2055. Such heat as is required in addition to that of the sun is most generally produced by the ignition of carbonaceous materials, which heat the air of the house, either directly, when hot embers of wood are left in a furnace or stove, placed within the house, as in Sweden and Russia; mediately, as when smoke and heated air, from or passing through ignited fuel, is made to circulate in flues; or indirectly, when ignited fuel is applied to boil water, and the hot vapour, or the water itself, is impelled through tubes of metal or other conductors, either to heat the air of the house at once, as in most cases, or to heat masses of brickwork, sand, gravel, rubble, or earth, tan, or even water (Hort. Trans., vol. iii.), which materials may afterwards give out the heat so acquired slowly to the atmosphere of the house. But heat is also occasionally supplied from fermenting vegetable substances, as dung, tan, leaves, weeds, &c., applied either beneath or around the whole or a part of the house, or placed in a body within it.