I 12 BACTERIOLOGY. 



phur grains in Beggiatoa and their subsequent 

 disappearance. The formation of sulphuretted 

 hydrogen by the breaking up of proteid or by 

 the reduction of sulphates is accomplished by 

 various kinds of bacteria ; the process of oxida- 

 tion may, be considered as taking place in two 

 stages : 



I. 2H 8 S + O 2 = 2H 2 O + S 2 

 sulphuretted hydrogen. 



II. S 3 + 2H 2 + 30 2 == 2H 2 S0 4 

 sulphur, sulphuric acid. 



The process of the reduction of sulphates is car- 

 ried on also by Beyerinck's Spirillum desul- 

 furicans, the sulphuretted hydrogen being able 

 to attack the iron salts with consequent forma- 

 tion of sulphide of iron. In this case, however, 

 ammonium carbonate cannot be- utilized for 

 synthesis, so that physiologically we are here 

 dealing with a higher stage of organization. 

 Engelmann suggests that intermediate stages 

 may perhaps be found among the splendid 

 purple-red Beggiatoa. 



Bacteria take part in yet another way in the 

 formation of the earth's crust. If cellulose be 

 decomposed by bacteria under water and in the 

 absence of air, not only acetic acid, butyric acid 

 and carbonic acid are formed, but also hydro- 

 carbons, methane (marsh-gas or fire-damp) and 

 coal ; bacterial processes of this nature, then, 



