FIX LIQUIDA. 621 



solution becomes brown, and at last microscopic crystals are obtained 

 with a bro^^^l residue like tar itself, which is no longer soluble in water. 

 A microscopical examination of tar which has been exhausted with 

 water, 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 

 be got which has more or less resemblance to coal. By heating fir- wood 

 enclosed with some water to 400° C, Daubree (1857) obtained a coal- 

 like substance, which yielded by a subsequent 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 Pitch. The aqueous portion 

 of the products consist principally of empyreumatic acetic acid, to 

 which tar owes its acid reaction. 



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

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

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

 pure, but is still united to a substance which, as shown by Erdmann,^ 

 is capable of yielding pyrocatechin. 



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

 jaelds 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 empyreumatic 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, Greasote 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 poi^icm, which separates from the other 

 products of distillation, forming what is called Impure Pyroligneous 

 Acid. This contains chiefly acetic acid and Methyl Alcohol or Wood 

 Naiihtha, CH''0 ; Acetoiie, C^ffO ; besides other liquid products abun- 

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

 catechin also occurs. 



The second class of pyrogenous products of wood consists of a 



homologous series of liquid hydrocarbons, sparingly soluble in water, 



and which therefore are chiefly retained in the hea^^ layer below the 



pyroligneous acid, forming the proper wood-tar. The liquid in question 



1 Liebig, Annalen der Cliemie u. Phnrmacie, Snppl. v. (1867) 229. 



