In passing onward through the various rooms, two changes are wrought in the air: it loses so much of the caloric with which it is charged for every foot it travels, and it becomes laden with the exhalations from the lungs of the bathers. A large proportion of carbonic acid is thrown into the air, and as the normal temperature of the human body remains, in a healthy person, at about 98° Fahr., and rises but a few points even when submitted to the action of heat, these exhalations, in addition to being heavier than air, are very much below the average temperature of a sudatory chamber. Consequently they fall, and must be extracted at the floor level. The total area of the outlets for vitiated air should be about equal to the area of the narrowest part of the shaft that conducts the fresh, hot air from the heating chamber. Thus, supposing the latter to be 5 superficial feet, and the size of outlet ventilators a clear 12 in. by 3 in., there may be 20 ventilators disposed round the bath-rooms, say 4 in the calidarium, 7 in the tepidarium, and 9 in the combined shampooing room and lavatorium. In the diagrams at Figs. 8 and 9 the foul-air conduit is the space comprised under the marble-topped benches running round the hot rooms. At the end of the laconicum they enter flues, which I have shown as running side by side with the smoke flues. Other methods of heating the air, besides those mentioned, include coils of iron flue-pipes in a brick chamber--a principle that has been frequently adopted in the past--and plain cylindrical iron radiating stoves, such as employed at the Hammam in Jermyn Street. [Illustration: FIG. 9. Section of Hot Room, showing Foul-air Conduit.] In the latter plan, however, a great expense is created by the large number of furnace-fires to be kept constantly burning. An exposed stove in a hot room, has, moreover, the objection to its use that it re-heats the air in the bath, which should never on any account be done.