450 PRACTICAL PHYSIOLOGY 



the size of a, split pea) of German yeast, and stir it in the urine 

 until a milky solution is obtained. Now transfer the fluid to a 

 Doremus ureometer (Fig. 244) so that the upright limb is completely 

 filled with fluid. Place this in an incubator, or in a warm place, as 

 on the mantelpiece, over night when it will be found that gas 

 carbon dioxide has collected in the upper portion of the vertical limb. 



Two control tubes one with a weak solution of dextrose and yeast, 

 the other with normal urine and yeast should be arranged so as to 

 prevent any fallacy due to inactive or impure yeast. 



Instead of using a Doremus ureometer a test tube inverted in a 

 trough of mercury may be employed. 



Lactose and pentose do not give a positive result by this test. 



2. The Phenyl Hydrazine Test. The method of employing this 

 is described on p. 275. The obtaining of characteristic dextrosozone 

 crystals is positive evidence of the presence of dextrose; glycuronic 

 acid (p. 454) also gives crystals, but less readily. 



Estimation of Dextrose in Urine. The polarimeter (see p. 282) 

 may be employed for the estimation of dextrose in urine. The main 

 objection to its use is that other optically active bodies besides 

 dextrose, e.g. glycuronic acid and oxybutyric acid, which are laevo- 

 rotatory, occur in diabetic urine, and therefore to a certain extent 

 vitiate the result. The other method is to determine the reducing 

 power of the urine. For clinical purposes the methods of Fehling, 

 Gerrard, and Pavy are employed. In Fehling's method the amount 

 of urine necessary to decolourise a measured quantity of standard 

 Fehling's solution is determined. It is, however, difficult to observe 

 when the blue colour of the Fehling's solution has disappeared owing 

 to the precipitation of red cuprous oxide. To obviate this difficulty 

 Gerrard prevents the precipitation of cuprous oxide by the addition 

 of potassium cyanide. The solution when reduced has a light yellow 

 tint, so that the end-point is a little troublesome, but for the use 

 of students this is probably the best method. In Pavy's method 

 ammonia is added to the Fehling's solution. There is then no 

 precipitation of cuprous oxide until the ammonia has been boiled 

 off. The reaction liquid is perfectly colourless when reduced, so 

 that the end-point is a good one, but the ammoniacal solution 

 absorbs oxygen from the air, so that the liquid has to be kept 

 boiling in a flask with a small opening. Moreover, owing to the 

 escape of ammonia there is only a limited time in which to perform 

 the titration before cuprous oxide begins to be precipitated. This 

 method, therefore, although very rapid, requires considerable practice. 



Fehling's Method. The standard solution contains 34 '64 grm. pure 



