THE CARBOHYDRATES 17 



occur in agricultural products, viz. the hexoses, glucose, and 

 fructose, and the bi-hexoses, sucrose, lactose, and maltose, 

 and only those will now be considered. 



Hexoses. Glucose and fructose, when submitted to ultimate 

 analysis, give exactly the same results, viz. 



Per cent. 



Carbon 40' i 



Hydrogen 6'6 



Oxygen 53*3 



TOO'O 



The empirical formula C 6 H 12 O 6 is obtained from these 

 figures and the molecular weight of the compounds. The 

 grouping of the atoms in the molecules is deduced from the 

 reactions and properties of the substances. Thus it appears 



(1) That they are both pentahydric alcohols. 



On treatment with CaO, BaO, and PbO, they form saccha- 

 rates, corresponding to the alcoholates, in which alcoholic 

 hydrogen is replaced by the basic elements. These compounds 

 are decomposed by CO 2 . 



They also form esters, i.e. compounds in which the alcoholic 

 hydroxyl is replaced by acid radicles. The pentacetate 

 (C 2 H 3 O2)5C 6 H 7 marks the limit of acetylation, and it is 

 evident, therefore, that the sixth atom of oxygen is differently 

 combined. 



Esters of this kind with various organic acids are of 

 common occurrence in plants. They are decomposed on 

 treatment with hydrolytic agents and also by the action of 

 ferments which are always associated with them in the plants. 

 They yield glucose as one of the products and are therefore 

 called glucosides (p. 87). 



(2) They are either aldehydes or ketones in respect of the 

 sixth atom of oxygen. 



Reducing agents convert them into hexahydric alcohols, 

 sorbite and mannite. 



They react with phenylhydrazine, forming hydrazones, 

 compounds in which the aldehydic or ketonic oxygen is 

 replaced by the hydrazine radicle. 



c 



