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 



