CHARCOAL — SOOT. 325 



phosphoric acid, but consist of silica, alumina, oxide 

 of iron, and a small quantity of sulphate of lime. 

 Their chief value appears to consist in the charcoal 

 and sulphate of lime which they contain. 



882. Although charcoal has been spoken of as 

 undergoing no change whatever under ordinary cir- 

 cumstances (90), and as having no tendency to oxi- 

 dize or form carbonic acid when exposed to the air, 

 yet in some conditions when exposed to moisture, 

 warmth, and air, the more porous kinds of charcoal 

 do slowly and gradually form carbonic acid; and this 

 action appears to go on more rapidly when charcoal 

 is mixed with humus (672) and other decaying organic 

 matters. 



883. Vv'^aste charcoal of all kinds is useful as 

 manure, particularly in close and retentive soils ; 

 hence it is often the custom to burn or char sawdust, 

 brushwood, and dry vegetable rubbish: a mixture of 

 ashes and charcoal is thus obtained, which is more 

 valuable than that which is left when such matters 

 are entirely burnt to ashes. Eor various purposes in 

 the arts, such as the manufacture of gunpowder, that 

 charcoal is best which contains the least quantity of 

 inorganic matter, but for manure it is just the re- 

 verse (162). 



884. In connection with ashes, soot, and the refuse 

 of gas-works may be mentioned, although very dif- 

 ferent in their nature. Soot consists principally of 

 finely-divided charcoal or carbon, but it contains a 

 considerable quantity of salts of ammonia; hence it 



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