UNIVERSITY 



FIG. 9. 



THE ORIGIN OF SOIL. 



page, 28), the two best known genera being Beggiatou (Fig. 



9) and Thiothrix. In addition to these are others frequently 



called " red bacteria" which belong to the more common types. 



Both of these classes of bacteria make use 



of H 2 S. This gas they oxidize, and the 



energy thus liberated serves the bacteria for 



their life energy. These organisms use H 2 S 



in much the same way as other bacteria, and 



other plants in general, use carbohydrates, 



as a source of energy liberated from the com- 



pound by oxidation. The red sulphur bac- 



teria can endure more H 2 S than the others 



and these alone are found where the amount 



of this gas is excessive. In certain waters 



there have accumulated great quantities of 



iron sulphite which covers the bottom in a thick, black layer. 



This is especially the case in some localities on the Black Sea. 



This sulphite is constantly liberating H 2 S and in the waters 



thus saturated the red sulphur bacteria grow luxuriantly. The 



other type of sulphur bacteria, on the other hand, are more 



common in the ordinary sulphur springs where H 2 S is not so 



abundant. 



This H 2 S is oxidized by the bacteria, probably according to 

 the formula : 



Sulphur bac 

 Beggiatoa ; B, Ophido- 

 monas. Both show sul- 

 phur masses in the rods. 

 ( Winogradsky.) 



In accordance with this reaction sulphur is set free and it is 

 seen to be deposited within the body of the bacteria. Small, 

 round grains of sulphur appear in these organisms when they 

 grow in the presence of H 2 S (Fig. 9). The sulphur appears to 

 be, not pure sulphur, but rather an amorphous form composed 

 mostly of CS 2 , but when the bacteria die this is converted into 

 pure crystal sulphur. When the bacteria are growing, how- 

 ever, the sulphur does not remain long within their bodies. 

 After a little it is oxidized by the bacteria, becoming converted 



