METHODS USED IN CULTIVATION OF BACTERIA 



191 



ductor, tends to prevent rapid changes of temperature within the 

 chamber as the result of changes in the external environment. Both 

 walls are perforated above by openings to admit thermometers into 

 the interior and one wall is perforated so that a thermo-regulator 

 may be inserted into the water jacket. The under surface of the 

 chamber is heated by a gas flame, the size of which is automatically 

 regulated by the thermo-regulator. 



A number of thermo-regulators are on the market, all of them con- 

 structed upon modifications of the same principle. One of the most efficient 

 of those in common use is that shown in Fig. 22. This consists of a long 

 tube of glass fitted with a metal cap through which an inlet tube (A) projects 



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i. 



FIGS. 22, 23. THERMO REGULATOR. 



into the interior. Slightly below the middle of the tube there is a glass 

 diaphragm separating its interior into two compartments. In the middle 

 of the diaphragm an aperture leads into a spiral of glass which projects 

 into the lower compartment. The lower compartment is filled with ether 

 and mercury. The lower end of the inlet tube (A) has a wedge-shaped slit. 

 The gas from the supply pipe passing through the tube (A) is conducted 

 through the slit-like opening in its lower end into the inner chamber and 

 passes out to the burner through the elbow (B). When the temperature is 

 raised, the ether and mercury in the lower chamber expand and the mercury 

 rises in the upper chamber, gradually restricting the opening through the 

 V-shaped slit in the inlet tube. Thus the gas supplied to the burner is 

 diminished, the flame reduced, and the temperature again falls. The tem- 

 perature can be arbitrarily adjusted by raising or lowering the inlet tube. 

 A scale at the upper end of the inlet tube allows exact adjustment. Complete 



