99 
anhydrous gypsum, but selenite is occasionally found im- 
bedded in veins. 
11. I now inquire what aid may be derived from physiolo- 
gical Botany and Chemistry, in the cultivation of these soils, 
and more particularly from a consideration of the recent 
opinions of Liebig. 
In the former paper read by me, it was stated, that the 
quality of soils could not be determined by an inspection of 
their analysis, or, in other words, that it was not known what 
combination of the earths produced the highest degree of 
fertility. Professor Liebig, however, does give us a very 
simple criterion by which their quality may be determined, 
which, if it proves to be correct, will be of inestimable value 
to the practical farmer. 
There must, in the first place, he free access of the atmos^ 
phere, as well as of moisture to the soil, Liebig says, " Land 
of the greatest fertility contains argillaceous earth and other 
disintegrated minerals, with chalk and sand in such a pro- 
portion as to give free access to air and moisture. A fertile 
soil in the next place must contain clay, or rather alumina.^* 
There must be, says he, " something in aluminous earth 
which enables it to exercise an influence on the life of plants, 
and assist in their development. Pure sand and pure lime- 
stone, in which there are no other organic substances except 
siliceous earth, carbonate or silicate of lime, form absolutely 
barren soils, but argillaceous earths form always a part of 
fertile soils. Now from whence come the argillaceous earths 
in arable land; what are their constituents, and what part do 
they play in favouring vegetation ? They are produced in 
the soil by the disintegration of aluminous minerals by the 
action of the weather ; and the influence which alumina ex- 
ercises on the life of plants depends on its invariably contain- 
ing potash and soda. Alumina exercises in another way. 
