336 PRINCIPLES OF CHEMISTRY 



or naphthalene or other hydrocarbons, be burned in the open air, soot is 

 formed in large quantities, because these substances contain a large 

 amount of carbon. Tar, pitch, and similar substances for this reason 

 burn with a smoky flame. Thus, soot is finely-divided charcoal, 

 separated during the imperfect combustion of the vapours and gases 

 of carbonaceous substances rich in carbon. Specially-prepared soot 

 (lampblack) is very largely used as a black paint, and a large quantity 

 goes for the manufacture of printers' ink. The quantity of organic 

 matter remaining undecomposed in the charcoal depends on the 

 temperature to which it has been submitted. Charcoal, prepared at the 

 lowest temperature, still contains a considerable quantity of hydrogen 

 and oxygen even as much as 4 p.c. of hydrogen and 20 p.c. of 

 oxygen. Such charcoal still preserves the structure of the substance 

 from which it was obtained. Ordinary charcoal, for instance, in which 

 the layers of the tree are yet visible, is of this kind. On submitting it 

 to further heating, a fresh quantity of hydrogen with carbon and 

 oxygen (in the form of gases or volatile matter) may be separated, and 

 the purest charcoal will be obtained on submitting it to the greatest 

 heat. 9 If it be required to prepare pure charcoal from soot, it is 

 necessary first to wash it with alcohol and ether, in order to get rid of 

 the soluble tarry products, and then submit it to a powerful heat to 

 drive off the impurities containing hydrogen and oxygen. Charcoal, 

 however, when completely purified does not change in appearance. 

 Everybody knows that charcoal is a black amorphous substance, 

 without any signs whatever of crystallisation (probably a colloid). Its 

 porosity, 10 bad conducting power for heat, capability of absorbing the 



9 As it is difficult to separate from the charcoal the admixture of ash that is, the 

 earthy matter contained in the vegetable substance used for producing charcoal in order 

 to obtain it in its purest condition, it is necessary to use such organic substances as do 

 not contain any ash, for example, completely refined or purified crystallised sugar, 

 crystallised tartaric acid, &c. 



10 The cavities in charcoal are the passages through which those volatile products 

 formed at the same time as the charcoal have passed. The degree of porosity of 

 charcoal varies considerably, and has a technical significance, in different kinds of 

 charcoal. The most porous charcoal is very light; a cubic metre of wood charcoal 

 weighs about 200 kilograms. Many of the properties of charcoal which depend exclu- 

 sively on its porosity are shared by many other porous substances, and vary with 

 the density of the charcoal and depend on the way it was prepared. The power which 

 charcoal has of absorbing gases, liquids, and many substances in solution, has refe- 

 rence to this. The densest kind of charcoal is formed by the action of great heat on 

 sugar. The lustrous gray dense charcoal formed in gas retorts is also of this character. 

 This dense charcoal collects on the internal walls of the retorts subjected to great heat, 

 and is produced by the vapours and gases separated from the heated coal in the retorts. 

 In virtue of its density, such charcoal becomes a good conductor of the galvanic current 

 and approaches graphite. It is principally used in galvanic batteries. Coke, or the 

 charcoal remaining from the imperfect combustion of coal and tarry substances, is also but 



