L24 BACTERIA AND SOIL MINERALS 
muds are saturated with hydrogen bisulphid gas. This reduction 
of sulphates is, in a way, comparable to denitrification since it is 
the result of deoxidation and since it also destroys substances that 
are already plant foods. Some species of bacteria appear able to 
attack pure sulphur, causing it to combine with hydrogen as H.S. 
From these several sources much sulphur is liberated into the air 
in the form of HS, the liberation in all cases being due to the action 
of bacteria, different classes acting on different compounds con- 
taining sulphur. 
Recombination of Sulphur Compounds.—The recombination 
of the H.S to form sulphuric acid and then sulphates is brought 
about in some cases apparently by purely chem- 
ical forces, since the gas will easily combine 
with oxygen if the conditions are right. But 
a large part of it enters new combinations 
through the agency of microérganisms. There 
is a group of bacteria that consume HS, 
oxidizing the gas within their bodies and utiliz- 
fie. 27-—Sulphur ing the energy thus liberated for their own life 
bacteria. A, Beg energy. They are as dependent upon the 
giatoa Ben presence of H.S as ordinary plants are de- 
sulphur massesinthe pendent upon COz. In the presence of this gas 
rods (Winogradsky)- they flourish, and as they oxidize the gas the 
sulphur is set free from its combination with hydrogen and 
separated as pure sulphur. The sulphur appears then within 
the bodies of the bacteria as minute reddish dots (Fig. 27). The 
bacteria that can perform this function seem to be of two types, 
one type belonging to the higher fungi (see page 13) and the 
other being true bacteria. The latter are sometimes called the 
“red bacteria’? because of the color produced by the sulphur 
grains within them. These bacteria may continue thus to liberate 
the sulphur and in waters where H,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 
