METHODS USED IN CULTIVATION OF BACTERIA 159 



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. 42. This con- 

 sists of a long tube of glass fitted with a metal cap through which an in- 

 let tube (A) projects into the interior. Slightly below the middle of the 

 tube there is a glass diaphragm separating its interior into two com- 

 partments. 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 irercury rises in the 

 upper chamber, gradually restric ting 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 

 temperature 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 shutting off of the gas is prevented by a 

 small circular opening placed in the inlet tube just above the slit. 



Another cheaper and simpler them o-regulator is shown in 

 Fig. 43. This consists of a long tube -open at the top and fitted 

 about 1^ inches fro'm the top with two hollow glass elbows. One of these 

 elbows remains open, the other, situated on a slightly lower level, is closed 

 by a brass screw-cap. The tube is filled with irercury to a point slightly 

 above the level of the elbow containing the screw-cap. The height of 

 the mercury can thus be increased or decreased by screwing in or out 

 upon the cap. Into the upper end of the tube there is fitted another 

 device which consists of a T-shaped system of glass tubes, one arm 

 of the T being open and the other closed, the perpendicular leg of the 

 T tapering to a minute opening at the bottom. The gas passes into 

 one arm of the T down through the tapering leg and into the space 

 immediately above the mercury. It then passes out through the open 

 elbow of the main tube. As the mercury rises, it gradually diminishes 

 the space between its surface and the small opening in the tapering tube 

 above it, finally completely shutting off the gas from this source. Gas can 

 now pass only through a minute hole perforating the vertical leg of the 

 12 



