144 
7- Vegetable and animal matters are known by 
their sensible qualities, and by their property of 
being decomposed by heat. Their characters may 
be learnt from the details in the last Lecture. 
8. The saline compounds found in soils, are com- 
mon salt, sulphate of magnesia, sometimes sulphate 
of iron, nitrates of lime and of magnesia, sulphate 
of potassa, and carbonates of potassa and soda. 
To describe their characters minutely will be un- 
necessary ; the tests for most of them have been 
noticed, p. 106. 
The silica in soils is usually combined with 
alumina and oxide of iron, or with alumina, 
lime, magnesia and oxide of iron, forming gravel 
and sand of different degrees of fineness. The 
carbonate of lime is usually in an impalpable 
form; but sometimes in the state of calcareous 
sand. The magnesia, if not combined in the 
gravel and sand of soil, is in a fine powder united 
to carbonic acid. The impalpable part of the soil, 
which is usually called clay or loam, consists of 
silica, alumina, lime and magnesia ; and is, in 
fact, usually of the same composition as the hard 
sand, but more finely divided. The vegetable or 
animal matters (and the first is by far the most 
common in soils,) exist in different states of decom- 
position. They are sometimes fibrous, sometimes 
entirely broken down and mixed with the soil. 
To form a just idea of soils, it is necessary to 
conceive different rocks decomposed, or ground 
into parts and powder of different degrees of fine- 
ness, some of their soluble parts dissolved by water, 
and that water adhering to the mass, and the whole 
