III.] THE CARBOHYDRATES. 21 



and finally red. It is better seen in reflected than transmitted 

 light. If no sugar be present, only a black colour may be 

 obtained. 



{g.) Add Fehling's solution; boil = a yellow or yellowish-red 

 precipitate of cuprous oxide or hydrate. [For the method of 

 making Fehling's solution, the precautions to be observed in using 

 it, and for some other tests for glucose, see " Urine."] 



(A.) Barfoed's Solution. —To 200 cc. of a solution of neutral acetate of 

 copper, containing i part of the salt to 15 of water, add 5 cc. of a 38 per cent, 

 solution of acetic acid. When heated with dextrose some red cuprous oxide 

 is precipitated, while lactose, cane-sugar, maltose, and dextrin, when they 

 are boiled with it for a short time, give no reaction. Hence this substance 

 has been used to distinguish dextrose from maltose. 



{i. ) Bdttger's Bismuth Test. — Heat the fluid with caustic soda and a small 

 quantity of dry basic bismuth nitrate = a grey or black reduction product of 

 bismuth oxide. For Nylauder's modification, see " Urine." In all reactions 

 depending on reduction, one must recollect that some substances which are 

 by no means related to the glucoses — e.g., uric acid, ki-eatinin, phenyl- 

 hydrazine— may cause reduction, and thus lead one into error. 



(C.) other Reactions. 



(,/.) Phenyl-Hydrazine Test. — Two parts of phenyl-hydrazine 

 hydrochloride and three of acetate of soda are mixed in a test-tube 

 with 6-10 cc. of the dextrose 

 solution. Boil for 20-30 

 minutes, and then place the 

 tube in cold water. If sugar 

 be present, a yellow crystaUine 

 deposit is formed, which, 

 microscopically, consists of 

 yellow needles either detached 

 or arranged in rosettes (fig. 

 6). The substance formed is 

 phenyl-glucosazone 

 (CjgHgoN^O^), with a melting- 

 point of 204° C. 



The arrangement of the 

 acicular crystals I find fre- 



,1 . CI i.' Fig. 6.— Crystals of Phenyl-Glncosazone, X 120. 



quently varies. bometimes 



they are in rosettes (see " Urine "), and at other times more 

 feathery. They are soluble in alcohol, and may be recrystallised 

 from it. 



This is an extremely important and reliable reaction. The best 

 proportioiis for the ingredients are i part dextrose, 2 hydrocliloride 

 of phenyl-hydrazine, 3 sodic acetate, and 20 water. The substance 



