Il6 BACTERIA AND SOIL MINERALS. 
these several sources much sulphur is liberated into the air in the 
form of H 2 S, the liberation in all cases being due to the action of 
bacteria, different classes acting on different compounds containing 
sulphur. 
Recombination of Sulphur Compounds. The recombination 
of the H 2 S to form sulphuric acid and then sulphates is brought about 
in some cases apparently by purely chemical forces, since the gas 
will easily combine with oxygen if the conditions aie right. But 
a large part of it enters new combinations through the agency 
of microorganisms. There is a group of bac- 
teria that consume H 2 S, oxidizing the gas within 
their bodies and utilizing the energy thus liber- 
ated for their own life energy. They are as 
dependent upon the presence of H 2 S as ordinary 
plants are dependent upon CO 2 . In the pres- 
ence of this gas they flourish, and as they oxidize 
the gas the sulphur is set free from its combina- 
FIG. 28. Sulphur 
bacteria. A, Beg- tion with hydrogen and separated as pure sulphur. 
Bo P show The sulphur appears then within the bodies of 
sulphur masses in the the bacteria as minute reddish dots (Fig. 28). 
rods (Winogradsky). . 
The bacteria that can perform this function seem 
to be of two types, one type belonging to the higher fungi (see page 
12) and the other being true bacteria. The latter are sometimes 
called the "red bacteria" because of the color produced by the sul- 
phur grains within them. These bacteria may continue thus to liber- 
ate the sulphur and in waters where H 2 S is abundant large quantities 
of pure sulphur may be deposited. These are the so-called sulphur 
springs around which deposits of sulphur may be found. As long as 
the gas is abundant the bacteria flourish; but if the gas disappears 
they appear to use up the sulphur in their own bodies, after which 
they die. In some way the sulphur in their bodies is in 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 
