DETERMINATION OE EREEZlNd POINT 



513 



forms a small drop there, the thermometer should be carefully inverted 

 and tapped srently so as to cause the mercury in the reservoir to coalesce 

 with the mercury in the top of the capillary, (c) The thermometer is 

 returned to the upright position by a gentle steady movement and its upper 

 end is struck a sharp tap against the palm of the hand, cai;sing 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 top 

 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 thernioiiefer l)i baths having temperatures equal 

 to the highest and loicest to be encountered in the erperivient, 

 and noting that the top of the colmnn of >nercicry 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 0° C, 

 the mercury stands not lower than the middle of the 

 scale. 



(5) Dry the thermometer and insert it along wnth 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 deter- 

 mined. 



(7) First make an approximate determination by placing FiG,i04.—T:ppcr portion 



. . ^ '- . , . ■ ^ ~ of the stem of a liuck- 



the freezing-point tube directly in the cooling bath so mann ' Thermometer, 

 that s. rapid fall of temperature occurs. ^^^ ""'^''"^y "' 



(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 ice by w^arming 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 temperature will 

 now begin to rise. 



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1-5! 



