ROLL CULTURE 115 



wet with Nessler's reagent above a bouillon culture of a given or- 

 ganism. If a yellow to brown color is produced ammonia is present. 



Nitrites are detected by growing the organism in a solution of a 

 nitrate (see other works for description). 



Incubate for a week and then add one cubic centimeter each of 

 the following solutions: 



a. Sulphuric acid .5 grams. 



Acetic acid 1 50 c.c. 



b. Amido naphthaline i gram. 



Water 20 c.c. 



Boil, filter, and add 180 c.c. of dilute acetic acid. 



If nitrites are present a pink color is produced by these reagents. 

 Enzymes may be detected by noting whether gelatine is liquefied, 

 or milk curd digested. Both these actions are evidences of the 

 presence of enzymes. 



Bacteria growing exclusively in the absence of oxygen are known 

 as anaerobes; to cultivate these successively various forms of appa- 

 ratus are necessary. 



The following methods are pursued in ordinary laboratory man- 

 ipulations. 



1. Exclusion of oxygen. 



2. Exhaustion of oxygen by means of an air pump. 



3. Absorption of oxygen by means of chemicals that absorb oxy- 

 gen from the air. A mixture of pyrogallic acid and sodium hydrate 

 absorbs oxygen rapidly, leaving nitrogen only in the chamber. 



4. Displacement of air by means of an air-pump and allowing 

 hydrogen to enter the vacuum. 



Under the first method we may either exclude oxygen by laying 

 sheets of sterile mica or a cover-glass on the surface of the agar 

 or gelatine plates (Fig. 36), thus excluding air, or deep punctures 

 may be made in tubes half filled with gelatine or agar, for growths 

 often occur in the depths of the medium, especially if the latter has 

 been boiled previously to expel the oxygen; or, instead of mica, 

 sterile paraffine may be poured over the top of the tube. The 



