332 PRINCIPLES OF CHEMISTRY 



tube heated to a high temperature. Organic substances, when burning 

 with an insufficient supply of air, give off soot that is, charcoal pro- 

 ceeding from carbon compounds in a state of vapour, the hydrogen 

 of which has, by combustion, been converted into water ; so, for instance, 

 turpentine, naphthalene, and other hydrocarbons which are with diffi- 

 culty decomposed by heat, easily yield carbon in the form of soot during 

 combustion. Chlorine and other substances, which, like oxygen, are 

 capable of taking up hydrogen, and also substances which are capable 

 of taking up water, can also separate carbon from (or char) most organic 

 substances. 



When organic substances are burning in an insufficient supply of 

 air, they both separate and leave charcoal. Part of it remains behind 

 as a residue, as, for instance, after burning wood in a stove some remains 

 in the form of charcoal. Wood charcoal is prepared in large quantities 

 in a similar manner that is, by the partial combustion of wood. 4 In 

 nature a like process of carbonisation of vegetable refuse takes place in 

 its transformation under water, as shown by the marshy vegetation 

 which forms peat. 5 In this manner, doubtless, the enormous masses 



4 The object of producing charcoal from wood has been explained in Footnote 1. 

 Wood charcoal is obtained in so-called stacks by partially burning the wood, or by 

 means of dry distillation that is, by heating wood in an enclosed space (in retorts, 

 Note 1) without the access of air. It is principally manufactured for metallur- 

 gical processes, especially for smelting and forging iron that is, for use in smithies. 

 The preparation of charcoal in stacks has one advantage, and that is that it may be 

 done on any spot in the forest. But in this way all the products of dry distillation 

 are lost. For charcoal burning, a pile or stack is generally built, in which the logs are 

 closely placed, either horizontally, vertically, or inclined, forming a stack of from six to 

 fifty feet in diameter and even larger. Under the stack there are several horizontal air 

 passages, and an opening in the middle to let out the smoke. The surface of the stack is 

 covered with sods to a considerable thickness, especially the upper part, in order to hinder 

 the free passage of air and to concentrate the heat inside. When the stack is kindled, the 

 pile begins to settle down by degrees, and it is then necessary to look after the turf 

 casing and keep it in repair. As the combustion spreads throughout the whole pile, the 

 temperature rises and real dry distillation commences. It is then necessary to stop the 

 air holes, in order as much as possible to prevent unnecessary combustion. The nature 

 of the process is, that part of the fuel burns and develops the heat required for sub- 

 jecting the remainder to dry distillation. The charring of the stack lasts about a fort- 

 night, and is brought to a termination by scattering earth over the incandescent charcoal 

 to stop the combustion. The charring is stopped when the products of dry distillation, 

 which are emitted, no longer burn with a brilliant flame, but the pale blue flame of 

 carbonic oxide appears. Dry wood in stacks yields about one-fourth of its weight of 

 charcoal. 



5 When dead vegetable matter undergoes transformation in air, in the presence of 

 moisture, there remains a substance much richer in carbon namely, humus, black earth 

 or mould. 100 parts of humus in a dry state contain about 70 p.c. of carbon. The roots, 

 leaves, and stems of plants which wither and fall off form a soil rich in humus. 

 The non-vital vegetable substances (ligneous tissue) first form brown matter (ulmic 

 compounds), and then black matter (humic substances), which are both insoluble 

 in water ; after this a brown acid is produced, which is soluble in water (apocrenic acid),. 



