170 
Besides this agency, which may be considered 
as mechanical, there is another agency between 
soils and organizable matters, which may be re- 
garded as chemical in its nature. The earths, and 
even the earthy carbonates, have a certain degree 
of chemical attraction for many of the principles 
of vegetable and animal substances. This is easily 
exemplified in the instance of alumina and oil ; if 
an acid solution of alumina be mixed with a solu- 
tion of soap, which consists of oily matter and 
potassa, the oil and the alumina will unite and 
form a white powder, which will sink to the bottom 
of the fluid. 
The extract from decomposing vegetable mat- 
ter, when boiled with pipe-clay or chalk, forms a 
combination by which the vegetable matter is 
rendered more difficult of decomposition and of 
solution. Pure silica and siliceous sands have 
little action of this kind ; and the soils which 
contain the most alumina and carbonate of lime 
are these which act with the greatest chemical 
energy in preserving manures. Such soils merit 
the appellation which is commonly given to them 
of rich soils ; for the vegetable nourishment is long 
preserved in them, unless taken up by the organs 
of plants. Siliceous sands, on the contrary, deserve 
the term hungry, which is commonly applied to 
them ; for the vegetable and animal matters they 
contain not being attracted by the earthy consti- 
tuent parts of the soil, are more liable to be decom- 
posed by the action of the atmosphere, or carried 
off from them by water. 
In most of the black and brown rich vegetable 
