130 BACTERIOLOGY. 



wards, and the ground at a depth of from four 

 to six meters is germ-free. From the soil the 

 bacteria may pass into the water, and we every- 

 where find in water the same germs as in soil, 

 unless the water be derived from the deeper 

 germ-free layers. In water, just as in soil, bac- 

 teria share in the decompositions of organic 

 substance and aid in the transformation of in- 

 organic compounds of sulphur and iron. In 

 the ooze at the bottom of rivers and lakes, 

 where there is lack of oxygen, they bring about 

 the formation of marsh-gas, hydrogen, and sul- 

 phuretted hydrogen, and so take the first step in 

 the beginnings of coal-formation. The num- 

 ber of individual germs, and of species, in water 

 and soil depends largely upon the amount of 

 decomposable substances present in the sur- 

 roundings. In the neighborhood of our dwell- 

 ings, in the tracts of low land about rivers and 

 lakes, and in river deltas, there is a high per- 

 centage of such substances, and in such local- 

 ities we meet active decomposition and most 

 luxuriant bacterial vegetation. Many species, 

 like B. termo, the hay bacillus, and the earth 

 bacillus, are apparently spread over the whole 

 world, others are very rare or limited to special 

 localities. The true home of the comma-bacil- 

 lus of Asiatic cholera, for instance, seems to lie 

 in the notorious Sunderbunds of the Ganges. 



