402 ILLUSTRATIVE EXPERIMENTS 



Thus suppose that the A observed is -2-3, the osmotic pressure of 

 this sample of urine would be 2-3 x 122-7 = 282-2 metres of water = 282-2/13-6 

 = 20-7 mm. of mercury. 



Precautions, (a) The temperature of the cooling bath must not be 

 too low. It should not exceed 3 below the freezing point of the liquid. 



(#) Excessive supercooling should be avoided. It should not be greater 

 than half a degree. 



(y) Stirring should not be too rapid say 1 up-and-down movement 

 per second, and it should be as uniform as possible. 



() If the liquid shows a tendency to give up heat without freezing and 

 that even vigorous stirring does not initiate solidification, the introduction 

 of a small crystal of ice through the side tube generally suffices to start 

 solidification. 



8. Osmotic Pressure by Barger's Method (Fig. 75). Trans. Chem. Soc. 

 85, p. 286. Prepare a number of capillary tubes by drawing out soft glass 

 tubing of \ inch bore into capillaries 1-2 feet long. These should be cut 

 into smaller pieces, having a smooth regular edge, in order that the tube 

 may be closed tightly with the finger while it is being filled. The internal 

 diameter of the capillaries should be between 1 and 2 mm., preferably 

 about 1-5 mm. 



The filling of the tubes requires a little practice. The tube is taken 

 between the middle finger and thumb, and its upper end, which should be 

 rounded, is closed with the index finger. The other end is then dipped 

 below the surface of solution A. By lifting the index finger very slightly 

 enough liquid is admitted into the tube to make a column of about 5 mm. 

 long. The finger is replaced on the end of the tube, which is then lifted 

 from the fluid and inverted so that the open end is uppermost. It is held 



/ 2 3 4 5 6 7 8 9 



FIG. 76. Barger's Method for determining Molecular Concentration. 



Upper figure,'actual size. Lower figure, as seen under the microscope; micrometer 



scale in eyepiece, j 



in a slanting position, and^by'diminishing the pressure of the index finger 

 on the lower end, the globule of liquid is allowed to slide down the tube, 

 its progress being regulated by the slant of the tube. The process is repeated, 

 using solution B and so on, using solution A and B alternately and finishing 

 with A. When all the drops are in, the collection is moved so that the last 



