Houses and other Buildings. 233 



can be limited, it should never, under any circumstances be 

 allowed to exceed 300°. Metals appear to be the best substan- 

 ces for heating air. The temperature is limited to 300° because 

 the animal and vegetable matter, which is found mechanically- 

 mixed with the air at all times, will be decomposed if the tem- 

 perature be raised a little higher. When this decomposition takes 

 place, as is very observable when the heated surface is red hot, 

 certain elastic fluids and vapours are produced, which give to 

 the air a peculiar odour, and a deleterious quality which never 

 fails to affect the health of those who inhale it for a length of 

 time. This oppressive sensation has been mostly felt in churches 

 and other places where large iron stoves are used and are 

 sometimes heated to redness. The peculiar odour accompany- 

 ing it has been erroneously attributed to the iron ; and on this 

 account, earthen ware or stone has been employed to form the 

 exterior surface of the stove. It will, however, be found that 

 whatever be the material, if the temperature at all approaches 

 a red heat, the same smell will be perceived ; as it arises 

 entirely from the decomposition of the matter which is in the 

 air, and not from the heating body. This matter is very visible 

 to the naked eye, in a sunbeam let into a dark room. 



When earthen ware or stone has been employed for stoves, 

 its inferior conducting power has seldom allowed the exterior 

 surface to get sufficiently hot, to produce the effect on the air 

 above alluded to. And hence it has been less objectionable as 

 affecting the purity of the air. 



It must however be admitted, that if the body used for heat- 

 ing the air, does not undergo any change, a metal from its being 

 a good conductor must be preferred to any other substance. 

 Silver or platina, if it were not for the expense, would set 

 aside every prejudice. But long experience h^s shown that iron 

 possesses every essential property. The slightly oxydated sur- 

 face which is common to all iron coming from the forge or 

 the mould in casting, is well fitted for receiving radiant heat. 

 And if its temperature be kept below a red heat, there does not 

 appear to be any limit to its durability. The latter point, there- 



