THE CARBOHYDRATES 35 



coloration with iodine. The composition of the dry substance, 

 free from ash constituents, corresponds approximately to the 

 formula, C 6 H 10 O 5 . Quince seed mucilage, on hydrolysis 

 with dilute sulphuric acid, 1 yields (i) an insoluble portion 

 (cellulose), (2) a soluble portion precipitated by alcohol (gum), 

 and (3) glucose which is not precipitated by alcohol. 



The mucilaginous products obtained by hydrolysis of 

 pecto-celluloses (p. 29) are precipitated from solution on 

 addition of alcohol in the form of a white, tasteless, stringy, 

 gelatinous mass called pectin. It shrinks greatly on drying, 

 and becomes very brittle. On prolonged boiling with water 

 it changes into an insoluble form called pectic acid. This 

 is the essential constituent of fruit jellies and jams. On 

 further boiling, the pectic acid dissolves and the solution, 

 which has a sour taste and reddens litmus, gives no precipitate 

 with alcohol. This product is called meta-pectic acid. 



The hypothetical mother substance, pectose, from which 

 pectin, pectic acid, and meta-pectic acid were supposed to 

 be derived, may now be regarded as identical with the actual 

 pecto-celluloses. These probably occur in unripe fruits, and 

 are gradually changed into pectic acid and meta-pectic acid 

 as the fruit becomes first ripe and then rotten. 



Substances which, like gums and mucilages, yield pentose 

 sugars on hydrolysis are called pentosanes to distinguish them 

 from the amyloses which yield hexose sugars, and are there- 

 fore called hexosanes. The pentosanes yield furfural on 

 boiling with hydrochloric acid. 



C 5 H 10 5 -> C 5 H 4 2 + 3 H 2 0. 

 (Cf. Oxycelluloses, p. 28.) 



APPENDIX TO CHAPTER III 



OPTICAL ACTIVITY 



ANY substance which causes rotation of the plane of polarised light 

 is said to be optically active. This property has been observed 

 only in substances which contain an asymmetric carbon atom, i.e. 



1 Cross and Bevan. 



