APPARATUS 



415 



Saturation of Medium with Water. It is necessary to saturate 

 the medium with watpr at a suitable vapor pressure in order to 

 minimize the tendency of the aqueous droji to lose water to the 

 medium and thus change its specific gravity. It has been found 

 sufficient to employ a 0.2 .1/ potassium bromide solution for the 

 i-ange of specific gravity from 0.99-1.01. About 1 ml. of the salt 

 solution is shaken thoroughly with about 10 ml. of the less dense 

 kerosene-bromobenzene mixture. The resulting suspension is poured 

 at once into the gradient tube, and as the drops fall they saturate 

 the medium. The high specific gravity of the drops carries them down 

 far enough in the tube to avoid any interference with subsequent 

 measurements. 



Gradient Calibration. Calibration of the gradient is accom- 

 plished by the simple expedient of placing in the tube small drops 

 (0.10-0.15 fA.) of potassium chloride solutions of known specific 

 gravities (Table VIII). It is well to store these standard solutions 

 under a 1 cm. layer of kerosene in stoppered vessels, such as 50 ml. 

 volumetric flasks, which restrict the surface exposed to the kerosene. 

 In some cases, such as in the determination of "reduced weight", 

 ( page 420) , standard drops composed of mixtures of doubly distilled 

 water and deuterium oxide have been used. The^O.10-0.15 fA. pipette 

 used to add the drops may be either the type shown in Figure 51 



TABLE VIII 

 Composition and Density" of Standards for Dilatometry 



" The densities are taken from the Landolt-Bornstein tables. 



(page 173) or the constriction pipette (Fig. 53, page 173). These 

 pipettes may be calibrated by measuring out strong acid of known 

 concentration and titrating it with a microburette, but for the 

 present purpose it is sufficient to calibrate them more roughly by 

 weighing the quantity of water they deliver directly on the pan of a 

 microbalance. 



