174 BAGTERIOLOOY. 



is shaken thoroughly with the gas and allowed to flow 

 back and forth from bulb to closed branch and the re- 

 verse several times to insure intimate contact of the 

 COj with the alkali. Lastly, before removing the thumb 

 all the gas is allowed to collect in the closed branch, so 

 that none may escape when the thumb is removed. If 

 COg be present a partial vacuum in the closed branch 

 causes the fluid to rise suddenly when the thumb is 

 removed. After allowing the layer of foam to subside 

 somewhat the space occupied by gas is again measured, 

 and the difference between this amount and that meas- 

 ured before shaking with the sodium hydroxide solution 

 gives the proportion of COj 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.— rAs 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 gelatin 

 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 



