The class of carbon compounds is so vast that it forms a separate branch of chemistry, known under the name of organic chemistry—that is, the chemistry of carbon compounds, or, more strictly, of the hydrocarbons and their derivatives.
If any one of these organic compounds be strongly heated without free access of air—or, better still, in a vacuum—it decomposes with more or less facility. If the supply of air be insufficient, or the temperature be too low for combustion (see Chapter III.), and if the first volatile products of transformation of the organic matter are subjected to condensation (for example, if the door of a stove be opened), an imperfect combustion takes place, and smoke, with charcoal or soot, is formed.2
The nature of the phenomenon, and the products arising from it, are the same as those produced by heating alone, since that part which is in a state of combustion serves to heat the remainder of the fuel. The decomposition which takes place on heating a compound composed of carbon, hydrogen, and oxygen is as follows:—A part of the hydrogen is separated in a gaseous state, another part in combination with oxygen, and a third part separates in combination with carbon, and sometimes in combination with carbon and oxygen in the form of gaseous or volatile products, or, as they are also called, the products of dry distillation. If the vapours of these products are passed through a strongly heated tube, they are changed again in a similar manner and finally resolve themselves into hydrogen and charcoal. Altogether these various products of decomposition contain a smaller amount of carbon than the original organic matter; part of the carbon remains in a free state, forming charcoal.3 It remains in that space where the decomposition took place, in the shape of the black, infusible, non-volatile charcoal familiar to all. The earthy matter and all non-volatile substances (ash) forming a part of the organic matter, remain behind with the charcoal. The tar-like substances, which require a high temperature in order to decompose them, also remain mixed with charcoal. If a volatile organic substance, such as a gaseous compound containing oxygen and hydrogen, be taken, the carbon separates on passing the vapour through a tube heated to a high temperature.
Organic substances when burning with an insufficient supply of air give off soot—that is, charcoal—proceeding from carbon compounds in a state of vapour, the hydrogen of which has, by combustion, been converted into water; so, for instance, turpentine, naphthalene, and other hydrocarbons which are with difficulty decomposed by heat, easily yield carbon in the form of soot during combustion. Chlorine and other substances which, like oxygen, are capable of taking up hydrogen, and also substances which are capable of taking up water, can also separate carbon from (or char) most organic substances.
Wood charcoal is prepared in large quantities in a similar manner—that is, by the partial combustion of wood.4
In nature a similar process of carbonisation of vegetable refuse takes place in its transformation under water, as shown by the marshy vegetation which forms peat.5
In this manner doubtless the enormous masses of coal were formed6 which, following the example set by England, are now utilised everywhere as the principal material for heating steam boilers, and in general for all purposes of heating and burning.7
Russia possesses many very rich coalfields, amongst which the Donetz district is most worthy of remark.8
During the imperfect combustion of volatile substances containing 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