Theory of Heat Engines

—Both steam and gas engines are forms of heat motor; since both operate by means of the expansive energy of gases. which have been subjected to the action of heat. A permanent gas, or the vapor from a liquid or solid substance, when exposed to heat tends to expand, and, in expanding, exerts an active pre ure in all directions. Thus, if a gas, or a readily volatized liquid, like water or alcohol, contained in a corked ve el,be exposed to heat, the expansion will be exhibited in the expulsion of the cork. In this fact is demonstrated the principle, on which all forms of heat engine operate: that heat may be transformed into mechanical energy through its effects on liquids and gases, promoting the change from fluid to gaseous state and then increasing the volume of the gas. No state of matter is entirely permanent, and, as a general rule, the absorption of a sufficient quantity of heat results in liquefying a solid, and in vaporizing a liquid. Gases subjected to heat, either when ignited, as with inflammable gases, or merely heated as with separated steam, tend to a ume greater volumes so long as the temperature is not allowed to fall. On the other band, modern science has succeeded in producing liquid air and liquid carbonic acid gas by the combination of extremely high pre ures and extremely low temperatures. It is sufficient to say that no re ure has yet been found sufficiently high to Rage air, without the cooperation of a temperature commensurately low. Conversely, also, no known degree of cold can produce this effect, apart from a high pre ure acting at the same time.