THE URINE. 197 



be screwed on too tight, since the glass itself under very 

 strong pressure may become optically active. With a new 

 tube the cap is usually only lightly screwed on. A series 

 of readings is always made and the numbers obtained noted ; 

 sometimes single observations fall entirely out of the series; 

 these may be stricken out without hesitation. 



The tube must be carefully cleaned immediately after 

 using and also be rinsed with distilled water. This is espe- 

 cially important in the examination of urine. In putting the 

 apparatus away the cover is only screwed on loosely in order 

 that the rubber ring may not stick to the glass. 



The urine must be perfectly clear and should always 

 be filtered; further, it must not be too strongly colored. If 

 it is not possible to get it perfectly clear by nitration or if it 

 is too strongly colored, it must be treated with precipitating 

 reagents, which also remove the coloring-matter. Neutral 

 lead acetate is most generally used for this purpose. The 

 urine is shaken with powdered lead acetate in a dry flask 

 (to 50 cc. of the urine about 1 g. of the acetate) and filtered 

 through a dry filter into a dry beaker. If the urine which 

 passes through at first is turbid, pour it back repeatedly on 

 the filter. Instead of this method of procedure we may also 

 mix four volumes of urine with one volume of a saturated 

 lead acetate solution and filter through a dry filter. The 

 dilution must, of course, be taken into consideration in cal- 

 culating the result. If the urine contains oxy butyric acid 

 which is always to be assumed when it contains ace to- 

 acetic acid a correction must be applied to the number 

 read for the amount of sugar, owing to the laevorotation 

 caused by the oxybutyric acid. The urine is allowed to 

 ferment, and then the rotation is determined. This is to be 

 added to the rotation caused by the glucose. Combined 

 glucuronic acid may also under certain circumstances cause 

 Isevorotation. (P. Mayer, Berl. klin. Wochenschr. 1900, 



