CHEMIC COMPOSITION OF THE HUMAN BODY. 11 



oxid, from which it can be precipitated by acids. It is an amorphous 

 powder ; dilute acids can convert it into dextrose. 



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 soluble in water and in hot 

 alcohol, from which it crystallizes 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 sugars 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 alka- 

 line 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 precipitate 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 ^ 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 



