2118. Steam affords a simple and effectual mode of heating hothouses, and, indeed, large bodies of air in every description of chamber; for no other fluid is found so convenient a carrier of heat. Steam was the first improvement on the old mode of heating by flues, and it is still occasionally used, though it has been almost superseded by hot water. The heat given out by vapour differs in nothing from that given out by smoke; though an idea to the contrary prevails among gardeners, from the circumstances of some foul air escaping into the house from the flues, especially if these are over-heated or over-watered; and from some vapour issuing from the steam-tubes when these are not perfectly secure at the joints. Hence flues are said to produce a burnt or drying heat, and steam-tubes a moist or genial heat; and in a popular sense this is correct, for the reasons stated. It is not, however, the genial nature of steam heat which is its chief recommendation for plant-habitations, but the equality of its distribution, and the distance to which it may be carried. Steam can never heat the tubes, even close to the boiler, above 212 degrees, and it will heat them to the same degree, or nearly so, at the distance of 1000, 2000, or an indefinite number of feet. Hence results the convenience of heating any range or assemblage of hothouses, however great, from one boiler, and the lessened risk of over or insufficient heating at whatever distance the house may be from the fireplace. The secondary advantages of heating by steam are, the saving of fuel and labour, and the neatness and compactness of the whole apparatus. Instead of a gardener having to attend to a dozen or more fires, he has only to attend to one; instead of ashes, and coal, and unsightly objects at a dozen or more places in a garden, they are limited to one place; and, instead of twelve paltry chimney-tops, there is only one, which, being necessarily large and high, may be finished as a pillar, so as to have effect as an object; instead of twelve vomiters of smoke and flakes of soot, the smoke may be burned by using some smoke-consuming furnace. The steam-tubes occupy much less space in the house than flues, and require no cleaning; they may often pass under paths where flues would extend too deep; there is no danger of steam not drawing, or circulating freely, as is often the case with flues, and always when they are too narrow or too wide, or do not ascend from the furnace to the chimney; steam is impelled from the boiler, and will proceed with equal rapidity along small tubes or large ones, and descending or ascending. Finally, with steam, insects will be effectually kept under in hothouses, with the greatest case, by merely keeping the atmosphere of the house charged with vapour from the tubes for several hours at a time.