SECTION XVII 



TARS AND EMPYREUMATIC OILS 



These are viscous or oily substances produced by the destructive 

 distillation of organic substances such as wood, coal, &c. 



TAR 

 (Wood Tar, Stockholm Tar, Fix Liquida) 



Source, &C. Tar or, as it is better termed, wood- tar (Fix liquida), 

 to distinguish it from coal-tar (Pix carbonis), is a bituminous liquid 

 obtained from the wood of Pinus sylvestris, Linne (N.O. Coniferce), 

 and other species of Pinus by destructive distillation. 



Various methods have been adopted for the destructive distillation 

 of wood. The simplest consists in constructing a mound of the 

 material and covering it with earth, leaving spaces by which air can 

 be admitted and through which the tar produced can drain off. The 

 wood is fired, and, the heat being carefully controlled, charcoal is 

 left as a residue in the mound, whilst a tarry and an aqueous liquid 

 are collected. 



More modern apparatus consists of either upright or horizontal 

 iron retorts arranged singly or in a battery of about twenty. These 

 are heated from without, and the products of distillation may be 

 ranged under three heads : 



(i) Gaseous products analogous to coal-gas ; these are conducted 

 back to the hearth and burnt, serving to economise fuel, 

 or they are stored in a gasometer until required for some 

 such purpose. 



(ii) Watery products ; water containing acetic acid, methyl 

 alcohol, acetone, &c. ; these pass over and are condensed. 

 They serve as a source of acetic acid, methyl alcohol, &c. 



(iii) Tarry products ; these separate from the watery liquid on 

 standing. 



Coniferous wood is chiefly employed, as it yields more tar than the 



