140 ATLAS OF BACTERIOLOGY. 



ish-red isatin sulphate. Upon the culture glass is 

 placed a tube open on both sides ; this is filled with 

 glass beads which are moistened with a one and a 

 half per cent solution of isatin in concentrated sul- 

 phuric acid. The presence of sugar in the nutrient 

 media diminishes or prevents the formation of mer- 

 captan. 



5. Reduction Processes. 

 (Eeduction of Coloring Matters, Nitrates, etc.) 



We have seen that aerobic bacteria in general 

 possess the power of converting powdered sulphur 

 into sulphuretted hydrogen and that nascent hydrogen 

 is necessary thereto. 



Similar processes, and probably also due in part 

 to nascent hydrogen, are the following : 



1. Reduction of blue litmus coloring matter, methyl 

 blue, and indigo when added to colorless leuco-prod- 

 ucts. The upper layer in contact with the air often 

 shows no reduction, only the deeper layers. On 

 shaking in the air the color is restored, but occasion- 

 ally the litmus coloring matter is restored with a red 

 color on account of the coincident production of acid. 

 The mode of experiment goes without saying ; bouil- 

 lon serves as the nutrient medium. According to 

 Cahen the reduction of litmus is effected by all lique- 

 fying bacteria. It is observed very beautifully, for 

 example, in bacillus fluorescens liquefaciens, but 

 there are also non-liquefying varieties (for example, 

 bacterium coli) which exhibit this characteristic. 



2. Eeduction of nitrates to nitrites and ammonia. 

 The former power seems to belong to very many bac- 



