EXAMINATION OF NORMAL AND ABNORMAL URINE. 471 



and that substances other than sugar may deoxidize cupric oxide at 

 the temperature of 100 C. (212 F.). 



Fehling's test differs from Trommer's test in merely using a pre- 

 viously mixed reagent instead of producing this reagent, as it were, 

 in the urine by adding to it cupric sulphate and an alkaline hydroxide 

 successively. This reagent, known as Fehling's solution, or as alkaline 

 cupric tartrate volumetric solution, is made by mixing exactly equal 

 volumes of the below-mentioned copper solution and the Kochelle 

 salt solution at the time required. 



Copper solution : 



Crystallized cupric sulphate ...'.. 34.64 grammes. 

 Water, sufficient quantity to make .... 500 c.c. 



Rochelle salt solution : 



Potassium sodium tartrate 173 grammes. 



Potassium hydroxide .125 " 



Water, sufficient quantity to make .... 500 c.c. 



Both solutions are preserved in small well-stoppered bottles, and 

 mixed only at the time needed, because the mixture is apt to decom- 

 pose when kept some time. 



The addition of sodium-potassium tartrate in Fehling's solution prevents the 

 precipitation of cupric hydroxide by the alkaline hydroxide. This action is 

 analogous to the formation of the soluble scale compounds of iron, where the 

 precipitation of ferric hydroxide is also prevented by tartaric or other organic 

 acids. 



While Fehling's solution is used chiefly for quantitative determina- 

 tions, it can also be used to advantage for qualitative tests. This is 

 done by heating about 10 c.c. of Fehling's solution in a test-tube, 

 and adding drop by drop the suspected urine ; if the latter contains 

 larger quantities of sugar a yellow or red precipitate of cuprous hy- 

 droxide and oxide will be produced very readily ; if but small quan- 

 tities are present, an equal volume of urine may be added to the 

 solution, and the boiling repeated several times before the reaction 

 takes place. 



Bdtger's bismuth test consists in adding to a mixture of equal 

 volumes of urine and potassium (or sodium) hydroxide solution a 

 few grains of subnitrate of bismuth and boiling for half a minute. 

 If sugar be present, a gray or dark-brown, finally black, precipitate 

 of bismuthous oxide, Bi 2 O 2 , or of metallic bismuth is formed. If but 

 very little sugar is present, the undecomposed excess of bismuthic 

 nitrate (or bismnthic hydroxide) mixes with the metallic bismuth, 



