This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
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, etc, and a heavy, dark, viscid, tarry liquid, coal-tar. The retort contains a porous, carbonaceous residue, coke.
Coal-tar is a nearly black, viscid liquid, with a strong, penetrating, disagreeable odour; it is heavier than water, its specific gravity varying from 1.1 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, anthracene, and pitch may be mentioned. For further details see Martin, ' Industrial Chemistry, Organic,' p. 415.
The chief medicinal use of coal-tar is as an antiseptic application in certain skin diseases.