III.] THE CARBOHYDRATES. 1 9 



volume of water in the other. Add to both an eqnal voUime of 

 sohition of iodine, and thus compare the difference in colour.] 



(c.) Precipitate some of its solution by adding alcohol. 



(d.) Render some of the dextrin solution alkaline by adding 

 caustic soda solution. No red-brown colour is obtained with 

 iodine. Acidify and the reddisli-brown colour appears. 



(e.) It is not precipitated by basic acetate of lead alone (unlike 

 glycogen). 



(/.) Precipitation occurs on adding ammonia and basic acetate of 

 lead. The ammonia gives a wliite precipitate with lead acetate 

 which carries down dextrin. 



There are several varieties of dextrin : — - 



5. Prepare Dextrin from Starch. — Make lo grams of starch into a paste 

 with 20 cc. of water, add 30 cc. of a 20 per cent, solution of sulphuric acid. 

 Mix, and heat in a water-bath at 90° C. Cool and precipitate the dextrin by 

 alcohol. Collect the white deposit, wash with alcohol, and dry it. 



6. III. Cellulose {CqRiqO^)7i occurs in every tissue of the higher plants, 

 where it forms the walls of cells, and the great mass of the hard parts of 

 wood. Cotton-wool may be used to test its reactions. 



(a.) It is insoluble in water and all the feebler solvents. 



{b.) It is soluble in Schweitzer's reagent, or a solution of ammonio-cupric 

 oxide. This is prepared by dissolving slips of copper in ammonia in an open 

 flask, or by dissolving precipitated hydrated oxide of copper in 20 per cent, 

 ammonia. The former is prepared by precipitating a solution of sulphate of 

 copper by soda in the presence of ammonium chloride, 



(c.) It is soluble in concentrated acids, and a gelatinous precipitate — called 

 amyloid — falls on the addition of water. The substance precipitated gives 

 a blue colour with, iodine. It is also soluble in zinc chloride. 



{d. ) It gives a blue colour with sulphuric acid and iodine, but not with the 

 latter alone. 



7 IV. Glycogen or Animal Starch n{CQE-^fi^). — Prepare a 

 solution (see "Liver"). Note the characters of the dry white 

 powder. 



(a.) Note that the solution is opalescent (unUke dextrin) ; add 

 iodine solution = red-brown or port- wine red colour. As in the 

 dextrin test, use two test-tubes, one with water and the other 

 with glycogen, to compare the difference in colour. The colour 

 disappears on heating and reappears on cooUng. It also dis- 

 appears on the addition of alkalies, which break up the feeble 

 compound. 



{fi.) Add caustic soda and copper sulphate solution = a blue 

 solution, boil = no reduction. 



((?.) Add basic lead acetate = a precipitate (unUke dextrin). 



(6?.) Add ammonia and basic lead acetate = a precipitate, as in 1 /. 



{^\) Boil with dilute hydrochloric acid = a reducing sugar. 

 Neutrahse the acid with dilute caustic soda, and test with Fehling's 

 eoUition for a reducing sugar, dextrose = a yellow precipitate. 



