186 METALLOIDS. 



454. OCCURRENCE. In the form of bi- 

 carbon occur? tuminous and anthracite coals, carbon ex- 

 ists in immense quantities, buried in the 



earth, in various countries ; as graphite, or plumbago, 

 it is also quite a common mineral ; as the diamond, it 

 is the rarest of all gems. It is one of the elements in 

 limestones, marbles and chalk, which are all carbonate 

 of lime. It forms nearly one half of all dried veget- 

 able matter, and more than half of all dried animal 

 matter. One two-thousandth of the air, also, is car- 

 bonic acid, of which carbon is a constituent. 



455. CHARCOAL. The 



Illustrate the 



preparation preparation of charcoal, one 



of charcoal. Qf the forms Qf carboilj may 



be illustrated by heating a small por- 

 tion of wood or cork, in a test-tube. 

 The other constituents of the wood, 

 and part of the carbon, are converted 

 into water, gases, and tan, and the larg- 

 est part of the carbon ramains behind, in the form of 

 charcoal. 



456. PREPARATION. In quantity, it is 

 coal made? commonly made by burning wood in large 

 heaps, previously covered with earth and 

 sod. It is necessary to admit a little air, through open- 

 ings in the heap, to maintain a partial combustion. 

 If too much air is admitted, the wood is entirely 

 consumed, and no charcoal is produced. Coke is 

 made from bituminous coal, by a similar process, and 

 is also obtained as a residue in the manufacture of 

 coal gas. 



