PLANTS, WITH GLASS ROOFS. 169 



readily be taken out or opened in order to free the flue from soot ; an 

 operation which will require to be performed at least once a year in all 

 houses, and in stoves twice a year, or oflener, according to the kind of 

 fuel used. 



As substitutes for smoke-flues, earthenware pipes or can-flues, as they 

 are called, have long been in use in Holland and France ; and as the 

 fuel used in those countries is almost always wood, which produces 

 little soot in comparison with coal, they are found to answer as per- 

 fectly as brick flues. When they are occasionally employed, the entire 

 surface of the pipes is exposed ;. but when they are used constantly, as 

 in houses for tropical plants, they are embedded in a casing of dry 

 sand, which forms a reservoir of heat capable of being increased to any 

 extent, even to that of the entire floor of the house, over which a floor- 

 ing for plants may be placed. Pipes of this kind might also be con- 

 ducted through a bed of small stones, so as to form a very effective 

 mass of heated material as a reservoir, while a portion of naked pipe 

 might serve for raising the temperature on occasions of extraordinary 

 cold. Iron pipes would, however, be the safest for burying under- 

 ground. In country situations, where wood for burning is not very 

 dear, or where coke from coal could be readily obtained, such flues might 

 be economically employed for drying up the cold damp of greenhouses, 

 and for a variety of purposes. We have said more on the subject of 

 smoke-flues than may be thought necessary at the present time, when 

 they are being so generally relinquished for hot- water pipes ; but our 

 object is to prevent our readers from being so completely prejudiced 

 against flues as not to have recourse to them in particular situations 

 and circumstances. The principal reason why so much has been said 

 against smoke-flues is, that gardeners till lately were not fully aware 

 of the importance of supplying moisture to the atmosphere of plant- 

 houses in proportion to the supply of heat, and of having reserve flues, 

 in consequence of which excessive heat would not become so fre- 

 quently requisite, and noxious gases would have less chance of being 

 driven through the top and sides of the flue into the atmosphere of the 

 house. Flues may also still be combined with boilers to utilize much 

 of that heat which is otherwise wasted up the chimney. 



Steam was the first substitute for flues employed in this country ; 

 and, under some circumstances, it may deserve a preference to either 

 flues or hot water. For example, where the heating apparatus must 

 necessarily be at a great distance from the structure to be heated, 

 steam can be conducted to it in a tube not more than an inch or two 

 in diameter, which may be so encased in non-conducting matter as to 

 occasion far less loss of heat than if either smoke or hot water were 

 employed. The disadvantages attending the use of steam in ordinary 

 cases are, the necessity of heating the water to the boiling-point, by 

 which more heat is driven up the chimney and lost than if the water 

 were raised to only half that temperature, and the want of a reservoir of 

 heat when the steam is not in action. The last disadvantage has been 

 supplied by passing the steam-pipes through brick flues filled with 

 stones, through pits, or through other large masses of stones, or through 



