CARBOHYDRATES. 37 



one molecule of water. This conception corresponds to the empirical 

 formulae: 



Ci 2 H 22 Oii = (C 6 H 12 6 ) 2 - H 2 or Ci 2 H 22 On + H 2 O = 2 (C 6 Hi 2 O 6 ). 



They correspond in their behavior to the monosaccharides. Cane- 

 sugar is an exception to this rule, as its alkaline solution is not capable of 

 reducing metallic oxides, whereas the other disaccharides retain entirely 

 the properties of the aldehyde alcohols. The exceptional behavior of 

 cane-sugar has been ascribed to a protected position of its aldehyde and 

 ketone groups. 



To this group belong sugars which are found in nature as such, besides 

 other sugars which are formed by cleavage from sugars of higher molecular 

 weight. Cane-sugar, milk-sugar, and maltose are of especial importance, 

 whereas the remaining members of the group are, according to their 

 occurrence and importance, of only limited interest. Worthy of men- 

 tion are trehalose, first found in ergot of rye ; gentiobiose, obtained from 

 gentianose by partial hydrolysis produced by invertin or very dilute 

 sulphuric acid; and cellose (cellobiose), which is believed to stand in the 

 same relation to cellulose as maltose to starch, but has not yet been found 

 in the vegetable kingdom. Melibiose is another hexobiose, and is formed 

 by partial inversion of either melitriose or of rafrinose, either by the action 

 of dilute acids or by certain varieties of yeast. 



Before discussing the above-mentioned, more important disaccharides, 

 we must mention two other hexobioses which have been obtained in a 

 peculiar manner, namely, isomaltose and isolactose. The former was 

 synthesized by Emil Fischer from grape-sugar by the action of cold, fuming 

 hydrochloric acid, and is said to be formed, on the other hand, together 

 with maltose by the breaking down of starch. It must be mentioned, 

 however, that the identification of isomaltose in most cases has not been 

 entirely satisfactory, so that we are still unable to tell much about the 

 building up and breaking down of carbohydrates in plant and animal 

 organisms, and especially because it seems probable that quite a number 

 of different products have been designated as maltose by various inves- 

 tigators. Isomaltose excited the interest of biologists in particular when, 

 in 1898, A. C. Hill l succeeded in obtaining it from grape-sugar by the aid 

 of the maltoglucase of yeast, and likewise by the so-called taka-diastase 

 from Aspergillus oryzae. Hill added the ferment to concentrated solutions 

 of grape-sugar, and showed that the fermentation reaction was to some 

 extent a reversible process. Hill himself regarded the product formed 



1 J. Chem. Soc. 73, 634 (1898) ; Ber. 34, 1380 (1901) ; Proc. Chem. Soc. 19, 99 (1901) ; 

 17, 184 (1901). 



