THE ORGANISM AS A MECHANISM 73 



tions, decompositions, or changes of the same kind as 

 that of the dilution of sulphuric acid by water, but 

 the mechanical energy appears to result directly from 

 chemical change without the intermediation of heat. 

 We shall return to this point in a later chapter, and 

 content ourselves with saying here that the chemical 

 compounds contained in the metabolic tissues of the 

 animal body undergo transformation from a state of 

 high to a state of low chemical potential, and that this 

 difference of potential is represented by the work done 

 and the heat generated. The proteid, fat, and carbo- 

 hydrate of the tissues represent the condition of 

 high potential ; and the carbon dioxide, the water, 

 and the urea, into which these substances are trans- 

 formed, represent the condition of low potential. 



Let us suppose a Carnot heat-engine in which the 

 temperature of the reservoir of heat is (say) I20°C, 

 and that of the refrigerator 5o°C. The heat of the 

 refrigerator can still be made a further source of energy 

 by constituting it the heat reservoir of another Carnot 

 engine which has a refrigerator at a temperature of 

 o°C. Our animal organism may be compared with a 

 Carnot cycle; its energy reservoir is the proteid, fat, 

 and carbohydrate ingested, and its refrigerator (or 

 energy sink) is the carbon dioxide and urea excreted. 

 Now the urea of the higher mammal becomes infected 

 with certain bacteria, which convert it into ammonium 

 carbonate. Another species of bacteria converts the 

 ammonia into nitrite, and yet another turns the nitrite 

 into nitrate. The main process of the animal is there- 

 fore combined with several subsidiary ones. 



