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 the subject, although 
it is not impossible that here, too, the bacteria of the soil, and 
especially of waters, may be of some significance. 
