CARBON: MARSH GAS 103 



(plumbago or black lead), distinguished by its softness, and hence used as 

 a writing material, is a black modification also capable of existing in a 

 crystalline form. The carbon of (i) animal and vegetable charcoal ; (2) 

 lamp-black, the soot produced by the incomplete combustion of oils 

 burning with a smoky flame ; (3) coke, the residue of the destructive dis- 

 tillation of coal in the manufacture of coal gas ; and (4) gas carbon, the 

 deposit in the upper parts of the retorts used for the same purpose, is the 

 black amorphous modification. " Charcoal burning," the process by which 

 wood is converted into charcoal, once an important industry, is still 

 carried on in woodland districts. Stacks of wood are set fire to, but by 

 covering the stacks with earth or turf the supply of air is so limited that 

 the wood is carbonised, but not burnt. 



The wood ashes, produced by the complete combustion of wood, have a 

 high value as a fertiliser, owing chiefly to the large quantity of carbonate of 

 potassium they contain. When lixiviated with water, and the solution 

 evaporated to dryness, a residue of impure carbonate of potassium is 

 obtained, constituting, when calcined, the pearl-ash of commerce. 



The attraction that exists between liquids and the surface of 

 solids has been already referred to (p. 102). Charcoal 

 exhibits this attraction in an extraordinary degree. Render 

 some water blue with solution of litmus, and shake with some 

 powdered animal charcoal for a few minutes. Now throw upon 

 a filter. Note that the filtrate is colourless, the dissolved 

 colouring matter having adhered to the charcoal. 



This force of adhesion is also exerted between solids and 

 gases, and its extent will largely depend upon the amount of sur- 

 face exposed by the solid. Owing to its porousness, wood charcoal 

 presents an enormous surface, and exhibits an enormous power 

 of absorbing gases. Heat a lump of wood charcoal in a tube 

 to bright redness, so as to expel absorbed gases, cork the tube, 

 and allow to cool. When cold, counterpoise on the pan 

 of the balance, then remove the cork, and watch the gradual 

 increase in weight. Place a small lump of charcoal in a flask 

 of water. Note that it swims on the surface. Now boil the 

 water till the air has been expelled from the pores of the char- 

 coal. Allow to cool, and notice that the density of charcoal, 

 when free from air, being greater than water, the lump sinks. 



