THE ASH OF PLANTS. 113 



either are acids in the ordinary sense of being sour to the 

 taste, or enact the part of acids by uniting to metals or 

 metallic oxides, to form salts. We may, therefore, desig- 

 nate them as the acid elements. They are Oxygen, Sulphur, 

 Phosphorus, Carbon, Silicon, and Chlorine. (Less com- 

 mon are Arsenic, Titanium, Iodine, Bromine, and Fluorine.) 



With the exception of Silicon, (and Titanium,) and the 

 denser forms of Carbon, these elements by themselves are 

 readily volatile. Their compounds vrith each other, which 

 may occur in vegetation, are also volatile, with two ex- 

 ceptions, viz.. Silicic and Phosphoric acids. 



In order that they may resist the high temperature at 

 which ashes are formed, they must be combined with the 

 m.etallic elements or their oxides as salts. 



Oxygen, Symbol O, atomic weight 16, is an ingredient 

 of the ash, since it unites with nearly all the other elements 

 of vegetation, either during the life of the plant, or in. the 

 act of combustion. It unites with Carbon, Sulphur, Phos- 

 phorus, and Silicon, forming acid bodies ; while with the 

 metals it produces oxides, which have the characters of 

 bases. Chlorine alone of the elements of the plant does 

 not unite with oxygen, either in the living plant, or during 

 its combustion. 



CAEBON AND ITS COMPOTTNDS. 



Carbon, Sym,. C, at. wt. 12, has been noticed already 

 with sufficient fulness, (p. 31.) It is often contained as 

 charcoal in the ashes of the plant, owing to its being en- 

 veloped in a coating of fused saline matters, which shield 

 it from the action of oxygen. 



Carbonic acid, Sym. C O^, molecular weight, 44, is the 

 colorless gas which causes the sparkling or effervescence 

 of beer and soda water, and' the frothing of yeast. 



It is formed by the oxidation of carbon, when vegetable 



matter is burned, (Exp. 6.) It is, therefore, found in the 



ash of plants, combined with those bases which in the liv- 

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