TAR 503 



wood of foliage trees (beech, birch, &c.). From the former 15 to 20 

 per cent, of tar may be obtained, from the latter 6 to 8 per cent. For 

 further details see Thorpe, ' Dictionary of Applied Chemistry ' : 

 Wood, Destructive Distillation of. 



Description. Tar is a dark-brown or blackish, semi-liquid 

 substance, with a peculiar aromatic odour and a bitter, pungent 

 taste ; it is heavier than water, its specific gravity varying from 

 1*02 to 1*15. By keeping, it becomes thicker, and acquires a granular 

 appearance, due to the formation of minute crystals (probably 

 pyrocatechin, resin acids, &c.). Water shaken with it acquires a 

 yellow colour and acid reaction (distinction from coal-tar, which 

 imparts to water an alkaline reaction). The filtered aqueous liquid 

 is coloured red by very dilute (O'l per cent.) solution of ferric chloride, 

 (distinction from juniper- tar oil). Tar is completely soluble in ten 

 volumes of alcohol. 



Constituents. The following are the chief constituents of tar : 

 benzene, toluene, xylene, and styrolene (styrol) ; phenol, cresol, 

 guaiacol and its homologues ; pyrocatechin and paraffin. The most 

 important of these, as far as the medicinal activity is concerned, are 

 probably pyrocatechin, phenol, cresol, guaiacol, and their homologues. 



Tar from Coniferous woods, which alone is official, is especially 

 rich in guaiacol and its homologues, beech-tar in guaiacol and 

 pyrogallol derivatives, birch-tar in guaiacol and benzophenol 

 derivatives. 



Uses. Tar is used as an external stimulant and antiseptic in certain 

 skin diseases ; given internally in the form of pills or syrup, it acts as 

 a disinfectant and deodorant of offensive discharges from the bronchial 

 tubes. 



COAL-TAR 



(Fix Carbonis) 



Source, &C. Coal-tar is obtained by the destructive distillation 

 of coal in iron or clay retorts. The gaseous products that are 

 formed during the process constitute ordinary illuminating gas ; 

 the liquid products are a watery solution of ammonium carbonate, 

 sulphide, &c., and a heavy, dark, viscid, tarry liquid, coal-tar. The 

 retort contains a porous, carbonaceous residue, coke. 



Description. Coal-tar is a nearly black, viscid liquid, with a 

 strong, penetrating, disagreeable odour ; it is heavier than water, its 

 specific gravity varying from I'l to 1*2. It is only slightly soluble 

 in water, to which it imparts an alkaline reaction (distinction from 

 wood- tar). It consists of a large number of substances, amongst 

 which benzene, toluene, xylene, phenol, cresol, naphthalene, 



