DISACCHARIDES. 217 



and LEFEVRE J have based their quantitative method for the estimation 

 of glucuronic acid. 



Glucuronic acid is best prepared from euxanthic acid, which decomposes 

 on heating it with water to 120 C. for several hours. The filtrate from 

 the euxanthon is concentrated at 40 C., when the anhydride gradually 

 crystallizes out. On boiling the mother-liquor for some time and evaporat- 

 ing further, the crystals of the lactone are obtained. In regard to the 

 quantitative estimation of glucuronic acid we must refer the reader to 

 the works of TOLLENS and his collaborators and of NEUBERG and NEI- 



MANN. 2 



2. Disaccharides. 



Some of the varieties of sugar belonging to this group occur ready 

 formed in nature. Thus we have saccharose and lactose. Some, on the 

 contrary, such as maltose and isomaltose, are produced by the partial 

 hydrolytic cleavage of complex carbohydrates. Isomaltose is also obtained 

 from dextrose by reversion (see page 219). 



The disaccharides or hexobioses are to be considered as glucosides, 

 each of which is derived from two monosaccharides with the exit 

 of 1 molecule of water. Corresponding to this, their general formula 

 is Ci 2 H 2 2On. On hydrolytic cleavage and the addition of water they 

 yield 2 molecules of hexoses, either 2 molecules of the same hexose or 

 one each of two different hexoses. Thus 



Saccharose + H 2 O = dextrose + levulose ; 

 Maltose + H 2 O = dextrose + dextrose ; 

 Lactose + H 2 O = dextrose -f galactose. 



The configuration of the dissaccharides has not been determined with 

 positiveness. 



The levulose turns the polarized ray more to the left than the dex- 

 trose does to the right; hence the mixture of hexoses obtained on the 

 cleavage of saccharose has an opposite rotation to the saccharose itself. 

 On this account the mixture is called INVERT-SUGAR, and the hydrolytic 

 splitting is designated as inversion. This term, " inversion/' is not only 

 used for the splitting of saccharose, but is also used for the hydrolytic 

 cleavage of compound sugars into monosaccharides. The reverse reaction, 

 whereby monosaccharides are condensed into complex carbohydrates, is 

 called reversion. 



We subdivide the disaccharides into two groups, first, the group to 

 which saccharose belongs, where the members do not have the property 



1 Ber. d. d. chem. Gesellsch., 40. 



2 Tollens, Zeitschr. f. physiol. Chem., 44, which cites also the older work; Neuberg 

 and Neimann, ibid., 44; Neuberg, ibid., 45. 



