THE CARBOHYDRATES. 281 



The specific gravity is first determined in the fresh urine, after 

 adding 2 grammes of tartrate of potassium and sodium, and 2 

 grammes of diacid sodium phosphate to every 100 c.c. To about 

 200 c.c. which have thus been prepared, from 5 to 10 grammes 

 of fresh yeast are added, and the mixture is set aside at a tem- 

 perature of from 20 to 25 C. until fermentation is completed. If 

 but little sugar is present, two or three hours will suffice; otherwise 

 the mixture is allowed to stand for twelve hours. Evaporation 

 is guarded against by closing the bottle with a perforated stopper 

 through which a finely drawn out tube passes, which is open at the 

 distal end. The specific gravity is then again determined at the same 

 time as before, and the difference multiplied by 0.230. The result 

 indicates the amount of sugar in per cent. 



The method yields good results unless very small amounts of 

 sugar are present, viz., less than 0.5 per cent. In such an event, 

 the reducing power of the urine is first ascertained according to 

 Knapp's method. It is then fermented, when the remaining re- 

 ducing substances are again determined. The difference may be 

 referred to sugar. 



FERMENTATION METHOD. In the clinical laboratory especially 

 constructed saccharimetric tubes are used, of which Lohnstein's is 

 probably the best. These are provided with a scale which enables 

 the percentage of sugar to be read off directly from the amount of 

 carbonic acid that has gathered in the upper end of the tube. The 

 instruments are accompanied by printed instructions for use, which 

 need not be considered at this place. 



KNAPP'S METHOD. This method is to be preferred to the older 

 method of Fehling, which furnishes results of value only in espe- 

 cially experienced hands. 



The method is based upon the observation that mercuric cyanide 

 in alkaline solution is reduced by sugar to metallic mercury. If 

 urine is then added to a solution containing a known amount of the 

 cyanide until this is entirely reduced, the corresponding amount of 

 sugar can be directly ascertained. 



The solution .which is generally employed for this purpose con- 

 tains 10 grammes of the chemically pure cyanide, and 100 c.c. of a 

 solution of sodium hydrate (sp. gr. 1 .145) in the liter : 20 c.c. cor- 

 respond to 0.05 gramme of glucose. 



The urine must be free from albumin and should contain not 

 more than 0.5 to 1 per cent, of sugar. This should first be ascer- 

 tained by a preliminary test. If more is present, the urine should 

 be correspondingly diluted. 



Twenty c.c. of the reagent are diluted with 80 c.c. of distilled 

 water, or with less if a smaller amount of sugar than 0.5 per cent, 

 is present. The solution is heated to the boiling-point, and then 

 titrated with the diluted urine, boiling for one-half minute after the 

 addition of every 2 c.c. or less of the urine. As the end-reaction 

 is approached, the mercury together with the phosphates settles to 



