194 BACTERIOLOGY. 



moved. After allowing the layer of foam to subside 

 somewhat the space occupied by gas is again measured, 

 and tlie difference between this amount and that meas- 

 ured before shaking with the sodium hydroxide solution 

 gives the proportion of CO2 absorbed. The explosive 

 character of the residue is determined as follows: the 

 cotton plug is replaced and the gas from the closed 

 branch is allowed to flow into the bulb and mix with 

 the air there present. The plug is then removed and 

 a lighted match inserted into the mouth of the bulb. 

 The intensity of the explosion varies with the amount 

 of air present in the bulb." 



Cultivation without Oxygen. — As we have 

 already learned, there is a group of organisms to which 

 the name ^'anaerobic organisms" has been given, which 

 are characterized by their inability to grow in the pres- 

 ence of oxygen. For the cultivation of the members 

 of this group a number of devices are employed for the 

 exclusion of oxygen from the cultures. 



Koch's method. Koch covered the surface of a gel- 

 atin plate, which had been previously inoculated, with 

 a thin sheet of sterilized isinglass. The organisms 

 which grew beneath it were supposed to grow without 

 oxygen. 



Hesse's method. Hesse poured sterilized oil upon the 

 surface of a culture made by stabbing into a tube of 

 gelatin. The growth that occurred along the track of 

 the needle was supposed to be anaerobic in nature. 



Methods of Libonus. Liborius has suggested two 

 useful methods for this purpose. The one is to fill a 

 test-tube about three-quarters full of gelatin or agar- 

 agar, which, after having been sterilized, is to be kept 

 in a vessel of boiling water for ten minutes to expel all 



