DRY DISTILLATION AND CHARCOAL. 53 



Heated in a closed chamber or covered with earth, as in charcoal 

 pits, the wood is prevented from burning and a variety of changes 

 occur, depending on the rate of heating. If the temperature is raised 

 gradually so that the wood is heated several hours before a temperature 

 of 600° F. is reached the process is called dry distillation. In this proc- 

 ess the wood is destroyed. It forms at first "red" or "brown" coal, 

 still resembling wood, and finally charcoal proper. This coal is darker, 

 heavier, conducts heat and electricity better, requires a greater heat to 

 ignite, and produces more heat in burning the higher the temperature 

 under which it is formed. 



One hundred pounds of wood (dried at 300° F.) leaves only about 30 

 pounds of charcoal. In common practice much less charcoal (18 to 20 

 per ceut) is produced. In this change from wood to coal the volume is 

 diminished by about one-half, so that a cord of wood which contains 

 about 100 cubic feet of wood solid would be converted into 50 cubic 

 feet at best. 



Of the 70 pounds of gaseous products which 100 pounds of wood 

 lose, during coaling, in being heated up to 700° F., about 63 pounds 

 become volatile before the temperature of 550° F. is reached. 



If condensed in a cooler, about three-fourths of the 63 pounds of 

 volatile matter first evolved is found to be wood-vinegar, from which 

 about 4 pounds of pure acetic acid, the only source of perfectly pure 

 vinegar, is obtained. Besides acetic acid, the liquid contains wood 

 spirits and a quantity of various allied substances. 



After the first stage of dry distillation, a large part of the products 

 developed can not be liquefied in the ordinary cooler. They are gases 

 like the illuminating gas, mostly belonging to the marsh gas series; 

 they lack oxygen and thus show that the available oxygen has been 

 nearly exhausted in the preceding part of the process. Products of 

 the later stages are tars and heavy oils, volatile only at high tem- 

 peratures. Here also belong the substances known collectively as wood 

 creosote, employed as antiseptics in wood impregnation. 



Warmed in dilute nitric acid with a little chlorate of potash, the cells 

 of a piece of wood may be separated, each cell remains intact, but its 

 wall is reduced in thickness and material; thelignin substances being 

 dissolved out, only the cellulose is left. In commercial cellulose manu- 

 facture, soda, sulphates, and of late chiefly sulphites are substituted 

 for the nitric acid. The wood is chipped, boiled in the respective solu- 

 tion under high pressures, the residue is washed, and the remaining 

 cellulose bleached and ready for use. As a matter of economy the 

 residual liquid is evaporated and the soda used over again. 



When resinous wood, "fat pine," "lightwood," such as the knots 

 and stumps of longleaf, pitch, and other pines, is heated in a kiln or 

 retort, the resins ooze out, are collected, and in distillation with steam 

 yield turpentine and rosin. The resins and their components vary with 

 the species; the balsam of fir is limpid, its turpentine remains clear on 



