82 LABORATORY GUIDE IN BACTERIOLOGY 



enon accompanying a fermentation. Carbohydrates are 

 fermented by many organisms without gas-formation. 

 The usual product in this case is an acid, mostly lactic 

 acid. Such fermentations produce turbidity only, and 

 no gas. It is therefore necessary, in describing them, 

 to characterize the process as fermentation without 

 gas-formation. '. 



Method oj analyzing the gas produced in the closed arm. 

 The gas consists approximately of carbon dioxid and 

 hydrogen, which may be proved by the following method: 

 Fill the bulb with a 2 per cent, solution of sodium hydrate, 

 and close the mouth with the thumb, taking care not to 

 leave any air between the thumb and the liquid. Now 

 slowly tilt the gas back and forth from the closed arm 

 to the bulb five or six times, and finally allow it to collect 

 again in the closed arm. The sodium hydrate combines 

 with the carbon dioxid, and consequently on releasing the 

 thumb the volume of gas will become smaller in propor- 

 tion to the amount of carbon dioxid absorbed. Now 

 measure the percentage again with the chart, and ascer- 

 tain the proportion of gas left in the closed arm to the 

 original amount. 



Example 



Total percentage of gas before addition of NaOH. ... 45 

 Percentage left after absorption by NaOH 30 



Difference 15 



30 per cent, then represents the amount of hydrogen 

 left, and 15 per cent, the amount of carbon dioxid ab- 

 sorbed. The proportion is expressed by the formula 

 H =3 o 2 



co a 15 r 



