Chemical Apparatus. 117 



thick smoke ; but if the furnace is gradually sup- 

 plied, almost the whole vapour will be inflamed. 

 Hence it will be manifest, that flaming bodies are 

 not proper when a moderate and equable heat is 

 required. 



The object of FURNACES is to confine the heat 

 as much as possible ; the air furnace consists of 



1. An ash-pit open. 



2. A chamber for the fire upon a grate. 



3. A door for fuel. 



4. A vent for smoke. 



In a furnace thus constructed, the air over the 

 fuel being rarefied rises through the vent and is 

 succeeded by fresh air rising through the grate, 

 and penetrating through the burning fuel. On 

 this the rising of the smoke in a common chim- 

 ney depends ; and on this principle may be ex- 

 plained Agricola's observation on the constant 

 circulation of air in mines. When miners open 

 a vein, they begin at the foot of the mountain, 

 as it is generally the richest, and make what 

 they call a level or drift in the side of it, inclining 

 a little upwards, for the convenience of draw- 

 ing off the water. After they have bored a little 

 way, they begin to want air, and their lights go 

 out. To remedy this they sink a perpendicular, 

 which they call a sliaft^ till it meets with the 

 level or drift. As soon as this communication 

 is made, there is a constant current of air press- 

 ing in at the drift and out at the shaft in winter, 

 and a contrary draught in summer. The rea- 



