154 LABOR A TOR Y G VIDE IN PHYSIOLOG Y. 



soluble in water. Stir or shake the mixture to bring 

 the starch into suspension in the water; pour upon a 

 filter. A clear filtrate passes readily through. Test 

 the filtrate for starch; result, negative; pour a few 

 drops of iodine upon the filter, starch present. Con- 

 clusions: 



(#) Potato starch is insoluble in cold water. 



() The granules of potato starch will not pass 

 through common filter paper. 



(3) Dilute a few cubic centimeters of starch paste; pour 

 it upon a filter; to the filtrate add iodine. The blue 

 color indicates that in the cooking of starch the grains 

 are broken up into particles sufficiently small to readily 

 pass through the meshes of common filter paper. 



(4) In order to determine whether dilute starch paste 

 will in response to the laws of osmosis pass through 

 an animal membrane, fill a dialyzer with dilute starch 

 paste. Set aside to be tested one or two days later. 



(5) Put a bit of absorbent cotton into a beaker or test 

 tube; add water, boil; add iodine. Cellulose, as repre- 

 sented by cotton fibers, is insoluble in water and does 

 not respond to the iodine test. 



(6) Put a few bits of ash-free filter paper into a test 

 tube; add water; boil; add iodine. Cellulose, as repre- 

 sented by the fibers of ash free filter paper, is insol- 

 uble in water and responds to the iodine test. One 

 must remember in this connection that in the prepa- 

 ration of ash-free filter paper mineral acids are used 

 to dissolve out the salts; and mineral acids, especially 

 sulphuric acid, so modify cellulose that it responds to 

 the iodine test with a blue color. 



(7) Add water to dextrin in a beaker; stir with a rod. 

 Dextrin is readily soluble in cold water. To a small 

 portion add iodine. The solution will probably as- 



