SOLID FUELS Solid fuels may be divided into two main groups as being "natural" or "manufactured." In the former category are coal, wood, peat and other vegetable matter; and in the latter, pro ducts such as coke and charcoal, obtained by destructive distilla tion, viz., by heating the raw material in the absence of air, by which process gas and tar are also yielded. Such processes form the basis of the manufacture of both metallurgical coke and gas, two very important industries.