VITAL PHENOMENA OF BACTERIA. 75 
Sulphuretted hydrogen may be formed: 
1. From albuminous substances. This power, ac- 
cording to Petri and Maassen, of forming sulphuretted 
hydrogen, particularly in liquid culture media contain- 
ing much peptone (5 to 10 per cent.) and no sugar, is 
possessed, though in different degree, by all bacteria 
examined by them; only a few bacteria form H,S in 
bouillon in the absence of peptone, while about 50 per 
cent. in media containing 1 per cent. peptone. 
2. From powdered sulphur. All bacteria produce in 
culture media to which pure powdered sulphur is added 
considerably more H,S than without this addition. 
Petri and Maassen suggest that this is due to the 
nascent hydrogen produced by the bacteria. 
3. From thiosulphates and sulphites. Studied par- 
ticularly in yeast, but demonstrated also by Petri and 
Maassen in several bacteria. 
The presence of sugar in the culture does not affect 
the production of H,S by bacteria, but saltpetre reduces 
it, nitrites being formed. The abacise of oxygen favors 
the production of H,S. Light diminishes the develop- 
ment of H,S by facultative anaérobes, sulphates being 
formed instead. 
Reduction Processes. All bacteria, as we have seen, 
possess the property of converting sulphur into sul- 
phuretted hydrogen, for which purpose is required the ~ 
presence of nascent hydrogen. The following pro- 
cesses depend also in part upon the action of nascent 
hydrogen : 
1. The reduction of blue litmus pigments, methylene- 
blue, and indigo to colorless substances. The superficial 
layer of cultures in contact with the air shows often no 
reduction, only the deeper layers being affected. By 
