XXIII.] URINE. 245 



to be necessary for the preliminary estimation 

 should give the amount within *5 c.c. If b be the 

 number of c.c. of diluted urine required to reduce 

 the 10 c.c. of Fehling the percentage of sugar in 



05 x 5 x 100 25 



the urine will be - , = -y- . 

 o b 



There are certain sources of error in the method given 

 above. ( 1 ) When the mixture is boiled, a little ammonia 

 is set free from some of the constituents of the iirine, 

 this dissolves some of the precipitated cuprous oxide ; 

 the cuprous oxide takes up oxygen from the air and 

 a cupric salt is formed giving a blue tinge to the 

 solution. The re-oxidation of the precipitated oxide 

 may be lessened by boiling in a small glass flask 

 instead of in an open porcelain dish, but since the 

 former is more easily cracked during boiling, and in it 

 the colour of the fluid is less readily seen, it is not recom- 

 mended for the beginner. (2) When the diluted urine 

 contains more than 1 p.c. or less than '5 p.c. of sugar, 

 the method is inaccurate; since 5 c.c. of a 1 p.c. solution 

 of sugar, or 10 c.c. of a *5 p.c. solution contain *05 gram 

 of sugar, i.e. will reduce 10 c.c. of Fehling's fluid, 

 the urine, for an accurate estimation, must be diluted 

 to such an extent that not more than 10 and not less 

 than 5 c.c. of the diluted urine are required to reduce 

 10 c.c. of Fehling's fluid. The Fehling's fluid is diluted 

 5 times because with either a greater or a less dilution 

 the results are inaccurate. 



