Thus, 



THE SUGARS. 359 



Starch, C,U,fi^ 



Cane-sugar, CijHjjOu 



Grape-sugar and fruit-sugar, CuHijOg 



2C,H,„0, + H,0 = C,,H^O,, 



Starch. Water. Cane-sugar. 



Cane-sugar. Water. 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) indirectlj' lermentable, (3) non-fermentable sugars. To 

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

 further notice here. 



The dii'ectlj' fermentable sugars are grape-sugar, fruit-sugar, 

 and inverted sugar. 



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

 of its turning tlie 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 per cent ; 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 Isevulose, is uncr3S- 

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



944. Inverted sugar occurs iu 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 iu the 

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



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 Ihey have first undergone a change 

 by which they are converted into other sugars. 



946. Cane-sugar occurs in the cell-sap of man}' plants, often 

 in large amount. The following percentages are regarded as 

 average ones : — 



1 According to Boussingault, 120 square metres of linden leaves yield in 

 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 : Chemie der Pflanzen, 

 18S2, p. 255). 



