DETERMINING BIOLOGICAL ACTIVITIES OF BACTERIA 165 



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 

 f NaOH solution, the mouth closed with a clean rubber stopper, and 

 the bulb inverted several times in order to mix the gas with the soda 

 solution. The tube is then again placed in the upright position, allow- 

 ing 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 CO 2 in the pre- 

 ceding experiment, at least when working with carbohydrate solutions, 



FIG. 47. TYPES OF FERMENTATION TUBES. 



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 Sulphide (H 2 S, Sulphuretted hydrogen). In alkaline 

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

 will form a sulphide with any alkali in the solution. For the estimation 

 of the formation of hydrogen sulphide, bacteria are cultivated in a strong 

 pepton solution to which 0.1 c.c. of a one per cent solution of ferric 

 tartrate or lead acetate has been added. The addition of these substances 

 gives rise to a yellowish precipitate in the bottom of the tubes. If, on 



