IRON 125 
as the gas is abundant the bacteria flourish; but if the gas disap- 
pears they appear to use up the sulphur in their own bodies, after 
which they die. In some way the sulphur in their bodies isin the 
end converted into sulphuric acid, which then combines with any 
lime that may be present to form sulphate of lime. Very little is 
known as to when or how the sulphur in the cell walls of these 
sulphur bacteria gets converted into sulphuric acid, or whether it is 
a purely chemical or a biological phenomenon. But although the 
whole process is not fully understood, it is evident that micro- 
6rganisms have a close relationship to the transformations of 
sulphur in the waters and soils. They liberate it from its com- 
bination 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 oxi- 
dize sulphites into sulphaies. Microérganisms are thus respon- 
sible 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. 
Other Transformations of Sulphur.—It has also been observed 
that if free sulphur is mixed with soil, it is gradually converted into 
sulphates. This is thought to be due in large part to bacteria. 
The process has been called sulphofication, but the term is not 
in such general use as the corresponding terms ammonification, 
nitrification and so forth. There is also evidence of other bacteria 
in soil that reduce sulphates to sulphites, a process very similar to 
the reduction of nitrates to nitrites. 
In general, much less is known about the transformations of 
sulphur than of those of nitrogen. The reason for this is that 
sulphur is almost never deficient in soils, and the subject has never 
been considered of sufficient practical importance to justify exten- 
sive investigations. 
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- 
