CARBON AND CARBONIC ANHYDRIDE. 



77 



bon is lost in this process, for it unites with the oxygen of 

 the air that is admitted in the openings, forming a gas, and 

 so passes out at the 

 upper openings with 

 the other matters that 

 are driven off by the 

 heat. About 40 per 

 cent, of the wood is 

 carbon, and the char- 

 coal obtained is from 

 20 to 25 per cent., so 

 that the loss of carbon 

 is nearly, often quite, 

 one half. The best 

 charcoal is made by 

 heating wood in tight 

 iron vessels till all the 

 vapors and gases are 

 driven off. The proc- 

 ess of making char- Fi - 1S - 

 coal can be illustrated by holding a burning slip of wood in 

 a test-glass, as represented in Fig. 19. 

 The portion within the glass, not hav- 

 ing a free access to air, is subjected 

 to a partial smothered combustion, 

 and therefore becomes charcoal. 



91. Soot. In burning wood there is 

 more or less smoke. This arises from 

 the imperfection of the combustion, 

 and is dense in proportion to that im- 

 perfection. If the combustion were 

 Fig. 19. perfect, there would be nothing visi- 



ble, for the substances passing off in the air would be, as you 

 will learn more particularly in another chapter, vapor and 



