There is nothing really better as a radiating surface than ordinary firebrick. From this material a soft heat is given off, differing in quality from that obtained from iron. An iron furnace, however, requires less thought in design, gives less trouble in fitting up, and is cheap, economical, and expeditious. Stoves, therefore, with an iron radiating surface, have been largely adopted in the past, in spite of the objection that, when super-heated, particles of metal are thrown into the air of the hot rooms. Of iron furnaces there are many placed before the public; but though all are doubtless suited to ordinary requirements, there are few that are capable of creditably fulfilling the conditions indispensable for the hygienic heating of the air of a Turkish bath. These conditions may be summarised as follows:-- 1. A maximum of heating-surface, with a minimum of grate space. 2. Perfect immunity from the danger of leakage from the furnace into the hot-air chamber or conduit. 3. Freedom from the defect of liability to overheat the air. 4. Inability to adulterate the air by throwing off matter from the heating surfaces. Such primary essentials must be constantly borne in mind by the designer of furnaces for the Turkish bath. Their importance must be obvious to all. Of the many iron stoves, Messrs. Constantine's "Convoluted" stove has been adopted the most frequently, as an eminently practical furnace for the effective heating of the sudatory chambers. The appearance of this stove is familiar to all architects, and it will be unnecessary, in these pages, to minutely describe its construction. [Illustration: FIG. 6. View of a small Furnace Chamber, with portion of wall broken away to show the "Convoluted" Stove.]