Coal tar is a thick, black liquid that is a byproduct of the production of coal. It is composed of hundreds of different compounds, including phenols, polycyclic aromatic hydrocarbons, and heterocyclic compounds. The term 'coal tar' is also used to refer to the residue left after the distillation of coal tar, which is known as eosin. Eosin is a red-brown dye used in histology and microscopy, as well as in the manufacture of certain pharmaceuticals.