PRACTICAL EXERCISES 



433 



there is an arrangement which rotates by a definite angie the plane 

 of polarization in one half of the field, but not in the other, e.g., a 

 small nicol occupying only half of the field. In the zero position of 

 the analyzer, both halves of the field are equally dark. The solution 

 to be investigated is placed in a tube of known length, the ends of 

 which are closed by glass discs secured by brass screw caps. The 

 glass discs must be slid on, so as to exclude all air. The tube 

 having been introduced between the polarizer and analyzer, the sharp 

 vertical line which indicates the division between the two half-fields 

 is focussed with the eye-piece, and then the analyzer is rotated till 

 the two halves are again equally shadowed. The angle of rotation, 

 0, is read off on the graduated arc, which is provided with a vernier. 



FIG. 133. MITSCHERLICH'S POLARIMETER. 

 (Half-Shadow Instrument.) (Simple Form.) 



Acetone. Put 250 to 500 c.c. of the urine suspected to contain acetone 

 into a litre flask. Add a few c.c. of phosphoric acid ; connect the flasl^ 

 with a worm (see Fig. 132) and distil over the urine into a small flask. 

 For qualitative tests it is best to collect only the first 20 to 30 c.c., as most 

 of the acetone is contained in this. Test the distillate for acetone by 



Lieberis Test. To a few cc. of the distillate in a test-tube add a few 

 drops of solution of iodine in potassium iodide and then sodium or 

 potassium hydrate. A precipitate of yellow iodoform crystals is thrown 

 down if acetone be present. 



Systematic Examination of Urine. In examining urine, it is con- 

 venient to adopt a regular plan, so as to avoid the risk of overlooking 



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