CAR

CAR

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nace appropriated for the purpofe, and the denfefmoak arifing from the combuftion is introducedinto chambers hung with facking, upon the fur-face of which the foot or lamp-black is depofited,and whence it is fwept off from time to timeand fold, without any further preparation. Com-mon lamp-black is nor indeed pure charcoal,being mingled with fome empyreumatic acidand a portion of refin in a ftate of imperfectdecompo.fition, but after it has been heated torednefs in a clofe veffel, all thefe impurities aredriven off, and there remains behind a charcoalin a ftate of almoft perfect purity, as it burnsaway with hardly any perceptible relidue.

Charcoal is an excellent conductor of elec-tricity and Galvanifm, but tranfmits caloric withvery great difficulty: a very fliort piece may beheld -by ,one end while it is heated red at theother, without conveying any notable warmthto the hand.

The combined a£tion of air and moifture atthe ufual temperature upon this fubftance canfcarcely be perceived, hence the advantage ofcharring the furface of flakes and piles that aredriven into moift ground, it being found by longexperience that thefe are much more {lowlyrotted than the foundeft timber deprived of thiscovering.

Charcoal is infufible by the greateft heat thatcan be applied. When expofed to a high tem-perature in dry and perfe£tly clofe veffels, itlofes little or nothing of its weight, but flirinksin fize and becomes proportionably harder,denfer, more compa£l, and of a very deep velvetblack colour. But although charcoal is thusunalterable by heat when the air is excluded,yet when made red hot and expofed to the air,it readily undergoes .combuftion, and by unitingwith oxygen is converted into a heavy gafeousacid, formerly called fixed, air , but in the re-formed nomenclature known -by the name ofCarbonic acid.

Pure oxygen gas produces, as might be ex-pelled, a much more rapid effebt on charcoalthan atmofpheric air does; a piece heated atone end to the loweft ftate of rednefs, andplunged in this gas, immediately burns with anintenfely white glow, unaccompanied by flame,and is fpeedily changed into carbonic acid.Twenty-eight parts of charcoal and feventy-twoof oxygen, according to the experiments ofLavoifier, form ioo of carbonic acid.

Water is very obftinately retained by char-,cpal, partly on account of its porous texture,And ii) part probably on account of a ftrong mu-

tual affinity, but no folution of one in the othertakes place even at a boiling heat: when how-ever charcoal is red hot, or nearly fo, it decom-pofes any water with which it may happen tobe in contact, and unites with both the elementsof this fubftance, producing with the oxygenousbafe, according as this is added to faturationor not, carbonic acid and gafeous oxyd of car-bon and with the hydrogenous bafe a heavyinflammable air, Carburetted Hydrogen.Common charcoal always contains more or lefsof moifture, and it is to the decompofition ofthis and the formation of the above-mentionedgaffes, that the blue flame is to be attributedwhich makes its appearance when charcoal iscalcined in a clofe crucible ; the ceflation of theflame being a proof that nearly all the moiftureis expelled.

Various gaffes feem to be abforbed with An-gular facility by charcoal. This fa£t had beenobferved by Fontana,Prieftley,Scheele,Morveau,and Morozzo, but it is to the Dutch chemiftsRouppe and Van Noorden that we are indebtedfor a moft interefting feries of new and accurateexperiments on this interefting fubjebl. It isabfolutely neceflary that the charcoal made ufeof in thefe refearches ffiould be entirely freefrom air and water, and two methods have beenadopted fuccefsfully to fecure this point. Theone is to heat a piece of charcoal till it is incan-defcent, and then to remove it from the fire asquickly as poffible, and extinguifh it in mercuryprevioufly dried, taking care to keep it in thisfituation for an hour or more, that it may becompleatly cooled. The gas to be operated uponis then to be introduced into a glafs jar overmercury, and the charcoal being ftill coveredby the mercury is brought beneath the mouthof the inverted jar, into which on being let goit immediately rifes, and thus comes in conta£lwith the gas. The fecond method differs fromthe firft merely in the manner of extinguifhingthe charcoal, by inclofing it while hot in an airtight copper box. This latter mode was thatchiefly employed by Meffrs. Rouppe and VanNoorden. d Thefe philofophers found that onepart of charcoal abforbed in four or five hoursthree times its bulk of atmofpheric air; thatoxygen gas was abforbed, at firft rapidly andthen more flowly, in the ratio of 2.8 times itsbulk; azot was taken up only in the proportionof 1.6, and hydrogen of 1.8, but in both thefecafes the abforption was inftantaneous. Ofnitrous gas 8.5 parts were flowly, and of car-bonic acid 14.3 p ar ts were very rapidly abforbed.

* An. de Chim. xxxii. p. 3.