REDUCING POWER OF BACTERIA. 231 



application of comparatively simple tests for the pres- 

 ence of the end-products. 



Test with Pigments. The reduction of various pig- 

 ments and aniline dyes is usually manifested by altera- 

 tions in the depth of color or in the complete decoloriza- 

 tion of the pigment. In some instances the reduction 

 is first manifested in the depth of the medium, and in 

 such instances the natural color of the pigment may 

 frequently be restored on shaking the medium. This 

 is manifestly a deoxidation of the pigment arising from 

 the avidity of the bacteria for oxygen when growing in 

 the depth of the medium. In other instances the reduc- 

 tion is more complete, and simple agitation of the 

 medium fails to restore the original color. 



Test for Hydrogen Sulphide. The reduction of sul- 

 phur compounds may be determined by growing the 

 bacteria in peptone solution containing ferric tartrate, 

 when the presence of hydrogen sulphide will be indi- 

 cated by the brownish-black or jet-black color of the 

 precipitated iron-sulphide. 



The complete reduction of nitrates is brought about 

 by many bacteria. Other bacteria are capable of carry- 

 ing the reducing action as far as the formation of ammo- 

 nia, while still others merely reduce the nitrates to 

 nitrites. These reducing functions are encouraged and 

 may be demonstrated by cultivating the bacteria in pep- 

 tone solution containing potassium nitrate. 



Test for Nitrites. The method of Griess,as modified by 

 1 1< >svay, is quite satisfactory. These reagents are required : 



( a) Naphthylamine, 0.1 gramme. 

 Distilled water, 20.0 c.c. 

 Acetic acid (25 per cent, solution), 150.0 " 



(b) Sulfanilic acid, 0.5 gramme. 

 Acetic acid (25 per cent, solution), 150.0 c.c. 



