CONSTITUENTS OF PLANTS 



347 



smaller percentages of water. Substances such as hays, meals, 

 etc., which are apparently dry, contain appreciable quantities of 

 water. 



The following table shows the percentages of water in some of 

 the different classes of substances : 



WATER IN PLANTS. 



Determination of Water. The usual method is to dry the sub- 

 stance at the temperature of boiling water until it no longer loses 

 weight (about five hours). The operation is conducted in a 

 water-oven. This consists of a double walled box. The space 

 between inner and outer walls is partly filled with water, and the 

 inner chamber is heated by the steam. There are certain sources 

 of error in the estimation of moisture which must be guarded 

 against, some of which are as follows : 



(1) Absorption of Oxygen. Certain fats and oils absorb 

 oxygen when heated in the air, thus gaining in weight ; they may 

 also become insoluble in ether. For instance, linseed oil, which 

 occurs in linseed meal, absorbs oxygen with avidity. If materials 

 containing such oils are dried in the air, the results on moisture 

 are too low, and the ether extract is liable to be too low also. The 

 remedy is to dry in a current of hydrogen, or of illuminating gas 

 which contains no oxygen. Gasolene gas is not suitable. 



(2) Chemical Changes. At the temperature of boiling water, 

 some compounds found in plants may undergo chemical change. 

 The sugar in ripe fruits may be decomposed, or caramelized. For 

 example, Snyder 1 found fresh tomatoes to contain 3.88 per cent. 



1 Bulletin 13, Minnesota Station. 



