CARBON AND THE HYDROCARBONS 841 



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 pro- 

 ducts of transformation of the organic matter are subjected to conden- 

 sation (for example, if the door of a stove be opened), an imperfect 

 combustion takes place, and smoke, with charcoal or soot, is formed. 2 



indeed be obtained from wood. As wood-charcoal and tar are valuable products, in some 

 cases the dry distillation of wood is carried on principally for producing them. For this 

 purpose those kinds of woods are particularly advantageous which contain resinous sub- 

 stances, especially coniferous trees, such as fir, pine, &c. ; birch, oak, and ash give much 

 less tar, but on the other hand they yield more aqueous liquor. The latter is used for the 

 manufacture of wood spirit, CH 4 O, and acetic acid, C 2 H 4 O<>. In such cases, the dry dis- 

 tillation is carried on in stills. The stills are nothing more than horizontal or vertical 

 cylindrical retorts, made of boiler plate, heated with fuel and having apertures at 

 the top and sometimes also at the bottom for the. exit of the light and heavy pro- 

 ducts of distillation. The dry distillation of wood in stoves is earned on in two ways, 

 either by burning a portion of the wood inside the stove in order to submit the remainder 

 to dry distillation by means of the heat obtained in this manner, or by placing the wood 

 in a stove the thin sides of which are surrounded with a flue leading from the fuel, 

 placed in a space below. 



The first method does not give such a large amount of liquid products of the dry 

 distillation as the latter. In the latter process there is generally an outlet below for 

 emptying out the charcoal at the close of the operation. For the dry distillation of 100 

 parts of wood from forty to twenty parts of fuel are used. 



In the north of Russia wood is so plentiful and cheap that this locality is admirably 

 'fitted to become the centre of a general trade in the products of its dry distillation. 

 Coal (Note 6), sea-weed, turf, animal substances (Chapter VI.), <fec., are also submitted 

 to the process of dry distillation. 



-' The result of imperfect combustion is not only the logs of a part of the fuel and the 

 production of smoke, which in some respects is inconvenient and injurious to health, but 

 also a low flame temperature, which means that a less amount of heat is transmitted to 

 the object heated. Imperfect combustion is not only always accompanied by the forma- 

 tion of soot or unburnt particles of charcoal, but also by that of carbonic oxide, CO, in the 

 smoke (Chapter IX.) which burns, emitting much heat. In works and factories where 

 large quantities of fuel are consumed, many appliances are adopted to ensure perfect com- 

 bustion, and to combat against such a ruinous practice as the imperfect combustion of i 

 fuel. The most effective and radical means consists in employing combustible gases 

 (producer and water gases), because by their aid perfect combustion can be easily 

 realised without a loss of heat-producing power and the highest temperature can 

 be reached. When solid fuel is used (such as coal, wood, and turf), imperfect combustion' 

 is most liable to occur when the furnace doors are opened for the introduction of fresh 

 fuel. The step furnace may often prove a remedy for this defect. In the ordinary; 

 furnace fresh fuel is placed on the burning fuel, and the products of dry distillation of! 

 the fresh fuel have to burn at the expense of the oxygen remaining uncombined with 1 

 the burnt fuel. Imperfect combustion is observed in this case also from the fact that 

 the dry distillation and evaporation of 'the water of the fresh fuel lying on the top of that 

 burnt, lowers the temperature of the flame, because part of the heat becomes latent. 

 On this account a large amount of smoke (imperfect combustion) is observed when a fresh 

 quantity of fuel is introduced into the furnace. This may be obviated by constructing] 

 the furnace (or managing the stoking) in such a way that the products of distillation pass 

 through the red-hot charcoal remaining from the burnt fuel. It is only necessary inj 

 order to ensure this to allow a sufficient quantity of air tor perfect combustion. All thi&| 



