THE SUGARS. 359 



Starch, C 6 H i(A 



Cane-sugar, C ] . 2 H 2 ,O 11 



Grape-sugar and fruit-sugar, C 6 H 12 O 6 

 Thus, 



2C 6 H 10 5 + H 2 = C 12 H, 2 O n 

 Starch. Water. Cane-suar. 



C ]2 H. 2 . 2 O n + H 2 == C 6 H 12 6 + C 6 H 12 6 



Cane-sugar. "\Vater. Grape-sugar. Fruit-sugar. 



The three following classes of sugars, based upon their rela- 

 tions to fermentation, have been made: (1) directly fermenta- 

 ble, (2) indirectly fermentable, (3) non-fermentable sugars. To 

 the third class belong Arabinose, Sorbit, etc., which need no 

 further notice here. 



The directly fermentable sugars are grape-sugar, fruit-sugar, 

 and inverted sugar. 



942. Grape-sugar^ otherwise termed glucose (or, on account 

 of its turning the plane of polarization to the right, dextrose), is, 

 as its name indicates, abundant in the grape, where it may form 

 from 10 to 30 per cent of the juice. Figs contain, on an aver- 

 age, 12 per cent; sweet cherries, 9 to 10 percent; apples and 

 pears, 7 to 10 per cent; plums, 2 to 5 per cent; and peaches 

 less than 2 per cent of this sugar. 



943. Fruit-sugar, sometimes known as laevulose, is uncrys- 

 tallizable. It is associated in most ripe fruits with dextrose. 



944. Inverted sugar occurs in some ripe fruits, where, as 

 Buignet has shown, it is formed from cane-sugar by the action 

 of a ferment and not of a fruit-acid. It is also found in the 

 so-called honey-dew of the leaves of the Linden. 1 



945. The indirectly fermentable sugars, of which common 

 cane-sugar may be taken as the best example, ferment under the 

 influence of yeast only when they have first undergone a change 

 by which they are converted into other sugars. 



946. Cane-sugar occurs in the cell-sap of many plants, often 

 in large amount. The following percentages are regarded as 

 average ones : 



1 According to Bouasiugault, 120 square metres of linden leaves yield hi 

 a single warm July day between two and three kilograms of honey-dew. As 

 to whether this substance is a product of an insect, or an exudation from leaves 

 under peculiar conditions, is not yet settled (Ebermayer : Chernie der Pflanzen, 

 1882, p. 255). 



