The term "xylene" refers to any of three benzene derivative isomers that share the same chemical formula, C₆H₄(CH₃)₂, but differ in their molecular structure. They are useful as solvents, as additives to improve the octane rating of aviation fuels, and as raw materials in the manufacture of fibers, films, dyes, and other synthetic products. Because of their high volatility they are classified as aromatic hydrocarbons (volatile compounds containing only carbon and hydrogen atoms). Xylene fractions were first isolated from coal tar in the mid-nineteenth century in Germany. Coal tar is the thick liquid product of the carbonization, or destructive distillation, of coal, a process in which coal is heated without air to temperatures above 1,600°F (862°C). Benzene and toluene, two aromatic hydrocarbons similar to xylene, are formed in the same process. Coal tar remained the source for xylene until industrial demand outgrew the supply. Later, techniques...