MICROLITER DIVER TECHNIQUE 347 



An investigation of various solutions which might be used as media 

 led Holter (1943) to the choice of a sodium nitrate-sodium chloride 

 solution. As Holter pointed out, a proper medium should have low 

 viscosity, chemical stability, transparency, biological innocuity, 

 glass-wetting properties, low and reproducible surface tension, and 

 very little gas solubility. Holter's medium has proved satisfactory 

 over a long period of use. 



Preparation of Holter Medium. Dissolve 27.2 g. sodium 

 nitrate, 13.7 g. sodium chloride, and 0.2 g sodium taurocholate in 

 59.1 ml. distilled water containing 3 drops of 0.1 N hydrochloric 

 acid. To a sample of the soln. add bromocresol purple and then 0.01 

 N hydrochloric acid to bring the color to that of the indicator at pH 

 5.8 to 6.0 in salt-free soln. From the quantity of acid required, cal- 

 culate the amount of 0.1 A" hydrochloric acid to add to the main 

 portion of the soln. to bring it to the same pH. Add the calculated 

 amount and filter the medium through lintless paper, refiltering the 

 first part of the filtrate. Any cloudiness appearing upon the addition 

 of the acid is due to precipitation of taurocholic acid and should 

 soon disappear. Check the density of the medium at the proper 

 temperature with an accuracy of 0.1%; a Mohr-Westphal balance 

 may be used. 



When the gas in the diver is air and no glass stoppers are used in 

 the-neck seals to inhibit diffusion (see page 368) the medium should 

 be saturated with air as described on page 374. When other gases 

 are used in the diver, the flotation medium need not be saturated 

 with these gases but the diffusion losses may be impeded by mouth 

 seals and glass stoppers. 



Dust is apt to contaminate the flotation medium after some time ; 

 hence, it is occasionally necessary to combine the contents of the 

 vessels, refilter the solution, check the density, and refill the vessels 

 with a pipette. The pipette is used to avoid getting the soln. on the 

 neck of the vessel where it might crystallize. 



(4) THE CONNECTING MANIFOLD 



The flotation vessels are connected to the manometer by means 

 of a thick-walled glass manifold tube (G, Fig. Ill), carrying about 



