PART III 



XXIX. THE CARBOHYDRATES 



In studying the chemistry of the principal constituents of the 

 earth's crust, oxygen is the element to which direction is 

 constantly directed : it is the oxides of the elements, and the 

 compounds of these oxides with each other, which form i-ts 

 principal constituents ; it is the properties and composition of 

 the oxides which especially serve to differentiate the elements. 

 In studying the chemistry of the animal and vegetable king- 

 doms, carbon is the element round which interest centres, and 

 it is the compounds of carbon with hydrogen, oxygen, and 

 nitrogen, obtained directly or indirectly from animal or veget- 

 able products, with which Part III. of these studies will 

 especially deal. If any dry animal or vegetable substance be 

 heated in a test-tube the substance chars, indicating that 

 carbon is a constituent. 



With respect to the constituents of plants, there are three 

 substances especially characteristic, viz. — (i) cellulose^ the 

 substance of which the fibre of plants mainly consists, and 

 which constitutes the membrane of the cells ; (2) starchy a 

 granular substance produced by all green plants, and deposited 

 in large quantities in the root, stem, or seed of certain plants ; 

 and (3) sugar ^ a soluble substance existing in the sap. Dry 

 specimens of these in the water oven and then heat in test- 

 tubes. Note that they decompose, charcoal and water being 

 produced, showing that carbon, hydrogen, and oxygen are 



136 



