350 GASOMETRIC-MANOMETRIC METHODS 



bottle coini)letely submerged and held to the bottom of the thermo- 

 stat by lead weights (Fig. 115). The vohmie of the air bottle should 

 be as large as possible (2-4 1.). 



(6) THE MICROLITER DIVER 



Dimensions of the Diver. The size and shape of different types 

 of diver are shown in Figure 116. The "standard" divers are most 

 generally employed; they have the dimensions: 



Inside neck diameter 0.8 to 0.9 mm. 



Neck wall thickness 0.1 mm. 



Neck length 8-10 ram 



Bulb diameter 2-3 mm. 



Tail length 5-8 mm. 



Total volume 10-12 Mi- 

 Neck volume about 5 /ul . 



Total weight 20-30 mg. 



These dimensions have been found to be preferable to those previ- 

 ously used, chiefly because the longer and narrower neck enables 

 reduction in the diffusion of gas through the seals. Regardless of how 

 large the divers are made, the inside neck diameter should not exceed 

 1-1.5 mm., although seals can be placed conveniently in necks up to 

 2 mm. On the other hand, the diameter should not be less than 0.6 

 mm., because with smaller diameters the difficulty of placing neck 

 seals is increased, due to the greater surface tension. It may not be 

 possible to go down to 0.6 mm. in some cases because the pipettes 

 which must be used may be too large to enter the neck. Usually the 

 smallest neck widths have to be 0.65-0.75 mm. 



In order to determine the minimum neck length, allow 1.5 mm. 

 between the seals, which, with their menisci, require about 1.5 mm. 

 each. The lowest seal must be about 1 mm. from the lower end of 

 the neck. 



Cylindrical divers {C, Fig. 116) are capable of the greatest re- 

 duction in size (down to a total vol. of about 3/a1.) and the conical 

 divers (D) furnish the greatest accuracy by a combination of a 

 relatively small gas volume with a reaction mixture having a rela- 

 tively large surface. The standard (^4) and long-necked {B) divers 

 are also shown in the figure. 



Making the Diver. Divers may be made according to the 

 method of Boell, Needham, and Rogers (1939) or as described by 

 Holter (1943). The technique of the former may be applied to 



