1 24 BACTERIOLOGY 



may run up too high. This sticking of the armature to the mag- 

 net is said to be due to the residual magnetism left in the core of 

 the magnet. When the current passing through the magnet is 

 broken by the excursion of the end of the thermoregulator away 

 from the set- screw at b, the armature is pulled away from the 

 magnet by a coiled spring. Another important precaution is 

 that the points at which contact is made and broken, b and c, 

 should be tipped with platinum. A small piece of platinum 

 wire inserted into the ends of the set-screws and a few square 

 centimeters of platinum foil riveted to the opposite point of con- 

 tact, meet the requirements. If platinum is not used at these 

 points oxidation takes place and prevents contact. The set- 

 screw at b is set by experiment for the temperature desired. 

 The further the point of the set-screw projects toward the free 

 arm of the regulator, the higher the temperature maintained. 



CULTIVATION OF ANAEROBIC BACTERIA. 



Deep Stab Culture. Bacteria which cannot grow in the pres- 

 ence of atmospheric oxygen may be successfully cultivated by 

 methods in which the oxygen is excluded or its concentration 

 diminished. The simplest procedure, first practised by Liborius, 

 is to make deep stab cultures into freshly solidified alkaline 

 glucose agar. The agar quickly closes over the needle track and 

 any traces of oxygen introduced into the depths of the agar are 

 absorbed and reduced by the glucose in the presence of the 

 alkali. The bacteria thus find at various points along the punc- 

 ture all variations in partial pressure of oxygen from almost 

 complete absence up to the concentration existing in the atmos- 

 phere at the surface of the medium. Obligate anaerobes begin 

 to grow near the bottom and, as the gases produced replace the 

 air above, the growth extends upward, often even entirely to the 

 surface. 



Veillon Tube Cultures. Isolated colonies of anaerobic bac- 

 teria may be obtained by a modification of this tube method of 



