1 8 PRACTICAL PHYSIOLOGY. [ill 



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

 Filter, and test the filtrate with iodine. It gives no blue colour. 



(h.) Boil starch with water = opalescent solution, which if strong 

 gelatinises or sets on cooling = starch paste. 



(f.) Add a solution of iodine^ = a blue colour, which disappears 

 on heating (the iodide of starch is dissociated by 

 heat) and reappears on cooling — provided it has not 

 been boiled too long. Direct a stream of cold water 

 upon the test-tube to cool it. 



{(1.) Render some of the starch solution alkaline 

 by adding caustic soda solution. Add iodine solu- 

 tion. ^0 blue colour is obtained. 



(e.) Acidify ('/.) with dilute sulphuric acid, then 



add iodine = blue colour is obtained. 



Fio. 4. — Potato (/.) To anotlier portion of the solution add a 



uie^^ view^'n ^®"^ drops of dilute cupric sulphate and caustic soda, 



polarised light and boil = no reaction (compare " Grape-sugar "). 



Kicoi's, x^cS^ (O-) To another portion of the solution add 



Fehhng's solution, and boil = no reaction. 

 (h.) Add tannic acid = yellowish precipitate, which dissolves 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. 



(a.) Does not filter. — Two dry filter papers are placed in two funnels about 

 5 cm. in diameter and filled with 2 per cent, solution of starch. Let one re- 

 main as a control, and to the other add any diastatic ferment — e.g., saliva 

 or liquor pancreaticus. The starch begins to filter, being converted into sugar. 



4. II. Dextrin (British Gum) (CgHjoOg) is an intermediate pro- 

 dact in the hydration of starch. There are two varieties — Erythro- 

 dextrin, which gives a red colour with iodine ; and Ach- 

 roodextrin, which gives no colour with iodine solution (see 

 *' Saliva "). 



Examine its naked eye characters. It is gummy and amor- 

 phous. Smell it. Dissolve some dextrin in boihng water, and 

 observe that the solution is not opalescent. 



(a.) This proves its solubility in water. 



(/;.) Add iodine solution = reddish-brown colour, which disap- 

 pears on heating and returns on coohng. [The student ought to 

 use two test-tubes, placing the dextrin solution in one, and an equal 



1 Solution of Iodine. — Dissolve 2 grams of potassic iodide in 100 cc. of 

 water, add i gram of iodine, and shake well. 



