6o THE CARBOHYDRATES 



(b) Fiicose. — Fucose, which is isomeric with rhamnose, may 

 be obtained by the hydrolysis of sea-weeds by means of di- 

 lute sulphuric acid ; it crystallizes in microscopic needles, and 

 yields a hydrazone, m.p. 172-173°. 



{c) Quinovose, another methyl pentose isomeric with rham- 

 nose, is produced by the hydrolysis of quinovite, a substance 

 formed by boiling quinovin contained in the bark of Cascarilla 

 hexandra with alcohol and hydrochloric acid. 



B. HEXOSES. 



There are no convenient general reactions for distinguish- 

 ing hexoses from any other group of sugars, but each of the 

 hexoses occurring in nature are readily identified by character- 

 istic reactions. 



GLUCOSE OR DEXTROSE. 

 The substance which is commonly known as grape sugar 

 occurs, together with levulose or fruit sugar, in a number of 

 sweet fruits, in honey, and in the seeds, leaves, roots, and 

 blossoms of a great many of the higher plants ; monosac- 

 charides also obtain in lower plants ; thus, Hunger describes 

 them as occurring in small granules near the plastids in 

 Dictyota. Glucose is formed by the hydrolysis of cane sugar, 

 of glucosides, and of many polysaccharides, such as starch, 

 cellulose, etc. 



Preparation of Glucose. 



The most convenient source for the preparation of glucose 

 on a small scale is cane sugar. One hundred and twenty c.c. 

 of 90 per cent alcohol mixed with 5 c.c. of fuming hydro- 

 chloric acid are heated at 45-50" ; 40 grams of powdered cane 

 sugar are now added, the mixture being kept thoroughly 

 stirred. After two hours the solution is allowed to cool, and 

 a little anhydrous glucose is added to induce crystallization. 

 In the course of a few days the resulting crop of crystals is 

 filtered off and washed with a little dilute alcohol ; it is recrys- 

 tallized by dissolving in half its weight of warm water and 

 adding twice as much 90-95 per cent alcohol, filtering warm 

 and setting aside to cool. 



On a commercial scale glucose is best prepared by heating 



