12 CHEMICAL PHYSIOLOGY. 



2. Prepare a Solution. Place 1 grm. of starch in a mortar, 

 rub it up with a little cold water, and then add 50 cc. of boiling 

 water, and rub up the whole together until the starch is 

 apparently dissolved and a somewhat opalescent fluid is obtained. 

 Allow the solution to cool. [In reality, the starch is only imper- 

 fectly dissolved by hot water.] 



(a.) Add powdered dry starch to cold water. It is in- 

 soluble. Filter, and test the nitrate with iodine. It gives 

 no blue colour. 



(6.) The above 'method shows that it is imperfectly dis- 

 solved in warm water. If more starch be used, a thick 

 " starch-paste," which sets on cooling, is obtained. 



(c.) To a portion of the above fluid add a solution of 

 iodine = a blue colour, which disappears on heating and re- 

 appears on cooling provided it has not been boiled too long. 

 Place the test-tube in cold water to cool it. 



(d.) Render some of the starch solution alkaline by add- 

 ing caustic soda solution. Add iodine solution. No blue 

 colour is obtained. 



(e.) Acidify (d.) with dilute sulphuric acid, then add iodine 

 = blue colour is obtained. 



(/.) To another portion of the solution, add a few drops 

 of dilute cupric sulphate and caustic soda (or Fehling's 

 solution), and boil = no reaction (compare " Grape Sugar"). 



(g.) Add tannic acid = yellowish precipitate, which dis- 

 solves on heating. 



3. Starch is a Colloid. Place some strong starch solution in a 

 dialyser or parchment-tube, and the latter in distilled water. 

 Allow it to stand for some time, and test the water for starch ; 

 none will be found. 



4. II. Dextrin (C 6 H 10 O 5 ). 



Prepare a Solution. Dissolve some dextrin in boiling water, 

 and observe that the solution is not opalescent. ^ 



(a.) This proves its solubility in water. 



