Reduction of Nitrates 55 



not grow on the ordinary culture media, but require special 

 solutions, free from the diffusible albumins, free indeed 

 from organic compounds of any sort ! Their supplies of 

 carbon are obtained by the dissociation of carbon dioxid. 

 It is highly noteworthy that they are thus able to nourish 

 without food more complex than ammonia, a fact which 

 is without parallel among organisms devoid of chlorophyl. 



1 1 . Reduction of Nitrates. A considerable number of 

 bacteria are able to reduce nitrogen compounds in the soil 

 or in culture media, prepared for them, into ammonia. 

 To the horticulturist this matter is of much interest. 

 Winogradsky * has described specific nitrifying bacilli which 

 he found in soil, and asserts that the presence of ordinary 

 bacteria in the soil causes no formation of nitrites so long 

 as the special bacilli are withheld. 



Reduction of nitrates can be determined experimentally by 

 the use of a nitrate broth made by dissolving in 1000 c.c. of 

 water, i gram of peptone and 0.2 gram of potassium nitrate. 

 The ingredients are dissolved, filtered, then filled into tubes, 

 and sterilized. The tubes are inoculated and the results 

 noted. As nitrites and ammonia are, however, commonly 

 present in the air and are taken up by fluids, it is always 

 well to control the test by an uninoculated tube tested 

 with the reagents in the same manner as the culture. 



Two solutions are employed f for testing the culture : 



I. Naphthy.arnin, I gram, 



Distilled water, 20.0 grams, j acetate. 

 II. Sulphanilic acid, 0.5 gram. 



Hydric acetate, diluted, 150.0 c.c. 



Keep the solutions in glass-stoppered bottles and mix equal 

 parts for use at the time of employment. 



About 3 c.c. of the culture and an equal quantity of the 

 uninoculated culture fluid are placed in test-tubes and about 

 2 c.c. of the test fluid slowly added to each. The develop- 

 ment of a red color indicates the presence of nitrites, the 

 intensity of the color being in proportion to the quantity 

 of nitrites present. If a very slight pinkish or reddish 

 color in the uninoculated culture fluid and a deeper red in 

 the culture develop, it shows that a small amount of 



* "Ann. de 1'Inst. Pasteur," 1891 ; "La Sernain^ medicale," 1892. 

 t "Journal of the American Public Health Association," 1888, p. 92. 



