40 



OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



is 11.05, which number might have been accepted as the mean capillary 

 depression of the manometer. But suppose when the volume of the 

 nitrogen in the manometer is determined, the meniscus stands 8.65 

 millimeters above the scratch, where the depression is in reality only 

 7.92 millimeters. The error, if the mean number 11.05 is used in 

 correcting for capillary depression, would be about 11.05 — 7.92 = 3.13 

 calibration units. The whole of the nitrogen in this manometer 

 amounts to only 400 calibration units. The error made in determining 

 the volume would therefore be 0.78 per cent. This example of what 

 might happen if the condition of the tube at 8.65 millimeters above 

 the scratch had escaped detection will serve to convince one of the 

 necessity of a detailed investigation of the capillary depression in tubes 

 of small bore; also of the advisability of using manometers of large 

 capacity, like those seen in Figures 19-21, in order to minimize errors 

 of capillary depression as well as those of meniscus. 



Fig. 22.— "Steel block" for the determination of gas volumes in manometers, for: the comparison 

 of instruments, and for the determination of capillary depression. 



(1) Mercury reservoir; (2) plunger for coarse adjustment of pressure; (3) plunger for fine adjust- 

 ments; (4), (5), and (G) manometers; (7), (8), (9), (10), and (11) packing; (12), (13), (14), 

 (15), and (16) nuts for compression of packing. 



Capillary depression appears twice as an important factor in the 

 measurement of osmotic pressure: (1) in determining the volume of 

 the nitrogen under standard conditions of temperature and pressure; 

 and (2) in correcting its volume under an unknown pressure, which 

 (i. e., the osmotic pressure) is a quotient of the two volumes. 



An instrument much used in the determination of capillary depres- 

 sion, and also in the comparison of manometers, is the " steel block" 

 seen in Figure 22. It contains a reservoir for mercury (1) and two 

 plungers, one of which (2) is large, and the other (3) small. The larger 

 one is employed for the coarser, and the smaller one for the finer, 

 adjustments of pressure in tubes 4, 5, and 6. The packing (7, 8, 9, 10, 

 and 11), which may be of leather or rubber, or partly of both, is com- 

 pressed in each case between the concave surfaces of two steel disks 

 and the required pressure is brought upon these by means of the 



