This corresponds to the loss of heat in the locomotive through the smoke passing out the smokestack, and in both cases the loss is greater when work is being done and less during inaction. The refuse products of the body (as the ashes of the locomotive) also carry away heat. This is the third portion of heat and is a large one.
Work is done in the locomotive by the expanding steam in the cylinders of the engine. The steam is cooled as it expands. Hence heat disappears when work is done; that is, is converted into mechanical energy, and a steam engine is hence called a heat engine; an engine for converting heat into work, according to the law of the conservation of energy. As the pistons are pushed to and fro by the tremendous pressure of the expanding steam, the reciprocating motion is communicated to the great drivers of the engine by strong arms of steel. But how is work done in the body? That is a question of prime importance and of surpassing interest. When muscle contracts and force is exerted, as when the body is lifted or an oar is pulled, muscular tissue (or material stored in muscular tissue) is oxidized; that is, burned, and heat is produced; yet not as much heat appears as would have appeared on the combustion of the same amount of body material if no work had been done. Apparently, then, heat has been converted into work. But we cannot trace the process with the same clearness as in the cylinder of a steam engine. Whether the potential energy of the body material is directly converted into work, or whether combustion first produces heat and a part of this heat is then converted into work, we do not know. In other words, we do not know whether the animal body as a machine for doing mechanical work is a heat engine or some other kind of engine. This is a fundamental question, as well as a very difficult one, and to a student of thermodynamics and physiology it prompts all sorts of speculation.
When one tries to picture to himself how the potential energy of food or body tissue can be directly converted into mechanical work, he is apt to turn to the other alternative and imagine that in some way the body is a heat engine. For we know that heat results from the oxidation of tissue, and we also know how heat can be converted into mechanical work. But we are at once confronted with a difficulty. One of the fundamental laws of thermodynamics requires that when heat is converted into work there shall be a difference of temperature between the source of heat and the place to which the heated material employed passes after doing the work. In other words, in a heat engine, whatever the mechanism, there must be a fall of temperature, which is greater as the relative amount of work, or efficiency, is greater. In the human body the efficiency perhaps surpasses that of the best steam engines; hence there should be a fall of temperature comparable with that between the boiler and condenser of a steam engine. This