6 THE CARBOHYDRATES. 



the markings which are thus brought out upon the gran- 

 ules which are least colored. 



4. Examine in the same way starch from other 

 sources: corn, wheat, buckwheat, etc. Observe the differ- 

 ence in the size and shape of the granules. Sketch these 

 and hand in the results. 



5. Prove that starch does not dissolve in cold water 

 by filtering after shaking powdered starch in a test-tube of 

 water. lodin gives no color to the filtrate. 



6. Add about a gramme of starch to 100 cubic centi- 

 meters of cold water, mix it thoroughly, and boil. The 

 starch has dissolved, as is shown by filtering and, after 

 cooling, testing a portion of the filtrate with iodin solution. 

 A deep-blue color is produced. It is destroyed by heating, 

 but reappears as purple or blue again upon cooling. 



V 7. Use a piece of parchment dialyzing tube to test 

 diffusibility. First see that this does not leak. It should 

 hold water when suspended by the two ends. Place inside 

 some of the starch solution made in the preceding experi- 

 ment and hang the whole in a small beaker of water, so 

 that the liquids inside and outside are at the same level. 

 ir Instead of the tube a piece of parchment can be placed in 

 a funnel from which the stem has been broken, as if the 

 liquid were to be filtered. Pour the starch solution into 

 this and suspend the whole in a beaker of water. Allow 

 it to stand several hours, then test the water outside with 

 iodin for starch. It does not pass through because it is a 

 colloid. Then put a little glucose in the dialyzer. It dif- 

 fuses out and can be found by Trommer's reaction. 



8. Examine under the microscope the starch-paste 

 which has been made by heating starch in water. The 

 granules have been burst open and destroyed. 



