364 GASOMETRIC-MANOMETRIC METHODS 



5. Remove the loop from the neck. 



For divers with smaller necks, the loop apparatus is clamped 

 horizontally, the diver is held horizontally in a diver clamp, and 

 the wax ring is placed by rotating the diver smoothly about its 

 long axis. A dissecting microscope assists in the observation and 

 control of the operation. 



Neck Seals. Neck seals are placed by bringing the end of the 

 pipette (type 1 or 2) into the neck so that the pipette stem is care- 

 fully centered. Then, by blowing cautiously a free-hanging drop 

 is formed which slowly grows {A, Fig. 127) until it touches the 

 neck wall (B) and finally forms the seal (C). If the centering is 

 inexact the liquid may spread between the pipette and the neck wall. 



With wide necks it may be necessary to add more liquid to form 

 the seal than is desired in the final seal. Should this be the case, the 

 tip of the pipette is raised from the bottom meniscus to the interior 

 of the seal, and the excess liquid is drawn up into the pipette. Hence, 

 pipettes used for neck seals must either have two marks, the vol- 

 ume between them being that desired in the final seal, or they must 

 have calibrated cylindrical bores such as are obtained when the 

 pipettes are made from broken thermometers. 



When coarse-tipped pipettes are employed or the divers have very 

 narrow necks, care must be exercised in removing the pipette from 

 the seal to prevent pulling up the liquid. The pipette should be 

 drawn out of the seal with a jerk. Similarly, when the ball-tipped 

 pipette is used, the ball should be removed from the drop deposited 

 by carefully moving the ball until it is just underneath the surface 

 of the drop, and then pulling it out with a jerk. 



When it is necessary to exchange neck seals without disturbing 

 the bottom drop, the seals are first carefully removed with small 

 rolls of filter paper, soft paper rolls being used to absorb most of 

 the liquid; hard lintless paper rolls which almost fill the neck are 

 used to effect the final cleaning and drying. The neck is then filled 

 with water by bringing the tip of the pipette to a point about 1 mm. 

 below the bottom neck seal and blowing out the water very care- 

 fully. Once the neck is filled, the cautious blowing is continued to 

 rinse out the neck with a slow stream of water. Finally the water is 

 removed from the neck and filter paper is used for drying. There 

 is danger of evaporation of the bottom drop as long as there is no 

 oil seal present, therefore the exchange of seals must take place 



