THE BACTERIA IX NATURAL WATERS 5 



to the distribution 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, therefore, the 

 bacteria are most numerous; and in other media their 





numbers vary according to the extent of contact with 

 the living earth. 



Classification of Waters. Natural waters, then, 

 group themselves from a bacteriological standpoint 

 in four 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, the surface-waters immediately exposed to such 



