PHYSIOLOGICAL CHEMISTRY 279 



CHAPTER II. 

 CARBOHYDRATES CONTINUED. 



OTHER REACTIONS OF CARBOHYDRATES. 



THERE are, however, other reactions of carbohydrates which do not 

 depend on their being aldehydes or ketones. The most important of 

 these are : 



/. Molisch test. This is an extremely sensitive test, being especially 

 suitable for the detection of minute traces of carbohydrate. For 

 example, most proteins (e.g. egg albumin) give it, on account of the 

 carbohydrate groups which they contain. 



EXPERIMENT I. To about 2 c.c. of a very dilute sugar solution, 

 or of a strong solution of egg albumin, add a drop of a saturated 

 alcoholic solution of a-naphthol. Then carefully pour about an equal 

 volume of pure concentrated H 2 S0 4 down the wall of the test tube so 

 that it forms a layer at the bottom. On standing a minute or so a 

 deep violet ring forms at the line of contact of the two fluids. The 

 greenish colour which also develops is due to the reagents and is no 

 part of the test. 



//. Fermentation with Yeast. By allowing yeast to grow on a 

 solution of dextrose, the latter is split up into alcohol and carbon 

 dioxide, 



C 6 H 12 6 = 2C 2 H 5 OH + 2C0 2 



Dextrose. Ethyl alcohol + carbon dioxide. 



All carbohydrates do not give this reaction, so that it is of value as 

 a distinguishing test for the presence of dextrose in the urine. Com- 

 mercially it is the agency employed in the preparation of alcoholic 

 beverages. 



To ascertain whether the addition of yeast to a solution produces 

 fermentation, the process should be allowed to proceed in an inverted 

 tube over mercury, or in a Southall's ureometer (see Fig. 244), so that 

 any carbon dioxide gas which develops may be collected, and if 

 necessary tested. 



EXPERIMENT II. Shake up a 1 per cent, solution of dextrose, 

 which has been previously boiled to expel the air and then cooled, with 

 a piece of yeast the size of a split pea. Pour the opalescent solution 

 thus obtained into a Southall's ureometer (p. 421) so that it completely 

 fills the vertical tube. Now place the tube aside in a warm place for 

 some time, when it will be found that a certain amount of gas has 

 collected at the top of the tube. This gas is C0 2 as can be shown by 



