It is not the province of the present work to enter into the chemical nature of the products of destructive distillation in any greater detail than is necessary to enable the general reader to know something of the recent discoveries in the utilization of these products. We shall, however, have occasion later on to make ourselves acquainted with the names of some of the more important raw materials which are derived from this source, and certain preliminary explanations are indispensable. In the first place then, let us start from the fact that coal--including carbonaceous shale and lignite--when heated in a closed vessel gives gas, tar, coke, and a watery liquor. A clear understanding must be arrived at concerning the manner in which these products arise. There is a widely-spread notion that the substances derived from coal and utilized for industrial purposes are present in the mineral itself, and that the art of the chemist has been exercised in separating the said substances by various processes. This idea must be at once dispelled. It is true that there is a small quantity of water and a certain amount of gas already present in most coals, but these are quite insignificant as compared with the total yield of gas and watery liquor. So also with respect to the tar; it is possible that in some highly bituminous minerals we might dissolve out a small quantity of tarry matter by the use of appropriate solvents, but in the coals mostly used for gas-making not a trace of tar exists ready formed, and still less can it be said that the coal contains coke. All these products are formed _by the chemical decomposition of the coal_ under the influence of heat, and their nature and quantity can be made to vary within certain limits by modifying the temperature of distillation.