CHAPTER II. 



THE MANOMETERS. 



The possibility of correctly determining osmotic pressure depends 

 upon four fundamental conditions, no one of which can be said to 

 exceed another in importance. They are (1) a suitable cell, i. e., a cell 

 which is able to support the membrane under high pressure and in 

 which the membrane is always deposited upon the interior surface of 

 the porous wall; (2) a truly semi-permeable membrane, i. e., a mem- 

 brane which does not leak the solute; (3) a perfectly automatic and 

 exact regulation of temperature; and (4) an accurate calibration of the 

 manometers. If any one of these conditions is unfulfilled, all efforts 

 to measure the force must lead to erroneous results, which are not only 

 futile but positively mischievous — mischievous because they furnish 

 the opportunity for an indulgence of the propensity of the over-hasty 

 and unwary to erect elaborate speculative structures upon foundations 

 of what may be justly called tainted facts. 



The manometers which are used for the measurement of osmotic 

 pressure have an external diameter of about 6 millimeters. The length 

 of the calibrated portion varies from 400 to 500 millimeters. The 

 diameter of the bore ranges from 0.45 to 0.72 millimeter. 



The reasons for using tubes of very small bore are : 



1. It is necessary to fill the upper ends of the manometers with short 

 columns of mercury, because in closing the instruments, after the intro- 

 duction of the gas, the caliber of the tubes in that region is affected to an 

 unknown extent. If the internal diameter is large, e. g., 1.0 millimeter or 

 more, the mercury is often dislodged by the severe tapping to which the 

 manometers are subjected at certain times. 



2. The compression of the small volume of gas which they contain 

 involves but little dilution of the cell contents. 



3. Relatively small volumes of mercury are required by manometers 

 of small bore. The importance of this fact will be better understood 

 when the subject of "thermometer effects" is discussed. 



The disadvantages of using manometers of small bore are: 



1. It is more difficult to deal satisfactorily with the meniscus in a 

 narrow tube. 



2. The capillary depression is large in small tubes and it varies 

 greatly with slight irregularities of bore. 



3. The movements of the mercury in narrow tubes are strongly 

 influenced by the presence of minute quantities of impurities, whether 

 the same are dissolved in the metal itself or are attached to the surface 

 of the glass. 



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