THE ASH OF PLANTS. 137 



the list includes all the elementary siihstances that belong 

 to agricultural yegetation. 



Hydrogen is never an ingredient of the perfectly 

 burned and dry ash of any plant. 



Nitrogen may remain in the ash under certain con- 

 ditions in the form of a Cyanide (compound of Carbon 

 and Nitrogen), as will be noticed hereafter. 



Besides the above, certain other elements are found, either occasion- 

 ally in common plants, or in some particular Itind of vegetation ; these 

 are Iodine, Bromine, Fluorine, Titanium, Boron, Arsenic, Lithium, 

 Rubidium, Barium, Aluminum, Zinc, Copper. These elements, how- 

 ever, so far as known, have no special importance in agricultural 

 chemistry, and mostly require no further notice. 



We may now complete our study of the Composition 

 of the Plant by attending to a description of those ele- 

 ments that are peculiar to the ash, and of those com- 

 pounds which may occur in it. 



It will be convenient also to describe in this section 

 some substances, which, although not ingredients of the 

 ash, may exist in the plant, or are otherwise important 

 to be considered. 



The Non-metallic Elements, which we shall first 

 notice, though differing more or less widely among them- 

 selves, have one point of resemblance, viz., they and their 

 compounds with each other have acid properties, i. e., 

 they either are acids in the ordinary sense of being sour 

 to the taste, or enact the part of acids by uniting to met- 

 als or metallic oxides to form salts. We may, therefore, 

 designate them as the acid elements. They are O.xygen, 

 Sulphur, Phosphorus, Carbon, Silicon, and Chlorine. 



With the eiception of 'Silicon, and the densef forms of 

 Carbon, these elements by themselves are readily volatile. 

 Their compounds .with each pther, which may occur in 

 vegetation, are also volatile.^with two exceptions, viz.. 

 Silicic and Phosphoric acids. 



In order that they mayNjesist the high temperature at 

 which ashes are formed, they must be combined with the 

 metallic elements or their oxides as salts. 



