ON THE SUGAR OP GRAPES. 343 



When the juice of grapes is boiled down as far as can its rob. 

 be done without danger of altering its qualities, it affords a 

 rob, the quantity of which is proportional to the saccharine 

 quality of the grape, and varies from 18 to 32| per cent. 

 It is difficult however to avoid some degree of empyreuma, 

 particularly if the juice be acidulous. This alteration di- 

 minishes the quality of fermenting in the rob redissolved in 

 water, though without annihilating it, as Beccher had 

 concluded from his experiments. The liquid sugar of the 

 cane tao, as Duthrone informs us, is much sooner altered 

 by boiling than the solid sugar. 



The rob boiled down to a certain point crystallizes in a Crystallizes. 

 short time. It congeals into a spongy mass, more or less 

 moistened with a sirup, that has a tendency to drain ofr. 

 Its crystals, when drained, are a mixture of tartar and cry- 

 stallizabie sugar. It was this product, extracted from the 

 muscat grape of Fuencarral, which, after halving under- 

 gone a few purifications, led me, instructed as I was by 

 Duthrone's excellent work on sugar, to treat the juice ef 

 grapes like that of the sugar-cane, 



As the juice contains acids, that hinder the extraction of Mode of ex- 

 the sugar, the first step is to free it from these. After the ^'■^^'^'"S *'**^ 

 must has been scummed, and while it is nearly boiling, a 

 lixivium of wood ashes is to be added by little and little, as 

 long as any effervescence takes place. The acids may be 

 known to be saturated by tasting the liquor, which will 

 then have only a saccharine taste. It is then to be boiled 

 down to about half, and left to cool in vats, or even in the 

 copper boilers, for there is no danger of verdigrease as in -^>-ie» 



preparing the" rob. While it thus stands, the tartar and 

 citric acid, if there were any, being converted into salts of 

 difficult solution, subside with the excess of the ashes, and 

 che sulphate of lime that was in the juice of the grape. 

 The malic acid, converted into malate of lime, remains in 

 the liquor in consequence of its great solubility. 



The must prepared in this way indicates 25° or 26 "^ on Not to be boiled 

 the areometer. If it were boiled down beyond this, the **^^ "^"^^* 

 subsequent clarification would not be so easy, on account ciaiification. 

 ^f its thickness. It is then to be beaten up with whites of 

 ggs or bullocks blood, heated, scummed, filtered, and 



boiled 



