168 PEACTICAL PHYSIOLOGY 



or two of liquor iodi : a blue colour results which disappears on heat.ng, 

 and returns again on cooling. Excessive heat must be avoided, since 

 the iodine is volatile. 



Starch granules also give this reaction under the microscope, as does 

 the cut surface of a potato. 



Hydrolysis can be effected by boiling with a weak acid or by the 

 action of ferments ptyalin and amylopsin (see above), and malt 

 diastase (see Advanced Course). 



EXPERIMENT XIII. Place some starch solution in a flask and add 

 to it a few drops of 25% sulphuric acid : boil for about a quarter of an 

 hour. Now apply the iodine test and notice that instead of a blue a 

 port wine colour is produced (due to dextrine). Apply Trommer's 

 test, and note that reduction is effected. 



The sugar produced by hydrolysing with an acid is dextrose, whereas 

 that produced by ferment action is maltose (see Advanced Course). 



Glycogen (C fi H 10 5 ) w . Just as plants store up excess of carbohydrate 

 in the form of starch, so do animals store it in the form of glycogen. 

 The chief seats of this storage are the liver and muscles, but in the 

 embryo it is more widely distributed. 



EXPERIMENT XIV. A simple method for the preparation of glycogen 

 is that introduced by Frankel. It consists in grinding up the fresh 

 liver in a mortar with about three times its volume of a 3 per cent, 

 solution of tri-chloracetic acid. This reagent coagulates the proteids. 

 The glycogen contained in the extract is precipitated by 90 per cent, 

 alcohol. Filter. Dissolve some of the glycogen in water and notice 

 that the solution is opalescent. Add to this a drop or two of iodine 

 solution. A port-wine colour results, which disappears on heating, and 

 returns on cooling. 



EXPERIMENT XV. Add some basic lead acetate; a precipitate results. 



EXPERIMENT XVI. Try Trommer's test ; no reduction occurs, but 

 the Cu(OH) 2 is kept in solution. 



EXPERIMENT XVII. To some of the solution add a few drops of 

 25% H 2 S0 4 , and boil for about ten minutes; dextrose is produced, 

 and reduction now occurs. 



Dextrine (C 6 H 10 5 ) n . During the hydrolysis of starch dextrine is 

 formed as an intermediate product. British gum is dextrine produced 

 by heating starch to 200 C. This substance is much employed in the 

 manufacture of envelopes. 



Dextrine is a fawn-coloured amorphous powder, soluble in cold water 

 and forming a dear solution with which the following reactions can be 

 obtained : 



