Cag 
macious of moisture ; turns up, when ploughed, in- 
to massive clods, and admits the entrance of roots 
with great difficulty. A calcarious soil is dry, fria- 
ble and porous; water enters and leaves it with fa- 
cility; roots penetrate it without difficulty, and — 
[being already greatly divided] less labor is neces- 
sary for it than for clay. Magnesian, like calcari- 
‘ous earth, is light, porous and friable, but like clay, 
when wet, takes the consistency of paste, and is 
very tenacious of water. It refuses to combine 
with oxigen or with the alkalies: is generally found 
associated with granit, gneiss, and schiste, and is 
probably among the causes of their comparative 
barrenness.(1) 
In these qualities are found the mechanical rela- 
‘tions between earths and vegetables. ‘To the divi- 
sibility of the former it is owing, that the latter are 
‘enabled to push their roots into the earth; to their 
density, that plants maintain themselves in an erect 
posture, rise into the air and resist the action of 
winds and rains; and to their power of absorbing 
and holding water, the advantage of a prolonged 
application of moisture, necessary or useful to ve- 
getable life. But besides performing these impor- 
tant offices, there is reason to believe that they con- 
ie food of vegetables. This opinion 
tribute to the 
rests on the fo lowing considerations and experi- 
ments : 
1. If earths do not contribute directly to the food 
(1) The opinion is general among the chemists of Europe, that magnesian earth 
is not only barren itself, but the cause of barrenness in other soils in which it may 
abound, unlegg saturated with carbonic acid. See Bosc, Tenant and Davy. 
