514 ILLUSTRATIVE EXPERIMENTS 



(13) If so, stir slowly and take readings of the temperature every few 

 seconds — tapping the thermometer each time before reading. Note the 

 highest temperature reached. 



(14) Again melt and repeat the determination. At least three determi- 

 nations of the freezing-point should be made, the mean being taken. The 

 deviations of the chosen readings from the mean should be less than 0-002° C. 



(15) The depression of the freezing-point or, in this case, the thermo- 

 metric readings may be converted into osmotic pressure in metres of water 

 by multiplying by the factor 122-7. 



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. 



Precautio))s. (a) The temperature of the cooling bath must not be too 

 low. It should not exceed 3" below the freezing-point of the li<]uid. 



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]"'l(i. 105. — Barfcer't: Method for Determining Moleeular C'oneentration. 



tplier figure, actual size. Lower figure, as seen under tlie microscope ; micrometer 



scale in eyepiece. 



(/3) Excessive supercooling should be avoided. It should not be greater 

 than half a degree. 



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

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



(8) 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 sufHces to start 

 solidification. 



(c) Osmotic Pressure by Barger's Method (Fig. 105). Trans. Chem. Soc. 

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

 tubing of ^ in. bore into capillaries 1-2 ft. 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 



