842 PRINCIPLES OF CHEMISTRY 



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- 

 may be easily attained by the use of step fire-bars. The fuel is fed into a hopper and 

 falls on to the fire-bars, which are arranged in the form of a staircase. The burning 

 charcoal is below, and hence the flame formed by the fresh fuel is heated by the con- 

 tact of the red-hot burning charcoal. An air supply through the fire grate, an eqnal dis- 

 tribution of the fuel on the fire-bars (otherwise the air will blow through empty spaces and 

 lower the temperature), a proper proportion between the supply of air and the chimney 

 draught, and a perfect admixture of air with the flame (without an undue excess of air), 

 are the means by which we can contend against the imperfect combustion of such kinds 

 of fuel as wood, peat, and ordinary (smoky) coal. Coke, charcoal, anthracite, burn with- 

 out smoke, because they do not contain hydrogenous substances which furnish the pro- 

 ducts of dry distillation, but imperfect combustion may occur with them also ; in that 

 case the smoke contains carbonic oxide. 



5 Under the action of air, organic substances are capable of oxidising to such an extent 

 that all the carbon and all the hydrogen they'contain will be transformed into carbonic anhy- 

 dride and water. The refuse of plants and that of animals are subjected to such a change 

 whether they slowly decompose and putrefy, or rapidly burn with direct access of air. But 

 if the supply of air be limited, there can be no complete transformation into water and 

 carbonic anhydride, there will be other volatile matters (rich in hydrogen), while charcoal 

 must remain as a non-volatile substance. All organic substances are unstable, they do not 

 resist heat, and change even at ordinary temperatures, particularly if water be present. It 

 is therefore easy to understand that charcoal may in many cases be obtained through the 

 transformation of substances entering into the composition of organisms, but that it is 

 never found in a pure state. 



However, water and carbonic anhydride are not the only products separated from 

 organic substances. Carbon, hydrogen, and oxygen are capable of giving a multitude of 

 compounds ; some of these are volatile compounds, gaseous, soluble in water they are 

 carried off from organic matter, undergoing change without access of air. Others, on the 

 contrary, are non-volatile, rich 'in carbon, unaffected by heat and other agents. The latter 

 remain in admixture with charcoal in the place where the decomposition takes place ; such, 

 for example, are tarry substances. The quantity of those bodies which are found mixed 

 with the charcoal is very varied, and depends on the energy and duration of the decom- 

 posing agent. The annexed table shows, according to the data of Violette, those changes 



