TAR, OIL OF TAR, AND PITCH 609 



IV. TAB, OIL OF TAB, AND PITCH. 



Tar, or Pix liquida, is a thick, viscid, brown-black, 

 aromatic liquid, obtained from the wood of Pimis sylvestris 

 and other pines by destructive distillation. Mineral or 

 Barbados tar has already been noticed. Coal tar (pix 

 carbonis), obtained from the destructive distillation of coal, 

 is a by-product in the manufacture of gas. Two descriptions 

 of wood tar are in use one got from hard exogens, such as 

 oak, birch, and ash, as a residual product in the making of 

 charcoal for gunpowder ; and the other as an empyreumatic 

 variety imported from Stockholm, Archangel, and America, 

 is got by roasting billets of the roots, branches, and refuse 

 coniferous timber stacked in shallow pits dug on a bank or 

 inclined plane. This old process is now superseded by 

 distillation of the refuse wood in cast-iron stills, whereby 

 nearly double the yield of tar is obtained ; 14 per cent, is 

 got from air-dried stems, 16 to 20 per cent, from roots. 

 When wood is thus distilled the condensed products separate 

 into two layers, the upper a mixture of methyl-alcohol, 

 pyroligneous acid, acetone, etc., in water ; the lower wood tar. 



Stockholm tar is soluble in alcohol, ether, oils, and alkaline 

 solutions, but not in water, which, agitated with it, acquires, 

 however, its odour, taste, and brown colour, and constitutes 

 tar water, once regarded as a valuable medicine. Tar con- 

 sists of pyroligneous acid, methyl-alcohol, creosote, and 

 various phenols, with toluene, xylene, and other hydro- 

 carbons. 



Tar when distilled yields oil of tar (oleum pieis liquidae), 

 an empyreumatic acid liquid, which, although colourless 

 when first distilled, speedily becomes yellow or brown, and 

 is soluble in alcohol. It contains the more volatile hydro- 

 carbons of the tar. There remains in the retorts pitch, or 

 pix nigra, a black, bituminous substance, solid and brittle, 

 with a shining fracture, dissolved by the same solvents as 

 tar, and consisting of modified resin, and a colourless, in- 

 odorous, crystalline substance, melting at 194 Fahr., called 

 retene (C 18 H 18 ) (Fliickiger). 



ACTIONS AND USES. Tar is antiseptic, stimulant, diuretic, 

 diaphoretic, expectorant, and parasiticide. Its active prin- 



2Q 



