128 BACTERIOLOGY 



that the points at which contact is made and broken, J 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. 



Electrically heated and electrically regulated incubators for 

 any desired temperature are now to be found on the market. 

 Their initial cost is rather high and, as a rule, they are "adapted 

 for use with only one kind of electric current. The exact current 

 available should be stated in ordering. 



In many places electric current is not constantly available and 

 in the field one often has to work without gas. Highly satisfactory 

 oil heated, water- jacketed incubators came into very general use 

 in field laboratories in England and 'France during the war. 



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. 



