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. and its lid were destroyed while the bottom of it and the included 

 metal were scarcely affected. Above the focus there is no combus- 

 tion, but the stream of heated air communicates its caloric to the 

 incumbent fuel, and ignites it to a considerable degree previous to 

 its descent to a lower part of the furnace, so that it is nearly at the 

 maximum when it begins to burn. As however the air must pass 

 for some space in contact with an extensive surface of heated coal ; 

 the same effect must be produced in our furnace as in those experi- 

 ments, where carbonic acid dissolves a proportion of its base and 

 becomes carbonic oxide. It is the substance wliich burns with a 

 blue flame over the iron furnaces, and hijures the power of those of 

 the chemist, not merely by combining with fuel of which it doubles 

 the expenditure, but still more by absorbing much caloric during its 

 formation. According to Ualton this gas gives out during its com- 

 bustion, I as much heat as charcoal, it must therefore absorb while 

 forming about half of the intire heat. In consequence of this the 

 upper strata of fuel are not by any means as much heated as they 

 would othei-wise be, and the gases which escape from the furnace, 

 if its depth be considerable and the fuel free from Hydrogen, are at 

 no high temperature. I have exposed various bodies in the flue of 

 a good air furnace, but never found them heated above moderate 

 redness, if the upper part of the furnace were closest). We must 

 next consider how a supply of air is directed through the fuel and 

 maintained ; the most obvious method is to apply mechanical power 



m2 



(f.) De Morveau proposjcl a furnace which on this principle is bad, it is iVIacquer's with a 

 liorizontal flue joining it to the chimney, in this a muffle is placed, which lie thought would 

 be heated to a great degree by the flame, even above the heat of a glass furnace ; but unless 

 he used half burnt charcoal, he could obtain no such result. 



