CARBOHYDRATES. 49 



make the iodine test. The power of the solution to produce a color 

 with iodine should rapidly disappear. When a negative reaction is 

 obtained cool the solution and neutralize it with concentrated potas- 

 sium hydroxide. Try Fehling's test (see page 27). This reaction 

 is now strongly positive, due to the formation of a reducing sugar. 

 Determine the nature of the sugar by means of the phenylhydrazine 

 test (see pages 24 and 25). 



5. Influence of Tannic Acid. Add an excess of tannic acid 

 solution to a small amount of dextrin solution in a test-tube. No 

 precipitate forms. This result differs from the result of the similar 

 experiment upon starch (see Starch, 8, page 46). 



6. Diffusibility of Dextrin. (See Starch, 9, page 46.) 



7. Precipitation by Alcohol. To about 50 c.c. of 95 per cent 

 alcohol in a small beaker add about 10 c.c. of a concentrated dextrin 

 solution. Dextrin is thrown out of solution as a gummy white 

 precipitate. Compare the result with that obtained under Dextrose, 

 5, page 25. 



CELLULOSE, (C 6 H 10 5 ) X . 



This complex polysaccharide forms a large portion of the cell 

 wall of plants. It is extremely insoluble and its molecule is much 

 more complex than the starch molecule. The best quality of filter 

 paper and the ordinary absorbent cotton are good types of cellulose. 



EXPERIMENTS ON CELLULOSE. 



1. Solubility. Test the solubility of cellulose in the ordinary 

 solvents (see page 23). 



2. Iodine Test. Add a drop of dilute iodine solution to a few 

 shreds of cotton on a test-tablet. Cellulose differs from starch and 

 dextrin in giving no color with iodine. 



3. Formation of Amyloid. 1 Add 10 c.c. of dilute and 5 c.c. 

 of concentrated H 2 SO 4 to some absorbent cotton in a test-tube. 

 When entirely dissolved (without heating) pour one-half of the 

 solution into another test-tube, cool it and dilute with water. Amy- 

 loid forms as a gummy precipitate and gives a brown or blue colora- 

 tion with iodine. 



After allowing the second portion of the acid solution of cotton 

 to stand about 10 minutes, dilute it with water in a small beaker and 

 boil for 15-30 minutes. Now cool, neutralize with solid KOH and 



1 This body derives its name from amylum (starch) and is not to be con- 

 founded with amyloid, the glycoprotein (page 106). 



5 



