DETERMINATION OF FREEZING-POINT 401 



the excess of mercury to break off from the end of the capillary. The 

 thermometer is again tested in the first bath. 



B. If, on the other hand, the amount of mercury in the bulb is so small 

 that the top of the column does not rise to the bottom of the scale, more 

 mercury will have to be drawn from the reservoir. The procedure is similar 

 to that outlined above, but at (c) the thermometer is replaced in the first 

 bath before breaking the mercury column. That is, the mercury in the 

 bulb is allowed to contract and draw in more mercury from the reservoir 

 before the connection between column and reservoir is broken by tapping. 



These operations are repeated till the proper level of mercury has been 

 attained. This is always tested by placing the thermometer in baths having 

 temperatures equal to the highest and lowest to be encountered in the experiment, 

 and noting that the top of the column of mercury remains on the scale. 



Method. (1) Set up the apparatus completely so as to ensure all parts 

 fitting properly. See that the stirrer in the inner tube is working smoothly 

 and does not strike against the bulb of the thermometer. 



(2) Remove the thermometer and stirrer from the tube. Clean and 

 dry the latter. 



(3) Pipette in 25 c.c. of urine. 



(4) Set the Beckmann thermometer so that, at C., the mercury stands 

 not lower than the middle of the scale. 



(5) Dry the thermometer and insert it along with the stirrer in the 

 freezing-point tube, so that the bulb of the thermometer is completely 

 immersed in the urine. 



(6) Fill the outer cooling vessel with water, ice and salt. The freezing 

 point of urine can now be determined. 



(7) First make an approximate determination by placing the freezing- 

 point tube directly in the cooling bath so that a rapid fall of temperature 

 occurs. 



(8) As soon as the urine shows signs of freezing remove the tube from the 

 freezing mixture, dry it quickly and place it in the air jacket in the cooling 

 bath. 



(9) Stir slowly and read the temperature when it becomes constant. 



(10) Withdraw the tube and melt the ice by warming with the hand, 

 trying to avoid raising the temperature more than 1 C. 



(11) Rapidly dry the tube and reinsert it in the air jacket and repeat the 

 freezing process, stirring slowly all the time. 



(12) When the temperature has fallen to from 0-2 to 0-5 below the 

 approximate freezing point found in (9) stir more vigorously. This 

 generally is sufficient to induce solidification to commence and the tempera- 

 ture will now begin to rise. 



(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 deter- 

 minations 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. 



B.B. 26 



