252 Pobnaise Method of Heating 



temperature of the external atmosphere, we do so at the ex- 

 pense of the oxygen and aqueous vapor the internal volume 

 may contain ; or rather, the bodies within the house capable 

 of giving off moisture, which is extracted from them by the 

 increased capacity of the air for moisture, is carried upwards 

 by the rarefied air, and escapes into the atmosphere through 

 the fissures of the structure, or is deposited on the glass by 

 condensation. In badly-glazed houses the heat thus lost 

 amounts to nearly one-fifth of the heat artificially diff'used ; 

 one-fifth more is lost by direct radiation from all parts of 

 the structure, and the remaining three-fifths constitute the 

 amount which supports the difference between the external 

 and internal atmospheres. This is one of the chief causes 

 why the atmospheres of hothouses are unnaturally arid. The 

 abstraction and loss of moisture by these means is far more 

 than would be supposed by those who have not calculated 

 the amount. This abstraction is more by Polmaise than by 

 any other system of heating. As the heat enters the house at 

 a high temperature, it makes an immediate demand upon 

 that portion of the house nearest to the current of ingress, 

 and as the plants and the soil around them are the only 

 bodies capable of giving off moisture, it is greedily abstracted 

 from them by the warm air, until their vital fluids are ex- 

 pended to furnish the supply. The plants become dried up 

 by a continuance of such treatment, the sap vessels are con- 

 tracted and hardened, and death is the consequence. 



The quantity of moisture a cubic foot of air will hold in 

 invisible suspension depends upon its temperature, and as its 

 temperature is increased, so is its capacity for moisture. Sup- 

 pose then that this capacity is doubled, between the temper- 

 atures of 40° and 60°, then it follows that the heated air and 

 the moisture it contained, thus escaping through the fissures 

 of the glass or condensed upon its surface, deprives the house, 

 and of course the plants within it, of double the quantity of 

 moisture the same equivalent of air contained previous to 

 its increase of temperature. Now, when a current of highly 

 rarefied air is introduced into a house, through an aperture in 

 any part of it, the air thus introduced will not diffuse itself 



