(c) =NATURAL GAS= obtained from natural accumulations occurring in subterranean pockets in various parts of the country. (d) =COAL GAS=, made artificially by the distillation of coal, commonly called “illuminating” gas. (e) =PRODUCER GAS=, some times known as “fuel gas,” produced by the incomplete combustion of coal in a form of furnace called a “producer.” (f) =BLAST FURNACE GAS=, the unconsumed gas from the furnaces used in smelting iron, somewhat similar to producer gas but lower in heat value. It should be noted that there is no essential difference between engines using a permanent gas or an oil as in either case the fuel is sent into the cylinder in the form of a vapor. In the case of oil fuel, the vapor is formed by an appliance external to the engine proper. In this book, the heat action of an engine using one form of fuel applies equally to the engine using another. The selection of a particular fuel for use with a gas engine depends not only upon its value in producing heat, but also upon its cost, the ease with which it meets the peculiar conditions under which the engine is to work, and its accessibility. Neglecting for the moment, all of the items that do not affect the operation of the engine from a power producing standpoint, the principal requirement of a fuel is the production of a high temperature in the cylinder since the output is directly proportional to the temperature range. Since a very considerable mass of air is to be raised to this high temperature, the heat value, or =CALORIFIC VALUE= of the fuel in British Thermal units is of as much importance as the temperature attained in the combustion. The calorific value of different fuels vary widely when based either on the cubic foot or pound, and a considerable variation exists even among fuels of the same class owing to the different methods of production or to the natural conditions existing at the mine or well from which they originated. The principal elements of gas engine fuels, carbon and hydrogen, exist in many different combinations and proportions, and require different quantities of air as oxygen for their combustion because of this difference in chemical structure. Since complete combustion is never obtained under practical working conditions, the actual evolution of heat and the actual temperatures are always much lower than those indicated by the =CALORIMETER= or heat measuring device. Besides the loss of heat due to imperfect combustion, there are many other losses such as the loss by radiation, connection, and slow burning, the latter being the principal cause of low combustion temperatures. From the statements in the foregoing paragraphs it will be seen that the theoretical or absolute calorific value of a fuel is not always a true index to its efficiency in the engine.