74 THE PHILOSOPHY OF BIOLOGY 



Carbohydrate, fat, proteid} Metabolism 

 break down into /of the animal 



* 



Carbon dioxide 

 Water 



Urea Urea \ Metabolism of 



Chemical y / urea bacteria 



Energy at passes into 



high potential ammonium 



carbonate ammonium j metabolism 



carbonate I of nitrifying 



sj, ) bacteria 



oxidises to 



nitrite Nitrite \ Metm. of 



^ (^ nitrify - 

 oxidises j ing 

 to nitrate; bacteria 



v, 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 ob- 

 tained 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, 



Other nitrogenous residues are contained in the 

 urine of animals, and several other excretory products 

 may be formed. But in all these cases we can easily 

 find subsidiary energy-transformations effected by 

 bacteria, as in the above scheme. This, then, is the 

 positive, or direct half, of that reversible Carnot cycle 

 with which we are comparing life. In it energy falls 

 in potential (or intensity, or level), and in this fall of 

 potential transformations are produced exhibit them- 

 selves, is perhaps a better way of putting it. We will 

 consider these transformations later ; in the meantime 



