CARBOHYDRATES. 353 



Gum-arabic dissolves slowly in 2 parts of water ; this solution 

 shows an acid reaction with litmus, and yields precipitates with lead 

 acetate or ferric chloride. 



Cellulose, C 6 H 10 O 5 , perhaps C 18 H 30 O 15 (Plant fibre, Lignine). Cellu- 

 lose constitutes the fundamental material of which the cellular mem- 

 brane of vegetables is built up, and forms, therefore, the largest 

 portion of the solid parts of every plant; it is well adapted to this 

 purpose on account of its insolubility in water and most other sol- 

 vents, its resistance to either alkaline or acid liquids, and its tough 

 and flexible nature. Some parts of vegetables (cotton, hemp, and 

 flax, for instance) are nearly pure cellulose. 



Pure cellulose is a white, translucent mass, insoluble in all the 

 common solvents, but soluble in an ammoniacal solution of basic 

 cupric carbonate ; it is not colored blue by iodine. 



Treated with concentrated sulphuric acid it swells up, and gradu- 

 ally dissolves ; water precipitates from such solutions a substance 

 known as amyloid, which is an altered cellulose giving a blue color 

 with iodine. Upon diluting the sulphuric acid solution with water 

 and boiling it, the cellulose is gradually converted into dextrin and 

 dextrose. 



Unsized paper (which is chiefly cellulose) dipped into a mixture 

 of two volumes of sulphuric acid and one volume of water, forms, 

 after being washed and dried, the so-called " parchment paper," 

 which possesses all the valuable properties of parchment. 



Pyroxylin, Pyroxylinum, C 6 H 8 O 3 (NO 3 ) 2 ( Cellulose dinitrate, Sol- 

 uble gun-cotton, Nitro-cellulose). By the action of nitric acid of 

 various strengths on cellulose, three different substitution products 

 (possibly compound ethers) may be obtained, which are distinguished 

 as cellulose mono-, di-, and trinitrate : 



C 6 H 10 5 + HN0 3 = H 2 + C 6 H 9 4 N0 3 . 

 C 6 H 10 5 + 2HN0 3 = 2H 2 + C 6 H 8 O 3 (NO 3 ) 2 

 C 6 H 10 5 + 3HN0 3 t= 3H 2 + C 6 H 7 O 2 (NO 3 ) 3 . 



Cellulose trinitrate is the highly explosive gun-cotton ; an intimate 

 mixture of gun-cotton and camphor is now extensively used under 

 the name of celluloid. Cellulose dinitrate or pyroxylin is soluble in 

 a mixture of ether and alcohol ; this solution is known as collodion. 



Experiment 54. Immerse 2 grammes of dry cotton for ten hours in a pre- 

 viously cooled mixture of 28 c.c. of nitric acid and 44 c.c. of sulphuric acid. 

 Wash the pyroxylin thus obtained with cold water until the washings have no 



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