CARBON. 357 



(b.) Unchanged by heat, in closed vessels, except that it grows 

 firmer, and harder, and blacker, and shrinks ; it will then very de- 

 cidedly scratch glass, and wear a file.* With the best pieces, one 

 can write his name on window glass. 



!c.) Unaltered by air and water, and exempt from decay. 

 d. If well prepared, it conducts electricity, but is a bad conduc- 

 tor of heat.f 



(e.) When once thoroughly made, it retains for a long time, 

 its power of conducting electricity. Heated without contact of air, 

 it emits inflammable gases and nitrogen^ % 



(f.) Jlfter being ignited, it absorbs gases without alteration ; this 

 is shewn by placing on the quicksilver bath, a piece recently extin- 

 guished, and covered by a jar. This power is much diminished by 

 pulverizing the charcoal. The following are the results of Saus- 

 sure, with box wood charcoal, the most powerful species ; the time 

 was from 24 to 36 hours ; the charcoal was first ignited, cooled in 

 mercury, and then placed in the gas. 



Gaseous ammonia 90 times the volume of the charcoal ; do. mu- 

 riatic acid 85 ; sulphurous acid 65 ; sulphuretted hydrogen, 55 ; 

 nitrous oxide, 40 ; carbonic oxide, 35 ; olefiant gas, 35 ; carbonic 

 oxide, 9.42 ; oxygen, 9.25 ; azote, 7.5 ; light gas from moist charcoal 

 5. ; hydrogen, 1.75 ; very light charcoal scarcely absorbs at all. 



The power of absorption in charcoal bears no relation to its chemi- 

 cal attraction for the gas or vapor, which, by heating the charcoal, is 

 in general recovered unaltered. 



Those gases that cannot be condensed into the liquid state, are 

 the least absorbed by charcoal, and the reverse is true, very nearly 

 in proportion to the ease with which they are condensed. Vapors- 



common charcoal ; although better for gun powder, it is not preferred by the iron 

 manufacturer. The loppings'of young trees, called crop wood, are now generally 

 used in England. Abundance of a substance like tar is produced, which Mr. Parkes 

 says is an excellent preservative of wood, against decay and insects. Essays, Vol. 

 I, p. 399. 



The proportion of charcoal obtained from different woods varies from 15 to 26 per 

 cent ; the average of 21 trials gave nearly 20 per cent. Parkes* Essays, Vol. I, 

 p. 408. 



Fir gave 18.17, lignum vitae 17.26, box 20.25, beech 15, oak 17.40, mahogany 

 15.75. Allen and Pepys. For a fuller table, see p. 363. 



Wood, burned in the open air leaves only about l-200th, or l-250th of the wood, 



but the charcoal is said to contain l-50th of its weight of alkaline and earthy salts. 



Turner. 



* Even in its common state, good charcoal will wear window glass. 



t Lampblack is prepared from the combustion of oils and resins. We may col- 

 lect it by receiving the smoke of a lamp upon a saucer, or by burning a piece of 

 pine knot or rosin, under suspended sacking. In the arts, the refuse resin and pitch 

 are burned in a peculiar furnace, furnished with long flues, terminating in a close 

 chamber, the ceiling of which is covered with porous cloth to catch the soot. 



t Mem. d' Arcueil, T. II, p. 484. 



Jour, de Phys. T. XXIII, and LVIII, and Ann. de Chim. T. XXXII. 



