6 ELEMENTS OF WATER BACTERIOLOGY. 



nature, the increasing size and closeness of the spiral on 

 the left-hand side indicating the progressive complexity 

 of organic matter as built up by the chlorophyll bodies 

 of green plants in the sunlight, and the other half of the 

 figure the reverse process carried out largely by the bac- 

 teria. In nature there are many short circuits, as, for 

 instance, when dead organic matter is used as food for 

 animals and built up into the living state again without 

 ever being nitrified and acted upon by green plants; but 

 the complete cycle of organic nitrogen is as indicated on 

 the diagram. 



We have dwelt thus at length upon the general relation 

 between bacteria and organic decomposition because in 

 this relation will be found the master key to the distri- 

 bution of bacteria in water as well as in other natural 

 habitats. It is true that certain peculiar forms may at 

 times multiply in fairly pure waters; but in general large 

 numbers of bacteria are found only in connection with 

 the organic matter upon which they feed. Such organic 

 matter is particularly abundant in the surface layer of the 

 soil. Here the processes of nitrification proceed most 

 rapidly. Here the bacteria are most abundant; and in 

 other media their numbers vary according to the extent 

 of contact with the living earth. Natural waters particu- 

 larly group themselves from a bacteriological standpoint 

 in three well-marked classes, according to their relation 

 to the rich layers of bacterial growth upon the surface of 

 the globe. There are first the atmospheric waters which 

 have never been subject to contact with the earth; second, 



