105 
the earth of flints, alumina or pure clay, lime, and 
magnesia. They are procured by incineration. 
The lime is usually combined with carbonic acid. 
This substance, and silica, are much more common 
in the vegetable kingdom than magnesia, and mag- 
nesia more common than alumina. The earths 
form a principal part of the matter insoluble in 
water, afforded by the ashes of plants The silica 
is known by not being dissolved by acids ; the cal- 
careous earth, unless the ashes have been very 
intensely ignited, dissolves with effervescence in 
muriatic acid. Magnesia forms a soluble and 
crystallisable salt, and lime a difficultly soluble 
one with sulphuric acid. Alumina is distinguished 
from the other earths by being acted upon very 
slowly by acids ^ and in forming salts very soluble 
in water, and difficult of crystallisation with them. 
The earths appear to be compounds of the pe- 
culiar metals mentioned page 46. and oxygene, 
one proportion of each. 
The earths afforded by plants, are applied to no 
uses of common life ; and there are few cases in 
which the knowledge of their nature can be of 
importance, or afford interest to the farmer. 
The only metallic oocides found in plants, are 
those of iron and manganesum : they are detected 
in the ashes of plants $ but in very minute quan- 
tities only. When the ashes of plants are reddish 
brown, they abound in oxides of iron. When 
black or purple, in oxide of manganesum ; when 
these colours are mixed, they contain both sub- 
stances. 
The saline compounds contained in plants, or 
