Heating. The attainment of a suitable temperature for any definite purpose may be brought about in various ways. The high temperatures necessary for steam boilers, for example, are produced by complete combustion of coal, gaseous or liquid fuel, in a confined space in close proximity to the boiler. A conservatory is generally of glass, through which the bright radiant heat from the sun may pass, without being able to return, and so the internal temperature is gradually rendered greater than the external. Ordinary English rooms are warmed by open grates containing burning coal or wood, a certain low percentage of the total heat emitted passing into the room as radiation. A gas-burner or oil lamp in a room radiates heat in all directions, and also allows the hot gaseous products of combustion to assist in warming; it is thus a fairly efficient heater, but a disagreeable one on account of the deleterious effects of the gases that pass into the air. Close stoves prevent these combustion-products from passing into the air, but they may be efficient heaters if their flues have a considerable surface-area within the room before leading to the chimney. These flues rapidly get heated, and so radiate heat into the room, the actual products of combustion being comparatively cool by the time they reach the chimney.