CARBON AM> THE HYDROCARBONS 



329 



This branch of chemistry is 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 

 access of air or, better still, in a vacuum it decomposes with more or 

 less facility. When organic substances are heated in air, it is well 

 known that they burn ; but if the supply of air be insufficient, or the 

 temperature be too low for combustion, and if the first volatile pro- 

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

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

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



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

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



There are many steps between the method of burning wood in stacks (Note 4), and 

 that of burning it in an enclosed space namely, those in which the burning of the char- 



FIG. 57. Apparatus for the dry distillation of wood. The retort a containing the wood is heated by 

 the flues c e. The steam and volatile products of distillation pass along the tube g through the 

 condenser ni, wln-iv they are condensed. The form, distribution, and dimensions of the apparatus 



vary. 



coal is accompanied by a certain production of tar. This is effected by means of trenches 

 dug in the earth and having sloping bottoms, by which contrivance the tar separated by 

 the charring of the wood flows into special receivers. This process is much used in 

 the north of Eussia. 



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 the dry distil- 

 lationof wood. Coal (Note 6), sea-weed, turf, animal substances (Chap. VI.), &c., are 

 also submitted to the process of dry distillation. 



2 The result of imperfect combustion is not only the loss 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 



