FIX LIQUIDA. 021 



solution becomes brown, and at last microscopic crystals are obtained 

 with a brown residue like tar itself, which is no longer soluble in water, 

 A microscopical examination of tar which has been exhausted with 

 \A'ater, shows that all crystals have disappeared. 



Chemical Composition — Dry wood may be heated to about 150" C. 

 without decomposition ; but at a more elevated temperature, it com- 

 mences to undergo a change, yielding a large number of products, 

 the nature and comparative quantity of which depend upon circum- 

 stances. If the process is carried on in a closed vessel, a residue will 



By heating fir- wood 



7) 



he got which has moi 



Hke substance, which yielded by a svibsequent increase of temperature 

 scarcely any volatile products. 



The results are widely different if a process is followed which permits 

 the formation of volatile bodies ; and these substances are formed in 

 largest proportion, if the heat acts quickly and intensely. At lower 

 degrees of heat, more charcoal results and more water is evolved. 



Among the volatile products of destructive distillation, those alone 

 which are condensed at the ordinary temperature of the air are of 

 pharmaceutical interest ; and of these, chiefly the portion not soluble in 

 water, or that which is called Tar or Liquid Fitch. The aqueous portion 

 of the products consist principally of empyi'eumatic acetic acid, to 

 which tar owes its acid reaction. 



The tissue of wood is chiefly formed of cellulose, intimately combined 

 with a saccharine substance, which may be separated if the wood is 

 boiled with dilute acids. The remaininfj cellulose is however not yet 



Erdmann 



pyi'ocatechin 



It is well-known that sugar subjected to an elevated temperature, 

 yields a series of pyrogenous products ; and the same fact is observed 

 if purified cellulose is heated in similar manner. But for tar-making, 

 wood is preferred which is impregnated with resins and essential oils, 

 and these latter furnish another series of enipyreumatic products. From 

 these circumstances, the components of wood-tar are of an extremely 

 complicated character, which is still more the case when other woods 

 than those of conifers form part of the material submitted to distilla- 

 tion. In the case of beech- wood, Creasote is formed, which is obtained 

 only in very small quantity from the Coniferce. Volatile alkaloids and 

 carbolic acid, which are largely produced in the destructive distillation 

 of coal, appear not to be present in wood-tar. 



The components of the latter may be considered under two heads : 

 —first, the lighter aqueous portion, which separates from the other 

 Foductsof distillation, forming what is called /m^w re Pyivhgneom 



A 



r 



Mcth 



Cff : Acetone. C'H 



tlantly soluble in water and acetic acid. In this portion, some pyro- 

 catechin also occurs. - e 



The second class of pyrogenous products of wood consists of a 

 nomologous series of liquid hydrocarbons, sparingly soluble m water, 

 firid which therefore are chiefly retained in the heavy layer below the 

 Pyroligneous acid, forming the proper wood-tar. The liquid m question 



' Liebig, Amialen der Chemk u. Phirmacie, Suppl. v. (1867)220. 



