PRACTICAL EXERCISES 23 



be soaked for a few minutes in a dilute solution of cupric sulphate. 

 Then the excess of the cupric sulphate should be poured off, and 

 sodium hydrate added, when the coagulated proteid will become 

 violet. Heat-coagulated proteids are insoluble in water, weak acids 

 and alkalies, and saline solutions, but fibrin is slightly soluble in the 

 latter. 



Carbo-hydrates. 



1. Glucose or Dextrose. Make a solution of dextrose in water, 

 and apply to it Trommer's test for reducing sugar. Put some of 

 the dextrose solution in a test-tube, then a few drops of cupric 

 sulphate, and then excess of sodium or potassium hydrate. The 

 blue precipitate of cupric hydrate which is first thrown down is 

 immediately dissolved in the presence of dextrose and many other 

 organic substances. Now boil the blue liquid, and a yellow or red 

 precipitate (cuprous hydrate or oxide) is formed. 



2. Cane-sugar. Perform Trommer's test with a sample of a solu- 

 tion. A blue liquid is obtained, which is not changed on boiling. 

 Now put the rest of the solution in a flask. Add -^th of its bulk of 

 strong hydrochloric acid, and boil for a quarter of an hour. Again 

 perform Trommer's test. It shows that much reducing sugar is now 

 present. The cane-sugar has been ' inverted,' i.e., changed into a 

 mixture of dextrose and levulose. 



3. Starch. (i) Cut a slice from a well-washed potato; take a 

 scraping from it with a knife, and examine with the microscope. 

 Note the starch granules with their concentric markings, using a 

 small diaphragm. Run a drop of dilute iodine solution under the 

 cover-slip, and observe that the granules become bluish. Examine 

 also with a polarization microscope. (2) Rub up a little starch in a 

 mortar with cold water, then add boiling water and stir thoroughly. 

 Decant into a capsule or beaker, and boil for a few minutes. After 

 the liquid has cooled, perform the following experiments : 



(a) Add a few drops of iodine solution to a little of the thin starch 

 mucilage in a test-tube. A blue colour is produced, which disappears 

 on heating, returns on cooling, is bleached by the addition of a little 

 sodium hydrate, and restored by dilute acid. 



(b) Test the starch solution for reducing sugar by Trommer's 

 test. If none is found, boil some of the mucilage with a little 

 dilute sulphuric acid in a flask for twenty minutes, and again 

 perform Trommer's test. Abundance of reducing sugar will now be 

 present. 



4. Dextrin. Dissolve some dextrin in boiling water. Cool. Add 

 iodine solution to a portion ; a reddish-brown (port-wine) colour 

 results, which disappears on heating and returns on cooling. The 

 colour is also bleached by alkali, restored by acid. If too little 

 iodine has been added there may be no restoration of the colour by 

 the acid. The addition of a little more iodine to the acid solution 

 will then cause the port-wine colour to return, arid this may be again 

 bleached by alkali, and will now be restored by acid. 



5. Glycogen. Seep. 511. 



