CARBOHYDRATES. 347 



46. CARBOHYDRATES. 



Constitution. The term carbohydrates or carbhydrates is not 

 well chosen, because it implies that these substances are carbon in 

 combination with water. Carbohydrates do contain hydrogen and 

 oxygen in the proportion of two atoms of hydrogen to one atom of 

 oxygen, or in the proportion to form water, but this does not exist as 

 such in the carbohydrates. 



The true atomic structure of carbohydrates is as yet but little 

 known. The compounds of the composition C 6 H 12 O 6 are now looked 

 upon as the aldehyde of the hexatomic alcohol mannite, C 6 H 14 O 6 , the 

 chief constituent of manna : 



C 6 H U 6 - 2H : C 6 H 12 6 . 



Mannite itself is formed from the saturated hydrocarbon C 6 H 14 , 

 by replacement of 6 atoms of hydrogen by 6OH ; its constitutional 

 formula is, therefore, (C 6 H 8 ) vi .(OH) 6 . 



Carbohydrates generally contain 6 atoms of carbon or a multiple 

 of 6. 



Properties. Carbohydrates are either fermentable, or can, in most 

 cases, be converted into substances which are capable of fermentation. 

 They are not volatile, but suffer decomposition when sufficiently 

 heated ; they have neither acid nor basic properties, but are of a neu- 

 tral reaction. Oxidizing agents convert them into saccharic and 

 mucic acids and finally into oxalic acid. (Soluble carbohydrates 

 have the property of bending the plane of polarized light.) 



Most carbohydrates are white, solid substances, and, with the ex- 

 ception of a few, soluble in water. The members of the first two 

 groups (glucoses and saccharoses) have a more or less sweet taste. 

 Many of them, especially glucoses, are good reducing agents, as is 

 shown by the fact that they deoxidize in alkaline solution salts (or 

 oxides) of copper, bismuth, mercury, gold, etc., either to a lower state 

 of oxidation or to the metallic state. 



Occurrence in nature. No other organic substances are found in 

 such immense quantities in the vegetable kingdom as the members 

 of this group, cellulose being a chief constituent of all, starch and 

 various kinds of sugar of most plants. Carbohydrates are also found 

 as products of animal life, as, for instance, the sugar in milk, in bees' 

 honey, etc. 



