160 



BIOLOGY AND TECHNIQUE 



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 ab- 

 solutely necessary but aids considerably in the maintenance of a con- 

 stant 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 is poured 

 into the cylinder to the depth of about two inches. An inlet pipe con- 



FIG. 44. MOITESSIER GAS-PRESSURE REGULATOR. 



ducts gas into the open space between the top of the glycerin and the bell. 

 From the top of the bell is suspended a conical piece of metal which hangs 

 free in the outlet pipe. As the gas pressure under the bell increases, 

 this is raised and the opening of the outlet pipe is gradually diminished 

 by the cone. Thus the relation between the pressure in the inlet pipe 

 and the actual quantity of gas passing through is equalized. A cup con- 

 nected to the top of the bell through the roof of the cylinder by a bar can 

 be rilled with birdshot and the pressure against the gas can thus be 

 modified to conform with existing conditions. 



