172 HEATING BY HOT WATER, HOT AIR, AND STEAM. 



must be made upon the moisture of the house, upon the plants, 

 and upon everything else within its influence capable of giving 

 off moisture. This is also the case with hot-water pipes. But 

 here the advantage of the latter is plainly illustrated ; for while 

 a hot-air stove abstracts the moisture, in excess, from that part 

 of the house nearest to the aperture of ingress, hot-water pipes 

 radiate the heat at a low temperature equally over the whole 

 surface, and, as the temperature at which the heat is radiated is 

 comparatively low, little or no moisture is abstracted. Some 

 suppose that they get a fine moist heat from hot-water pipes. 

 This, however sound and sensible it may appear, is, nevertheless, 

 a practical fallacy, the fact of the case being this, that, instead 

 of the moisture of the house being taken up by the air, as in 

 the case of Polmaise, and other stoves, the warm air of the 

 pipes being so much lower in temperature than that of the 

 stoves, it cannot take it up, and hence the moisture remains 

 with the plants and the atmosphere in its original purity. In 

 fact, there is no difference between the heat radiated from stone, 

 brick, or iron, unless it be mixed with extraneous gases, by heat- 

 ing these bodies to a high temperature. 



To supply the moisture required by the heated air, water may 

 be placed in evaporating pans, in connexion with the current 

 of ingress; but, as we have already shown, though moisture 

 may be supplied, the hydrogen of the rarefied air still remains 

 uncombined, and, until the air be replaced by a fresh volume 

 from the external atmosphere, its impurities still remain. 



With regard to the motion and circulation of the atmosphere 

 of a hot-house, the system of heating by hot air possesses, the- 

 oretically, some advantages over all others. Strictly speaking, 

 however, this has scarcely a practical foundation. If hot air be 

 admitted in currents, the atmosphere will be agitated, certainly, 

 but the house will be very unequally heated, as the heated air 

 will pass upward in currents, at the aperture of its entrance, 

 without diffusing itself over the lower surface of the house. 

 Air expands, when heated, ^^ of its bulk for each degree of 

 Fahrenheit, and the velocity of its motion is equal to the addi- 

 tional height which a given weight of heated air must have, in 

 order to balance the same weight of cold air ; and as all rare 



