116 FUEL. 



lets of wood in it into a pyramidal heap, with several 

 spiracles or flues formed through the pile. Chips and 

 brushwood are put into those below, and the whole is so 

 constructed as to kindle through in a very short time. It 

 must then be covered all over with clay or earth beaten 

 close, leaving openings at all the spiracles or flues. The 

 pile is then ignited, and carefully watched and kept from 

 bursting into a flame, by instantly closing the flues should 

 such happen. Whenever the white watery smoke issuing 

 from the flues is observed to be succeeded by a thin, blue, 

 and transparent smoke, the holes must be immediately 

 stopped ; this being the indication that all the watery vapor 

 is gone, and the burning of the true coaly matter com- 

 mencing. Thus a strong red heat is raised throughout the 

 whole mass, and all the volatile matters are dissipated by 

 it, and nothing now remains but the charcoal. The holes 

 being all stopped in succession as this change of the smoke 

 is observed, the fire goes out for want of air. The pile is 

 now allowed to cool, which requires many days, for char- 

 coal being a very bad conductor of heat, the pile long 

 remains red hot in the centre, and if opened in this state 

 would instantly burn with great fury. Even when it is 

 opened, the heat retained by some of the larger pieces 

 often ignites it, to guard against which water should be 

 provided to instantly extinguish it when observed. 



PROPERTIES OF CHARCOAL. 



Although charcoal is so combustible, it is, in some re- 



