THE CARBOHYDRATES 29 



cellulose, C 12 H 2 oO 10 -{- 2NaOH. This compound is com- 

 pletely decomposed by water, and partially by alcohol. With 

 the latter reagent only half the alkali is removed. 



Cotton cellulose dissolves readily in ammonia cupric 

 oxide, 1 and is reprecipitated in the gelatinous form on neutralis- 

 ing the ammonia with acids, or even by diluting with water. 

 It is also dissolved by concentrated (40 per cent.) solutions of 

 zinc chloride, and is reprecipitated on diluting. The precipitated 

 hydrates give a blue coloration with iodine. A reagent made 

 by adding iodine dissolved in potassium iodide to the zinc 

 chloride solution gives the blue colour by direct action on the 

 cellulose. 



Judging from the various reactions and properties of the 

 compound, Cross and Bevan 2 have proposed the following 

 formula as 'probably representing the constitution of the 

 cellulose molecule : 



/C(OH) 2 -C(OH) 2X 

 0=C< >CH 2 



\CH 2 CH/ 



It is probable, however, that it may be a multiple or 

 aggregate of such groups. 



Flax is not a simple substance like cotton, but a com- 

 bination. It is perfectly homogeneous, but when boiled with 

 alkali solutions it is resolved into cellulose and a mucilaginous 

 product called pectic acid. The cellulose is practically 

 identical with that of the cotton type in all except that it 

 is slightly less resistant to the action of hydrolytic agents. 

 The original flax appears to be a genuine chemical compound 

 not a mere adventitious association of this cellulose with 

 the group which is hydrolysed to pectic acid. 



Such compounds are of common occurrence in plants and 

 form a natural sub-group called pecto-celluloses or muco-cellu- 

 loses. These substances, which are probably identical with 

 the hypothetical pectose (p. 34), prevail in monocotyledons, 



1 To make this reagent, freshly precipitated cupric hydrate is dissolved 

 in strong ammonia solution. 



2 Cellulose. 



