152 THE CARBOHYDRATES 



2. Acids. Nitric acid (sp. gr. 1-25) at 1 80 converts cellu- 

 lose into oxycellulose, a substance of a weak acidic character, 

 which reduces Fehling's solution (see below under oxidizing 

 agents). Concentrated nitric acid, or a mixture of this acid 

 with concentrated sulphuric acid, converts cellulose into nitrates, 

 the composition of which varies with the conditions of the 

 experiment ; di-, tri-, tetra-, penta- and hexa-nitrates,* which 

 are of considerable technical importance, are known. If dilute 

 sulphuric acid is allowed to act for some hours at 1 00 C. 

 on cotton, it does not alter the structure of the fibre, but 

 makes it friable. This was at one time thought to be due to 

 the formation of a definite substance, hydrocellulose, but, ac- 

 cording to Stern, f the elementary composition of the cellulose 

 is not altered ; the friability is explained by the fact that cer- 

 tain portions of the fibre are more easily attacked than others, 

 and when these are converted into soluble products, the whole 

 fibre falls to pieces. 



Concentrated sulphuric acid dissolves cellulose, gradually 

 converting it into dextrin and ultimately 'into dextrose. If 

 the solution, as soon as made, is diluted with water, a gelatinous 

 hydrate is precipitated ; \ this substance is known as amyloid, 

 since it resembles starch in giving a blue colour with iodine. 

 The same substance is formed by the action of chlorzinc iodide, 

 the reaction being used as a test for cellulose. 



When boiled with 40 per cent hydrochloric, or 73 per 

 cent sulphuric acid, cellulose is hydrolysed with formation 

 of esters of polysaccharoses, II which contain acidic hydroxyl 

 groups. The claim made by Willstatter and Zechmeister IF 

 that cellulose can be quantitatively hydrolysed to glucose is 

 therefore inadmissible. 



The combined action of glacial acetic acid and acetic 



* See footnote, p. 159. 



f Stern: " J. Chem. Soc.," 1904, 84, 336. 



jThis reaction is made use of in the preparation of parchment paper. For 

 this purpose paper is rapidly drawn through a mixture of four parts of concen- 

 trated sulphuric acid with one part of water; the paper is then thoroughly 

 washed with water until it is free irom acid. 



This substance must not be confused with a compound of the same name 

 which occurs naturally in several plants (cf. p. 139). 



|| Cunningham : "J. Chem. Soc.," 1918, 113, 173. 



M Willstatter and Zechmeister : " Ber. deat. chem. Gesells.," 1913, 46, 2401, 



