348 GASOMETRIC-MANOMETRIC METHODS 



eight side tubes which communicate with the flotation vessels 

 through pieces of rubber pressure tubing about 15 cm. long, and the 

 standard-taper joints (A'), which are slightly greased with vaseline. 

 Each of the side tubes (5 cm. long and 5 cm. apart) has a glass stop- 

 cock to enable one flotation vessel at a time to be connected to the 

 manometer. The three-way tap (H) enables either the flotation 

 vessels or the manometer to be connected to the air. 



(5) THE MANOMETER 



The glass manometer tube has an inside diameter of about 2 mm. ; 

 each arm is around 150 cm. long. The manometer fluid is adjusted 

 by means of the pressure regulator illustrated in Figure 114. This 

 pressure regulator has been found to be more practical than the 

 syringes originally used, chiefly because the latter tend to leak and 

 also give rise to air bubbles in the manometer tube. The pressure 

 regulator is made of a piece of rubber tubing, 30 cm. long, 2 mm. in 

 wall thickness, and 11 mm. in inside diameter, which lies in a metal 

 trough {F) . The pressure screws are held by bridges (K) fastened to 

 the trough. The metal cylinder (D) used for coarse adjustment is 

 15 cm. long and the width of the trough. The pitch of screw B 

 causes a 1 mm. motion of D for each half-turn, corresponding to a 

 pressure change of about 25 cm. in the manometer. The pitch of the 

 fine screw (C) is about half that of B and a half-turn results in a 

 pressure change of about 1 cm. The ends of the tubing (A) are 

 closed with rubber stoppers. The reservoir (G) has a 20 ml. capacity. 

 L and M are three-way taps, FI and J stopcocks, and E a metal shoe. 



Manometer Fluid. Brodie solution is used as the manometer 

 fluid. It consists of 23 g. sodium chloride and 5 g. sodium tauro- 

 cholate dissolved in 500 ml. distilled water. Dyes may be used to 

 color the solution if desired, and it is well to store the solution over 

 a few crystals of thymol. 



The manometer is filled by drawing the solution up through J into 

 G. The levels in the arms are then adjusted so that both menisci are 

 about at eye level between the screws B and C. 



Air Bottle. In order to render the measurement of the equi- 

 librium pressure independent of barometric changes and, to a large 

 degree, of temperature changes in the thermostat, one end of the 

 manometer is connected by thick-walled capillary tubing to an air 



