In all practical heat engines, the heat energy is developed by the process of combustion, which is a chemical combination of the oxygen of the air with certain substances, such as coal or gasoline, known as “fuels.” The heat producing elements of the fuels are generally compounds of carbon and hydrogen, which when oxydized or burnt by the oxygen form products that are unlike either of the original components. It is due to this chemical change that heat energy is evolved, for the heat represents the energy expended by the sun in building up the fuel in its original form, and as energy can neither be created nor destroyed, heat energy is liberated when the fuel is decomposed. The heat energy thus liberated is applied to the expansion of the working medium to obtain its equivalent in the form of mechanical power. During the period of expansion, the heat obtained by the combustion is absorbed by the working medium in proportion to its increase in volume, and as this increase is proportional to the mechanical effort exerted by the engine, it will be seen that the output of the engine in work is a measure of the heat applied to the medium. The quantity of heat absorbed by the medium represents the energy required to set the molecules of the medium into their new positions in the greater volume, or to increase their paths of travel. In the conversion of heat, each heat unit applied to the medium results in the production of 778 foot pounds of energy, providing that there are no heat or frictional losses. In explanation of these terms or units, we wish to say, that the unit of heat quantity, called the =BRITISH THERMAL UNIT= is the quantity of heat required to raise one pound of water, one degree Fahrenheit, and the =FOOT POUND= is the work required to raise one pound through the vertical distance of one foot. As the British Thermal Unit = 778 foot pounds it is equivalent to the work required to raise 778 pounds one foot or one pound 778 feet, or any other product of feet and pounds equal to the figure 778. As liquids expand more than solids with a given temperature, and gases more than either, the mechanical work returned for a given amount of thermal energy (the =EFFICIENCY=) will be greater with an engine using gas as a working medium than one using a solid or liquid working medium. The steam engine and the gas engine are both examples of heat engines using gaseous working mediums, the medium in the steam engine being water vapor and in the gas engine, air and the gaseous products of combustion. For this reason the working medium will be considered as a gas in the succeeding chapters.