50 



OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



2. Suppose the open end of the manometer, e. g., to the middle of 

 bulb No. 1, is filled with air and it is desired to replace it with mercury. 

 A quantity of mercury is poured into the cup, the manometer is inserted 

 and, with the instrument tipped so as to expose the open end, the air 

 is exhausted until the mercury begins to run out. On bringing the 

 manometer again to the upright position, so as to immerse the open 

 end, and readmitting air, the mercury flows into the tube to replace 

 the air which has been withdrawn. 



Fig. 28. 

 Apparatus used in emptying, filling, 

 and cleansing the uncalibrated 

 portion of the manometers. 



(1) Reservoir for solutions or 

 mercury; (2) place for at- 

 taching tubes containing 

 absorbents. 



w 



3. Suppose, again, the manometer has been used in a measurement 

 of pressure, and the open end — perhaps also a small portion of bulb 

 No. 1 — is filled with the solution. Before the instrument can be used 

 for another experiment, this must be removed and replaced, either by 

 mercury or by some of the solution whose pressure is to be determined. 

 The necessary manipulation is as follows: (1) the old solution is 

 removed and replaced by air; (2) the air is replaced by the new solu- 

 tion, and this, in turn, is replaced in succession by other portions of the 

 same solution, until there is no danger that the concentration of the 

 new solution will be affected by the older one. If mercury is to be 

 substituted for a solution, the tube must be washed and dried before 

 introducing it. In this case, portions of the wash liquids— water, 

 alcohol, and redistilled ether — are introduced and removed in exactly 

 the same manner as when one solution is to be substituted by another. 

 The final drying of the manometer is accomplished by attaching a 

 tube containing drying agents or absorbents to the stem (2) and alter- 

 nately exhausting and readmitting air. When manometers of the 

 forms seen in Figures 20 and 21 are to be dealt with, the instrument 

 (Figure 28) is attached to the vent. The manipulation is then more 

 complex but not less effective. 



The time consumed in preparing a manometer for the measurement 

 of osmotic pressure is usually about one month. 



