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BIOLOGY AND TECHNIQUE 



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 thermo-regulator is shown in Fig. 24. This 

 consists of a long tube open at the top and fitted about 1^ inches from 

 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 mercury 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. 



FIG. 24. MOITESSIER GAS PRESSURE REGULATOR. 



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 T an inch above its end. The flame decreases and the 

 temperature again sinks. 



Since gas pressure in laboratories is apt to vary, it is convenient 

 to interpose between the gas supply and thermo-regulator some one 

 of the various forms of gas-pressure regulators. The use of these 

 is not absolutely necessary but aids considerably in the maintenance 

 of a constant temperature. The one most commonly employed is the 

 so-called Moitessier apparatus. This consists of a cylindrical metal 

 chamber within which there is fitted an inverted metal bell. Glycerin 



