498 POPULAR SCIENCE MONTHLY. . 



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 cylin- 

 ders of the engine. The steam is cooled as it expands. Hence heat dis- 

 appears when work is done; that is, is converted into mechanical en- 

 ergy, 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 communi- 

 cated 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 ex- 

 erted, 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 oxida- 

 tion 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 en- 

 gine, 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 o-f a steam engine. This 



