DETERMINING BIOLOGICAL ACTIVITIES OF BACTERIA 197 



bon dioxide produced by bacteria, cultures are grown in fermen- 

 tation tubes containing sugar-free broth, (see page 150) to which one 

 per cent of pure dextrose, lactose, saccharose, or other sugars has 

 been added. The tubes are incubated until the column of gas 

 formed in the closed arm no longer increases (twenty-four to forty- 

 eight hours). The level of the fluid in the closed arm is then 

 accurately marked and the column of gas measured. 



The bulb of the fermentation tube is then completely filled with 

 ^ NaOH solution, the mouth closed with a clean rubber stopper, 

 and, the bulb inverted several times in order to mix the gas with 



rr 



FIG. 27. TYPES OF FERMENTATION TUBES. 



the soda solution. The tube is then again placed in the upright 

 position, allowing the gas remaining to collect in the closed arm. 

 The gas lost may be roughly estimated as consisting of C0 2 . 



Hydrogen. The gas remaining, after removal of the C0 2 in the 

 preceding -experiment, at least when working with carbohydrate 

 solutions, may be estimated as hydrogen. When allowed to collect 

 near the mouth, further evidence of its being hydrogen may be 

 gained by exploding it with a lighted match. 



Hydrogen Sulphid (H 2 S, Sulphuretted hydrogen). In alkaline 

 media, sulphuretted hydrogen, if formed, will not collect as. gas, 

 but will form a sulphid with any alkali in the solution. For the 

 estimation of the formation of hydrogne sulphid, bacteria are cul- 



