Let us suppose a Carnot heat-engine in which the temperature of the reservoir of heat is (say) 120°C., and that of the refrigerator 50°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 0°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 therefore combined with several subsidiary ones. _Carbohydrate_, _fat_, _proteid_ } Metabolism break down into } of the animal ↓ Carbon dioxide Water Urea------_Urea_ } Metabolism of Chemical ↓  } urea bacteria Energy at passes into high potential ammonium \ carbonate----_ammonium_ } metabolism \ _carbonate_ } of nitrifying \ ↓ bacteria \ oxidises to Work done by \ nitrite----_Nitrite_ } Metm. of the system \ ↓  } nitrifying \ oxidises } bacteria \ to nitrate} chemical energy at low potential The arrows show that energy is descending the incline indicated by a direct Carnot cycle. There is no more work to be obtained from the carbon dioxide and water excreted by the mammal, but more work can be obtained from the urea when it is used by bacteria, and “ferments” to ammonia. Work can again be obtained from the ammonia by bacteria, which convert it into nitrite, and yet again from the nitrite by other bacteria, which convert it into nitrate. The nitrate represents the energy-zero so far as the organisms considered are concerned.