EXAMINATION OF MILK. 13 



indigo white to indigo blue. Heat again and the solution 

 will be again decolorized. This process may be repeated 

 until all the sugar is used up by oxidation. 



All these reactions are given by glucose as well as by milk- 

 sugar. The conduct towards yeast is the simplest method 

 of distinguishing between the two sugars. Glucose is very 

 quickly converted into alcohol and carbon dioxide by yeast, 

 whereas milk-sugar is not, or at least very slowly and incom- 

 pletely. In order to carry out the experiment shake a quan- 

 tity of the 2 per cent, milk-sugar solution in a test-tube with 

 a piece of compressed yeast as large as a hazel-nut, fill a fer- 

 mentation-tube with the mixture (mercury seal), and put the 

 tube in a warm place (about 35). Make a similar test with 

 a 2 per cent, solution of glucose as a check. After some 

 hours the glucose solution will be found in fermentation, as 

 shown by the development of carbon dioxide, which fills a 

 part of the tube; the milk-sugar solution does not ferment. 

 It is advisable to set up a third tube, which contains only 

 water and yeast. No fermentation should take place in it 

 during twelve to twenty-four hours. Gradually a slight 

 development of carbon dioxide makes itself apparent (spon- 

 taneous fermentation of yeast). 



The following tests may also be used to distinguish between the 

 two sugars. 



1. Rubner's Test. Dissolve 4 g. of neutral lead acetate by warm- 

 ing with about 5 cc. of the 2 per cent, solution of milk-sugar, boil for 

 one to two minutes, then add an excess of ammonia and heat again. 

 A deep-red solution will be formed and gradually a precipitate of the 

 same color appears if sufficient ammonia has been added. Glucose 

 acts in the same way at first, but the color soon becomes yellow 

 (chamois) . 



Perhaps the following slight modification of the test is somewhat 

 simpler: mix about 3 cc. of milk-sugar solution (2 per cent.) with the 

 same volume of basic lead acetate solution and 1 cc. of ammonia, and 

 boil for some time. The milky fluid becomes first yellow, then brick- 

 red, and then remains unchanged, further addition of ammonia in- 



