WARMING AND VENTILATION (ante). In its aspect as to health, it may be assumed that no system of heating is advisable which does not provide for a constant renewal of the air in the locality warmed. All heating apparatus depends upon the transference of heat from the fire to 1,1ie various parts of the building which it is intended to warm, and this transfer may be effected by radiation, by conduction, or by convection. Radiant heat is emitted and absorbed in an accelerating ratio in proportiou as the difference of temperature between the radiant and the recipient increases; and, with the same difference of temperature between the recipient and the radiant, the effect of the radittnt will be greater according to the increased temperature of the recipient. In other words, the ratio of the emission of heat increases with the temperature. It is thus easier to effect the warming of a given space by means of a highly-heated surface than by a surface emitting a lower temperature. An open fire acts by radiation ; it warms the air in a room by first warming the walls, floor, ceiling, and articles in the room, and these in their turn warm the air. Therefore, in a room with an open fire the air of the room is, as a rule, less heated than the walls. In this case the warming of the air depends on the capacity of the surfaces to absorb or emit heat; except that the heat received by the wall may be divided into two parts, one heating the air in contact with the wall, and the other passing through the wall to the outer surfaces, where it is finally dissipated and wasted. Fire-places arc sometimes,constructed to assist the warming of the air of a room. For instance, in Sylvester's grate, iron bars, of which one end rminates under the fire, are laid so as to form a projecting radiating hearth. The venti lating fire-place warms the fresh air before its admission into the room by means of gills cast on the back of ate grate. In a close stove, heated to a moderate temperature, the
heat as it passes from the fire warms the surface of the materials which inclose and are in contact with the fire and the heated gases. The materials next transfer the heat to the outer surface in contact with the air, and the air is warmed by the agency of this outer surface. If heated to a high temperature a stove gives out radiant heat, which passes through the air to warm the objects on which the rays impinge. With hot-water pipes, the heat from the water heats the inner surface of the pipe, and this surface transfers its heat to the outer surface through the material of the pipes. The rate at which the heat CID pass from the inner to the outer surface, and be thus utilized instead of passing away straight into the chimney, depends on the heat evolved by the fire, ou the extent of sur faces exposed to the heat and their capacity to absorb and emit heat, and on the quality of the material between the inner and outer surfaces as a good or bad conductor of heat. This passage of heat through a body by conduction varies directly with the quality of material, and with the difference between the temperature of the inner surface exposed to the heat and the outer surface exposed to a cooling influence, and inversely as the thickness between the surfaces. Other things being equal, copper is a better material than iron for conveying the heat from the fire to water or air; and'coverings of brick work, wood, or woolen fabrics are better adapted than iron for retaining the heat. The property which appears more than any other to make materials good non-conductors of heat is their porosity to air, and the retention of the air in their pores.