310 CONSIDERATION OF CARBON COMPOUNDS. 



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 6 , the chief constituent of manna : 



C 6 H U 6 - 2H = C 6 H 12 o 6 . 



Mannite itself is formed from the saturated hydrocarbon 

 C 6 H 14 , by replacement of 6 atoms of hydrogen by 6HO ; its 

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



Carbohydrates generally contain 6 atoms of carbon or a mul- 

 tiple of 6. 



Properties. Carbohydrates are either fermentable or can 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 neutral reaction. Oxidizing agents convert them into sac- 

 charic and mucic acids and finally into oxalic acid. (Soluble 

 carbohydrates have the property of bending the plane of polar- 

 ized light.) 



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

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

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

 taste. 



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. Carbo- 

 hydrates are also found as products of animal life, as, for in- 

 stance, the sugar in milk, in bees' honey, etc. 



