126 BACTERIA AND SOIL MINERALS 
tion to a circulation of iron in nature, somewhat similar to that of 
sulphur. These bacteria are commonly seen covered with a de- 
posit of hydroxid of iron, giving them a reddish-brown color. It 
has been thought that they used iron in their activities much as 
sulphur uses iron. But the most recent work indicates that this 
is probably an error, and makes the agency of bacteria doubtful. 
At all events, nothing reliable is known to-day upon the subject, 
although it is not impossible that here, too, the bacteria of the 
soil, and especially of waters, may be of some significance. 
