CARBON AND THE HYDROCARBONS 847 



carbon and hydrogen, the hydrogen and part of the carbon first burn, 

 and the remainder of the carbon forms soot. 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. 

 It is prepared by burning tar, oil, natural gas, naphtha, &c. 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 structure of the tree is still 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 remove 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. Its porosity, 10 bad conducting power for heat, 



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 char- 

 coal 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 exclusively 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 property which charcoal has of 

 absorbing' gases, liquids, and many substances in solution, is a case in point. The 

 densest kind of charcoal is formed by the action of great heat on sugar and other fusible 

 substances. The lustrous grey dense coke formed in gas retorts is also of this character. 

 This dense coke 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 coke becomes a good conductor of the galvanic current 

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

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

 slightly porous, brilliant, does not soil or mark paper, is dense, almost devoid of the 

 faculty of retaining liquids and solids, and does not absorb gases. The light sorts of 

 charcoal produced from charred wood, on the other hand, show this absorptive power in 



