312 PHYSIOLOGY AT THE FARM. 



able kingdom, and constitute important constituents of food, 

 namely, cane-sugar, the starch of wheat, gum, pectine or 

 vegetable jelly, and celluline. The whole group, with very few 

 exceptions, have this peculiarity of constitution in common, that 

 the proportion of the two elements, oxygen and hydrogen, 

 exists exactly in that necessary to produce water. This 

 peculiarity is the origin of the name sometimes applied to 

 them viz., hydrates of carbon. It is not to be understood, 

 however, that they are merely compounds of carbon with 

 water, but rather that each of them contains as much oxygen 

 as is requisite to convert the whole of its hydrogen into water. 

 The difference between them in ultimate composition is most 

 commonly to be expressed by a larger or smaller amount of 

 the elements of water, and one of them is often capable of 

 being converted into another by the addition or subtraction of 

 so many atoms of water. Thus, if the carbon in each be re- 

 presented by 12 atoms, cane-sugar contains 11 atoms of 

 water, starch 10 atoms of water, starch-sugar or glucose 14 

 atoms of water. Hence also the conversion, for example, of 

 starch into glucose or starch-sugar may be described as the 

 combination of starch with four atoms of water. It is, as it 

 would seem, owing to this peculiarity of constitution that so 

 many members of this group are so prone to the changes pro- 

 duced by fermentation. Most of the members of this group 

 afford oxalic acid under the action of nitric acid. 



Sugar. Of the varieties of sugar known to chemists the 

 most important are the cane-sugar or sucrose, grape-sugar or 

 glucose, called also starch-sugar, fruit-sugar or fructose, and 

 milk-sugar or lactose. It will be sufficient for our present 

 purpose to take notice of these chief varieties. 



Cane-Sugar. The sugar of which cane-sugar is the type 

 abounds throughout the vegetable kingdom. The sugar-cane, 

 beetroot, and the sugar-maple are perhaps the only sources 



