390 GASOMETRIC-MANOMETRIC METHODS 



3. Make the diver go up and down quickly a few times by suck- 

 ing and blowing to free it of air bubbles which might be adhering. 



4. Pipette the diver and enough medium into the flotation vessel 

 so that the column of medium exceeds the length of the diver by 

 about 1 mm. 



Replacement of Mo. Some evaporation of Mo during the filling 

 of the diver necessitates replacement of the medium in M2 in order 

 that its density be the same as that of the surrounding liquid. The 

 replacement is effected by applying alternate suction and pressure 

 three times to the flotation vessel. The air space (L3, Fig. 134), 

 which is next to M2, is relatively large and the expansions and 

 contractions of the air force the medium out and then into the end 

 of the diver. The effect on Mi is naturally very much less. The 

 process of filling the diver and placing it in the flotation vessel 

 takes 20-30 min. 



(5) MEASUREMENT 



For the observation of the diver during an experiment, Zeuthen 

 used the horizontal microscope with the vertical micrometer move- 

 ment illustrated in Figure 151. A particular mark on the diver, such 

 as one of the menisci, is made to coincide with a reference mark in 

 the ocular of the microscope to establish the equilibrium position. 

 For the finest measurements, the diver is kept in a floating position 

 throughout the experiment. This is not very difficult, since the 

 motion of the diver is quite slow when pressure changes on the 

 medium fall within 1-2 mm. (water) of the equilibrium pressure. 

 In experiments in which the greatest accuracy is not required, the 

 diver may be left to sink to the bottom of the medium after a 

 measurement. Should the diver stick to the lower medium-air inter- 

 face, making it difficult to raise it, gentle tapping of the flotation 

 vessel will free it. Before taking another reading, the diver should be 

 made to float for a fixed interval (about three minutes) to allow 

 time for complete equilibration of the pressure. 



The equilibrium pressure is taken as the mean of the pressures 

 required to cause the diver just to begin to rise and to sink. The 

 two pressures are within 1-3 mm. water from one another, and the 

 readings require about one minute. While the accuracy of microliter 

 diver measurements is ±0.5 mm. water, that of the capillary diver 

 is ±1 mm. 



