25 



and for the preparation of wagon grease and other low-grade lubri- 

 cants. It is also a suitable raw material for the preparation of anilin 

 colors, but finds no industrial application for this purpose, because of 

 the low price of coal tar and the fact that the composition of the latter 

 is better known. 



In addition to the tar separated by settling, the crude pyroligneous 

 acid contains considerable tar held in solution by the acids and alcohol 

 present, which is recovered when the crude acid is distilled, and 

 constitutes what is known as " boiled tar." It may be sold as such 

 or burned under the retort, or it may be mixed with the raw tar and 

 subjected to any desired treatment. 



Wood tar, which varies in character with the kind of wood from 

 which it is obtained, is a thick, dark-colored, viscous material still con- 

 taining some acetic and other acids, and has a specific gravity of about 

 1.05 to 1.15. It consists when derived from hardwood chiefly of 

 paraffins, toluene, xylene, cresol, guaiacol, phenol, and methyl deriva- 

 tives of pyrogallol. The tars derived from coniferous woods con- 

 stitute the chief tars of commerce, and are particularly rich in 

 terpenes, contain considerable quantities of rosin, and have a much 

 greater commercial value than those derived from hardwoods. By 

 far the greater part of the tar derived from hardwood distillation 

 is burned under the retorts. 



The general composition of various tars is shown in the following 

 table : 



General composition of various tars. 



The crude or raw tar may be handled in several ways. It may be 

 burned under the retorts, sold as crude tar, or subjected to fractional 

 distillation for the isolation of its several constituents. To effect this 

 the tar is placed in a suitable still and heated. When mixtures of 

 volatile liquids are heated sufficiently high the distillate does not, as 

 a rule, have the composition of the mixture in the still, but the vari- 

 ous constituents pass over in a more or less pure form between certain 

 definite temperatures. This method of separating the product in the 

 still into its various components is known as fractional distillation, 



[Cir. 36] 



