THE CARBOHYDRATES. 295 



appearance. With free glucuronic acid a similar compound may 

 be obtained, according to Thierfelder, which may also be recognized 

 by its melting-point, while the conjugate glucuronates are inactive in 

 this respect. Pentoses likewise give rise to the formation of osa- 

 zons, but the melting-point of the resulting crystals serves to distin- 

 guish these also from the osazons of the hexoses. As a general 

 rule, however, neither the pentoses nor glucuronic acid interferes 

 with the reliability of the test. If doubt should arise, a special 

 examination should be made to ascertain whether pentoses or glu- 

 curonates are present in amounts sufficient to react with the reagent. 

 A further objection to the phenylhyclrazin test has been urged on 

 the basis that its delicacy is such that a positive reaction is obtained 

 even under normal conditions. This, however, I must deny. 



The test is conveniently conducted as follows i 5 drops of pure 

 phenylhydrazin are mixed in a test-tube with 10 drops of glacial 

 acetic acid and 1 c.c. of a saturated solution of common salt. To 

 this are added 3 c.c. of urine, when the mixture is boiled for two 

 minutes and is then set aside to cool. In the presence of more 

 than 0.5 per cent, of glucose, crystals of phenyl-glucosazon begin 

 to separate out after one or two minutes. Should smaller amounts 

 be present, it is necessary to wait. The sediment is then exam- 

 ined microscopically. As we are generally only dealing with glu- 

 cose in the urine, a further examination is usually not necessary, 

 especially if the substance crystallizes out in large needles, which are 

 often collected in stars and sheaves. To identify these further, how- 

 ever, their melting-point must be determined. As has been stated, 

 this differs in the different osazons, with the exception of laevulose 

 and glucose, which have the same melting-point. Lsevulose, how- 

 ever, is found only under exceptional conditions. Its presence may 

 be established as shown below. The melting-points of the various 

 osazons which may be encountered are as follows : 



Glucose 204-205 C. 



Lsevulose 204-205 C. 



Galactose 193 C. 



Maltose 206 C. 



Isomaltose 150-453 C. 



Lactose 200 C. 



Arabinose 159 C. 



Xylose 159 C. 



Glucuronic acid 114-115 C. 



The glucosazon is insoluble in water, but dissolves with ease in 

 hot alcohol, from which it can be precipitated on cooling in crystal- 

 line form, by diluting with water. The crystals are then collected 

 on a filter, dried over sulphuric acid, and further examined if 

 desired. 



POLARIMETRIC EXAMINATION. The polarimetric examination 

 for the presence of sugar should always be controlled by one or 

 more of the tests that have just been described. Dextrorotation, 



