340 



PRINCIPLES OF CHEMISTRY 



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. 



it is necessary to obtain a very high temperature by burning it, then even as little as 

 25 p.c. of charcoal from 100 parts of wood may be advantageous. Charcoal (from wood) 

 develops on burning 8,000 heat units, whilst wood dried in air does not develop more 

 than 2,800 units of heat ; therefore seven parts of charcoal give as much heat as tfwenty 

 parts of wood. As regards the temperature of combustion, it is far higher with charcoal 

 than with wood, because twenty parts of burning wood give, besides the carbonic anhydride 

 which is also formed together with charcoal, eleven parts of water, the evaporation of 

 which requires a considerable amount of heat. 



The composition of the growing parts of plants, the leaves, young branches, shoots, &c., 

 differs from the composition of the wood in that these vital parts contain a considerable 



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

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

 condenser m, where they are condensed. The form, distribution, and dimensions of the apparatus 



quantity of sap which contains much nitrogenous matter (in the wood itself there is very 

 little), mineral salts, and a large amount of water. Taking, for example, the composition 

 of clover and pasture hay in the green and dry state ; in 100 parts of green clover there 

 is about 80 p.c. of water and 20 p.c. of dry matter, in which there are about 8'5 parts of 

 nitrogenous albuminous matter, about 9*5 parts of soluble and about 5 parts of insoluble 

 non- nitrogenous matter, and about 2 p.c. of ash. In dry clover or clover-hay there is about 

 15 p.c. of water, 18 p.c. of nitrogenous matter, and 7 p.c. of ash. This composition of 

 grassy substances shows that they are capable of forming the same sort of charcoal as 

 wood itself. It also shows the difference of nutritive properties existing between wood 

 and the substances mentioned. These latter serve as food for animals, because they 

 contain those substances which are capable of being dissolved (entering into the blood) 

 and forming the body of animals ; such substances are proteids, starch, &c. Let us 

 remark here that with a good harvest an acre of land gives in the form of grass as much 

 organic substance as it yields in the form of wood. 



One hundred parts of dry wood are capable of giving, on dry distillation, besides 

 25 p.c, of charcoal and 30 p.c. or more of tar, 40 p.c. of watery liquid, containing acetic 

 acid and wood spirit, and about 25 p.c. of gases, which may be used for heating or 

 lighting purposes, because they do not differ from ordinary illuminating gas, which can 



