40 HOW CROPS GROW. 



To drive off all water from vegetable matters, the chemist usually 

 employs a water-oven, Fig. 9, consisting of a vessel of tin or copper 

 plate, with double walls, between which is a space that may be half 

 lillcd with water. The substance to be dried is placed in the interior 

 chamber, the door is closed, and the water is brought to boil by the 

 heat of a lamp or stove. The precise quantity of water belonging to, 

 or contained in, a substance, is ascertained by first weighing the sub- 

 stance, then drying it until its weight is constant. The loss is water. 



In the subjoined table are given the average quantities, per cent, of 

 water existing in various vegetable products when air-dry. 



WATER IN AIK-DRY PLANTS. PER CENT. 



Meadow grass (hay) 15 



Red clover hay 17 



Pine wood 20 



Straw and chaff of wheat, rye, etc 15 



Bean straw 18 



Wheat (rye, oat) kernel 14 



Maize kernel 12 



That portion of the water which the fresh plant loses 

 by mere exposure to the air is chiefly the water of its 

 juices or sap, and, on crushing the fresh plant, is mani- 

 fest to the sight and feel as a liquid. It is, properly speak- 

 ing, the free water of vegetation. The water which 

 remains in the air-dry plant is imperceptible to the senses 

 while in the plant, can only be discovered on expelling 

 it by heat or otherwise, and may be designated as the 

 hygroscopic or combined water of vegetation. 



The amount of water contained in either fresh or air- 

 dry vegetable matter is somewhat fluctuating, according 

 to the temperature and the dryness of the atmosphere. 



2. The Carbhydrates. This group falls into three 

 subdivisions, viz. : 



a. THE AMTLOSES, comprising Cellulose, Starch, Inu- 

 lin, Glycogen, the Dextrins and Gums, having the 

 formula (C 6 H 10 5 )n. 



t>. THE GLUCOSES, which include Dextrose, Levulose, 

 Galactose and similar sugars, having the composition 

 C 6 H 12 6 . 



c. THE SUCROSES, viz. : Cane Sugar or Saccharose, 

 Maltose, Lactose and other sugars, whose formula in 

 most cases is Ci 2 H 22 On. 



