366 GASOMETRIC-MANOMETRIC METHODS 



The Paraffin Oil Seal. Highly refined colorless paraffin oil 

 having a viscosity of about 20 Engler degrees at 20° and a density 

 of about 0.87 will suffice for most purposes. Of many liquids tested, 

 this has been found to be the best for sealing off the reacting sys- 

 tem from the outside. 



Seals Containing Living Organisms. In order to place a living 

 object in a seal without damage, it is first necessary to place a seal 

 of the aqueous medium in the neck and then introduce the object 

 into the seal with a braking pipette. 



The Mouth Seal. The mouth seal serves to prevent the loss of 

 gas from the diver, and it enables an adjustment of the equilibrium 

 pressure of the diver to a chosen value. In general, the length of 

 the seal must be several mm. to prevent significant gas loss, and 

 the length must be adjusted with an accuracy of 0.1 mm. to obtain 

 an equilibrium pressure within about 5-10 cm. of the desired value. 

 The length of the mouth seal is determined by measuring the dis- 

 tance between its lower meniscus and the diver mouth by means of 

 a traveling microscope (Fig. 128). The mouth seal is made as 

 follows: 



1. After placing the oil seal, bring the tip of an "air pipette" 

 (a braking pipette with a finely drawn delivery tip having an inside 

 diameter of about 100 /a and an outside diameter of about 150 jx) 

 into the diver neck to a point about 1 mm. below the position at 

 which the lower meniscus of the mouth seal is to be placed. 



2. Adjust the measuring microscope until the horizontal cross 

 hair in the eyepiece coincides with the top point on the rim of the 

 diver's mouth. Then lower the microscope by means of the mi- 

 crometer screw until the cross hair comes to the point where the 

 bottom meniscus of the seal is to'be. 



3. Place a seal of diluted flotation medium around the stem of 

 the braking pipette at the mouth of the diver with a fine hand 

 pipette. (The diluted medium is employed to avoid crystallization 

 in the mouth seal. It is prepared by diluting the ordinary medium 

 with an equal volume of water and adding sodium taurocholate to 

 0.1 to 0.2 % to insure that the menisci will be properly formed.) 



4. Apply suction to the braking pipette to draw the seal down 

 until the bottom meniscus reaches a position about 0.5 mm. below 

 the microscope cross hair; then raise the seal by blowing until the 



