24 PHYSIOLOGICAL CHEMISTRY 



coated with the solution. Continue the rotation in the inverted position until 

 the collodion ceases to flow. After the solution has evaporated such that the 

 collodion skin on the rim is dry and stiff, cut or loosen it around the edge of the 

 rim. With a pipette or wash bottle run in a few cubic centimeters of water be- 

 tween the membrane and the wall of the flask or test-tube. Shake the inclined 

 vessel while rotating on its longitudinal axis, thus detaching the membrane. 

 Now withdraw the detached bag and fill with water, to determine whether or not 

 it contains defects. 1 



All monosaccharides and disaccharides are diffusible, but many polysac- 

 charides are not. 



5. Influence of Alkali (Moore's Test). To 2-3 c.c. of sugar solution in a test- 

 tube add an equal volume of concentrated KOH or NaOH, and boil. The solution 



FIG. 2. DIALYZING APPARATUS FOR STUDENTS' USE. 



darkens and finally assumes a brown color. At this point the odor of caramel 

 may be detected. 



This test is of little practical value for the detection of glucose. The alkali 

 brings about condensation and decomposition. The brown color is due to the 

 formation of condensation products. Among the decomposition products are the 

 potassium or sodium salts of certain organic acids. 



6. Reduction Tests. To their aldehyde or ketone structure many 

 sugars owe the property of readily reducing alkaline solutions of the 

 oxides of metals like copper, bismuth and mercury; they also possess 

 the property of reducing ammoniacal silver solutions with the separa- 

 tion of metallic silver. Upon this property of reduction the most 

 widely used tests for sugars are based. When whitish-blue cupric 

 hydroxide in suspension in an alkaline liquid is heated it is converted 

 into insoluble black cupric oxide, but if a reducing agent like certain 

 sugars be present the cupric hydroxide is reduced to insoluble yellow 

 or red cuprous oxide. These changes are indicated as follows: 



l Gies: Quoted by Clark. Bioch. Bull., i, 198, 1911. 



