82 LABORATORY GUIDE IN BACTERIOLOGY 



air between the thumb and the liquid. Now tilt the 

 gas back and forth slowly from the closed arm to the 

 bulb and back to the closed arm five or six times, and 

 finally allow the gas to collect again in the closed arm. 

 The NaOH combines with the carbon dioxid, and con- 

 sequently, on releasing the thumb, the volume of gas 

 will become smaller in proportion to the amount of 

 carbon dioxid absorbed. The percentage of gas is 

 measured again with the chart, and the relation deter- 

 mined of the gas left in the 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 represents the amount of hydrogen and 

 15 per cent the amount of absorbed carbon dioxid. 

 The proportion is expressed by the formula 



co 2 15 r 



The fact that the gas remaining in the closed arm is 

 probably hydrogen may be proved by tilting it into the 

 bulb, previously filled with water and closed by the 

 thumb. Hold a burning match over the mouth and 

 release the thumb. A slight explosion takes place 

 from the combination of the hydrogen with the oxygen 

 of the atmosphere. 



The gas produced by yeasts usually consists chiefly 

 of carbon dioxid; the gas produced by intestinal 

 bacteria consists chiefly of two-thirds hydrogen and 

 one-third carbon dioxid; and the gas produced by the 

 proteus group consists chiefly of one-third hydrogen 



