300 CARBON. 



substance from which it is derived, and the accidental circum- 

 stances of its preparation. The following are the principal 

 varieties : gas-carbon, lamp black, wood charcoal, coke, and 

 ivory black. 



1. Gas-carbon has the metallic lustre, and a density of 1.76 ; 

 it is compact, generally of a mammillated structure, but some- 

 times in fine fibres, and considerably resembles graphite, but is 

 too hard to give a streak upon paper. It is the product of a 

 slow deposition of carbon from coal gas at a high temperature 

 and is frequently found to line the gas retorts to a considerable 

 thickness, and to fill up accidental fissures in them.* 



2. Lamp black is the soot of imperfectly burned combusti- 

 bles such as tar or resin. Carbon is deposited in a powder of 

 the same nature, when alcohol vapour or a volatile oil is trans- 

 mitted through a porcelain tube at a red heat ; and the lustrous 

 charcoal which is obtained on calcining starch, sugar and many 

 other organic substances, which fuse and afford a bright vesi- 

 cular carbon of a metallic lustre, is possessed of the same cha- 

 racters. It is deficient in an attraction for organic matters in 

 solution, which ordinary charcoal possesses. 



3. Wood charcoal. Wood was found by Karsten to lose 

 57 per cent of its weight when thoroughly dried at 212 and 10 

 per cent more at 304. The remaining 33 parts of baked wood 

 afforded, when calcined, 25 of charcoal, while 100 parts of the 

 same wood calcined, without being previously dried, left only 

 14 per cent of carbon. It is the absence of this large quantity 

 of water which causes the heat of burning charcoal to be so 

 much more intense than that of wood. When calcined at a 

 high temperature, charcoal becomes dense, hard and less inflam- 

 mable. The knots in wood sometimes afford a charcoal which 

 is particularly hard, and is used in polishing metals, but it con- 

 tains silica. From the minuteness of its pores, the charcoal of 

 wood absorbs many times its volume of the more liquefiable 

 gases, such as ammoniacal gas, hydrochloric acid, sulphuretted 

 hydrogen and carbonic acid, condensing 35 volumes of the 

 last. It also absorbs moisture with avidity from the atmos- 

 phere, and other condensible vapours, such as odoriferous efflu- 

 via. From this last property freshly calcined charcoal, when 

 wrapt up in clothes which have contracted a disagreeable odour, 



* Dr. Colquhoun, Annals of Philosophy, New Series, vol. 12, p. 1. 



