuch, however, as fires
at public baths must be kept banked up overnight, this is not a matter
of importance. It is this very slowness of increase in temperature that
constitutes the safeguard against that overheated air, the presence of
which we can, with practice, detect by the smell in so many baths. The
difficulties involved in the construction of a furnace of this nature
relate to the prevention of cracking and consequent escape of sulphurous
fumes and carbon into the air. The very simplicity of the construction
of the flues and air-chambers constitutes the chief danger, as the
chances are that, unless the architect stands by and sees every joint
made, the work will be done badly. Absolutely faultless workmanship must
be employed throughout, and the fireclay materials must be literally of
the very best and soundest description. Every single joint must be
perfectly made with fireclay cement or paste. The fireclay bricks, &c.,
must be selected with regard to the amount of indestructible silica in
the clay, consistent with hardness and toughness. Homogeneity of
material must be obtained, having regard to expansion and contraction.
The same material used for the bricks, &c., worked into a paste, must be
employed for the joints.

The design for a furnace on the principle shown at Fig. 10 must be
prepared with constant regard to expansion and contraction in heating
and cooling. Should this warning be disregarded, fractures will result.
It will be seen, upon reference to the plans, that the block of flues
and air spaces is left quite free, to allow of any expansion, the
connection with the smoke-shaft being by means of an iron flue-pipe,
which, being provided in considerable length before passing through the
party-wall of laconicum and stokery, by its flexible nature permits any
slight movement in a vertical direction. If an "expansion" joint were
provided, there would be a sufficient length of iron pipe if it passed
direct from the junction with the heating appara