88 CARBOHYDRATES. 



boiling-point is reached, a reddish precipitate, consisting of cup- 

 rous oxide, will form. Levulose, galactose, lactose, and maltose 

 have reducing power similar to that of dextrose, but differing in 

 degree ; thus, the power of lactose as compared with dextrose is 

 but that of 7 to 10, while maltose has one-third less power than 

 dextrose. Cane-, maple-, and beet-sugar have no reducing power, 

 and must first be converted into dextrose before the reaction will 

 take place. 



Fermentation-test. This test depends upon the fact that under 

 the influence of yeast dextrose is decomposed into ethyl alcohol 

 and carbonic anhydrid. 



Fehling's Test. This test is based on the same principle as that 

 of Trommer, namely, the property possessed by dextrose to reduce 

 metallic oxids. It is employed not only to determine the presence 

 of dextrose, but also to measure the quantity present. The test- 

 solution is liable to undergo changes which invalidate the result ; 

 it should, therefore, be freshly prepared, or at least be boiled be- 

 fore it is used. The principal change which takes place is the 

 formation of racemic acid from the tartaric acid of the solution, 

 and this has the same reducing action as the sugar. If after 

 boiling the solution is clear, it may be inferred that decomposition 

 has not taken place, and it may be used. The solution is prepared 

 in the following manner : 



34.639 grams of pure recrystallized copper sulphate are dis- 

 solved in distilled water, which is made up to 500 c.c. This solu- 

 tion should be kept separate from the second solution, which is 

 made by dissolving 175 grams of crystallized Rochelle salts and 

 60 grams of sodium hydroxid in distilled water, and likewise 

 made up to 500 c.c. It is found by experience that when these 

 two solutions are mixed the resulting mixture does not keep well. 

 When the test is to be made equal quantities of the two solu- 

 tions are mixed. Prof. Bartley's method of applying this test in 

 urine is as follows : 10 c.c. of the solution are measured into a 

 suitable flask. To this 10 c.c. of a freshly prepared 10 per cent, 

 solution of potassium ferrocyanid are added, and about 30 c.c. 

 of water. The mixture is heated on a water-bath, and the urine, 

 previously diluted with water if it contains much sugar, is run 

 in from a faucet, drop by drop, until the blue color just dis- 

 appears. The addition of the slightest excess of sugar shows 

 itself by the solution becoming quickly brown. By careful com- 

 parative tests Prof. Bartley has found this method to be reliable 

 and accurate provided the solution is not boiled during the reduc- 

 tion. The best temperature he finds to be between 80 and 90 C. 



Polariscope. This is also known as a polarimeter. It may be 

 employed to determine the presence of dextrose. In order to 

 understand the use of this instrument it will be necessary to con- 

 sider briefly the subject of the polarization of light. 



