IRON. 117 



process is not fully understood, it is evident that microorganisms 

 have a close relationship to the transformations of sulphur in the 

 waters and soils. They liberate it from its combination in proteid, 

 they oxidize the liberated gas into sulphur and finally into the 

 form of sulphuric acid which soon forms a sulphate. It is also 

 claimed that some kinds of bacteria can oxidize sulphites into 

 sulphates. Microorganisms are thus responsible for the constant 

 metamorphosis of sulphur compounds that keeps the soil properly 

 supplied with this element. Of their activities in ordinary cultivated 

 soil we know little or nothing. 



IRON. 



A small quantity of iron is needed by plants, and a group of 

 bacteria, called iron bacteria, has been supposed to have some rela- 

 tion to a circulation of iron nature, somewhat similar to that of 

 sulphur. These bacteria are commonly seen covered with a deposit 

 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 trie subject, although 

 it is not impossible that here, too, the bacteria of the soil, and 

 especially of waters, may be of some significance. 



