62 AGRICULTURAL BACTERIOLOGY. 



into sulphuric acid, the acid being liberated from the bacteria 

 to enter into combinations with the soil bases as already de- 

 scribed. Sometimes the action of these sulphur bacteria re- 

 sults in the deposition of large amounts of pure sulphur which 

 adheres to all sorts of solid objects in sulphur waters, forming 

 " sulphur grasses," etc. The sulphur deposits from sulphur 

 springs are thus, in considerable degree at least, due to bacterial 

 action which oxidizes the H 2 S, thus liberating the sulphur. 



From these facts it will be seen that sulphur is in a condi- 

 tion of circulation in nature and that bacteria are very impor- 

 tant factors in that circulation. Pure sulphur itself is utilized 

 by certain bacteria converted first into H 2 S, and eventually into 

 sulphuric acid which unites with soil bases to form sulphates. 

 The sulphates are seized by the common plants and built into 

 the proteid molecules. At the end of the life of the plant the 

 proteid suffers decomposition under the action of bacteria and 

 the sulphur is liberated in the form of H 2 S. The sulphur bac- 

 teria in the soil and water seize the H 2 S, using it as a source 

 of energy, oxidizing the gas and setting the sulphur free. The 

 same bacteria, as well as others, oxidize the free sulphur into 

 sulphuric acid which enters the soil again as sulphates. Mean- 

 time some of the soil sulphates may have been reduced to H 2 S 

 by different species of bacteria, the H 2 S thus liberated furnish- 

 ing more food for the sulphur bacteria. Various other low 

 sulphur compounds (sulphites, etc.) are also utilized by the 

 sulphur bacteria, and the total result of the whole action is that 

 the sulphur is in the end in considerable degree reduced to soil 

 sulphates where it can be utilized by the plants as a source of 

 the necessary sulphur for building proteids. The cycle is 

 complete. 



Iron Bacteria. Iron compounds are also metamorphosed 

 by bacterial action. One class of bacteria has such a close 

 dependence upon iron that it is called the class of iron bacteria. 

 The organisms belong to the higher bacteria and there are 



