828 PRINCIPLES OF CHEMISTRY 



that it forms a separate branch of chemistry, which treats of the com- 

 pounds of carbon, part of which are met with in plants and animals. 



evaporated during its combustion. The above-mentioned composition of wood shows 

 that it would be far more profitable to use charcoal for heating purposes than wood, if it 

 were possible to obtain it in such quantities as correspond with its percentage ratio that 

 is, forty parts per 100 parts of wood. Generally, however, the quantity produced is far 

 less, not more than 30 p.c., because part of the carbon is given off as gas, tar, &c. If 

 wood has to be transported great distances, or if 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 (wood) develops on burning 8000 heat units r 

 whilst wood dried in air does not develop more than 2800 units of heat ; therefore 

 seven parts of charcoal give as much heat as twenty parts of wood ; but twenty parts of 

 wood are only capable of yielding five parts of charcoal. This datum may be used for com- 

 paring the heating power of wood and charcoal. As regards the temperature of combus- 

 tion, it is far higher with charcoal than with wood, because twenty parts of burning wood 

 give, besides the carbonic anhydride which is also formed 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, <tc. T 

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

 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 3'5 parts of 

 nitrogenous 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, 13 p.c. of nitrogenous matter, and 7 p.c. of ash. This com- 

 position of grassy substances shows that they are capable of forming the same sort of char- 

 coal 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, by means of dry distillation, 

 besides 25 p.c. of charcoal and 10 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 caji 

 indeed be obtained from wood. As wood-charcoal and tar are costly 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 watery liquid. The latter is used 

 for the manufacture of wood spirit, CH 4 O, and acetic acid, C 2 H 4 Oo. In such cases 

 the dry distillation is carried on in stills. Stills are nothing more than hori- 

 zontal 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 products of distillation. The dry distillation of wood in stoves is carried 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 means 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 



