156 



CHAPTEE VII 



BACTERIOLOGICAL ANALYSIS OF BAETH AND OF Pt'TREP-XING 

 SUBSTANCES 



Micro-organisms in the soil. — The examination of soil 

 proves that very many micro-organisms which thrive in 

 the air and in water can also grow in earth. Moreover, in 

 every putrefactive process on the surface of the ground 

 there occurs an oxidation, or resolution of organic matter 

 with the aid of atmospheric oxygen ; consequently, in all 

 these processes a considerable number of micro-organisms 

 are afforded the possibility of maintaining themselves and 

 multiplying. In agriculture the land is manured with the 

 view of enabling highly complex organic substances to 

 undergo decomposition on the surface of the ground into 

 simple combinations, capable of serving as nutriment for 

 plants and of being assimilated by them in order that they 

 may be once more converted into higher combinations. In 

 these putrefactive processes the action of bacteria takes a 

 very prominent part. That it is the surface of the soil which 

 is so very rich in varieties of germs becomes evident from 

 the fact that at so short a distance as one to two meters 

 beneath the surface the amount of bacteria present rather 

 suddenly decreases, and that further down, at a depth of 

 three or four meters, the earth is found to be completely 

 free from germs. The ground-water level of the soil is 

 tolerably pure in this respect, and hence also only very few 

 micro-organisms are found in the water of springs. For 

 the same reason fountains fed by pipes, if properly con- 



