MEASUREMENT OF OSMOTIC PRESSURE 555 



(/) the movement of the mercury is much influenced by 

 impurities in the mercury or attached to the surface 

 of the glass. 



An improved manometer, specially suitable for measuring 

 large pressures, has a capillary which is enlarged in the lower 

 part of the tube to sixteen times the normal sectional area ; this 

 leaves a much longer column of gas to be measured at the 

 higher pressures but has not been used in the present series of 

 measurements. The bulb (3) is intended to prevent the escape 

 of nitrogen from the calibrated portion of the tube when the 

 pressure is reduced ; the traps (1) and (2) serve to catch minute 

 particles of solution which are carried forward by the mercury 

 during fluctuations of pressure ; these traps effectually prevent 

 the disaster which results when such particles work their way 

 into the calibrated portion of the tube, compelling a dismantling 

 and cleaning of the whole apparatus. The short column of 

 mercury (4) at the top of the tube serves to prevent contamina- 

 tion of the nitrogen while the instrument is being closed and 

 afterwards keeps the gas out of the portion of the tube the cali- 

 bration of which has been affected to an unknown extent by fusing 

 off the ends. Two very fine marks are etched on each mano- 

 meter, one near the bottom of the calibrated portion of the 

 instrument and the other higher up : these are the only reference 

 lines, as any attempt at graduation would interfere with the 

 accurate location of the meniscus. The distance between the 

 two marks is known, so that when one is out of sight in 

 the bath, readings can be taken from the other line and then 

 referred back to the first. 



The tubes are selected from large batches of the best 

 commercial qualities, one end of each selected tube being 

 marked and cut off so as to be available for sealing on to the 

 manometers if and when required. The tube is then calibrated, 

 either in a vertical or in a horizontal position or both, by means 

 of (a) a short thread of mercury of known weight, which is 

 measured in a series of positions along the tube and (6) a long 

 thread which fills the tube between the reference marks near the 

 ends of the tube and which is also weighed. The two weighings 

 do not give the same figure for the weight of mercury per 

 millimetre of the tube because of the curvature of the meniscus ; 

 but from the difference the volume-error due to the meniscus 

 can be calculated and applied to the subsequent readings of 



