10 HUMAN PHYSIOLOGY. 



2. DEXTROSES, C 6 H 12 O 6 . 



Dextrose, glucose, or grape-sugar is found in grapes, most sweet fruits, 

 and honey, and as a normal constituent of liver, blood, muscles, and other 

 animal tissues. In the disease diabetes mellitus it is found also in the urine. 



When obtained from any source, it is solule in water and in hot alcohol, from 

 which it crystalizes in six-sided tables or prisms. As usually met with it is in 

 the form of irregular, warty masses. It is sweet to the taste; less so, however, 

 than cane sugar. It is dextro-rotary, turning the plane of polarized light to 

 the right. In alkaline solutions dextrose absorbs oxygen, and hence in the 

 presence of metallic salts, copper, bismuth, silver, etc., it acts as a reducing 

 agent. On this property the various tests for dextrose, as well as other 

 which have the same property, are based. 



Fehling's Test. The solution usually employed for both qualitative and 

 quantitative purposes is a solution of cupric hydroxid made alkaline by an 

 excess of sodium or potassium hydroxid, with the addition'of sodium and 

 potassium tartrate. This solution, originally suggested by Fehling, bears 

 his name. It is made by dissolving cupric sulphate 34. 64 grams, potassium 

 hydroxid 125 grams, sodium and potassium tartrate 173 grams in i liter of 

 distilled water. 



The reaction is expressed by the following equation: 



CuSO 4 + 2KOH = Cu(OH) 2 + K 2 SO 4 . 



The object of the sodium and potassium tartrate is to hold the Cu(OH) 2 

 in solution. If a few cubic centimeters of this deep blue solution be boiled 

 and dextrose then added and the solution again heated to the boiling-point, 

 the cupric hydroxid is reduced to the condition of a cuprous oxid, which 

 shows itself as a red or orange-yellow precipitate. The color of the' pre- 

 cipitate depends on the relative excess of either copper or sugar, being red 

 with the former, orange or yellow with the latter. The delicacy of this test 

 is shown by the fact that a few minims of this solution will detect in one c.c. 

 of water the 1/15 of a milligram of sugar. 



For quantitative analysis, ten c.c. of Fehling's solution, diluted with 

 forty c.c. of water, are heated in a porcelain capsule, to which the dextrose 

 solution is cautiously added from a buret until the blue color entirely dis- 

 appears. The strength of this solution is such that 10 c.c. is decolorized 

 by 50 milligrams of sugar, from which the percentage of sugar in the urine 

 can be determined. Thus if 0.8 c.c. of urine decolorizes 10 c.c. of Fehling's 

 solution then it contains 50 milligrams of sugar. 



Fermentation Test. If to a solution of dextrose a small quantity of the 

 yeast plant be added, and the solution kept at a temperature of 25 C., it 

 will gradually undergo fermentation; that is, will be reduced to simpler 



