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ELEMENTS OF AGRICULTURE. 
32. All earths that contain carbonate of lime are calca* 
reous, and an excess is injurious, as the plants would fire. 
On the contrary, if the carbonate of lime is in suitable pro¬ 
portion, it is advantageous to the soil, and renders it better 
for cultivation. 
33. 'Chalk, marble, limestone, shells, ar§ calcareous. 
When exposed to the action of heat, the carfipnic acid es- 
£apes^ and they are converted into lime. Some of these 
substances, that become friable from the effects of frost, 
may improve the soil by increasing its depth. 
34. Carbonate of lime is found almost pure in marble, 
but in other bodies it is often united with foreign sub¬ 
stances. It can only be effective in the soil when reduced 
and pulverized by the action of heat. If it remained in 
its primitive state of stone, it would be more injurious 
than beneficial —- impeding the plough, breaking imple¬ 
ments, occasioning loss of time, and extensive repairs. 
This applies, however, to hirge stones only ; for if they 
are very small they improve clay lands, by rendering them 
less tenacious. 
35. We know that lands containing clay in large quan¬ 
tities, are very compact, and difficult to cultivate. Car¬ 
bonate of lime, reduced to an earthy texture, serves to 
loosen and render them more permeable to air, and to give 
them, consequently, properties favorable to vegetation. 
36. There are also soils containing organic remains, 
that decompose with great difficulty, and from which, 
consequently, cultivated vegetables can derive but little 
benefit. Thus, with soils that have been a long time in 
fallow, and are clothed with fern, sedge, rushes, etc., if we 
content ourselves with merely turning them under, without 
the application of such substances as lime to favor their 
decomposition, a long time may elapse before the vegeta- 
ion, .so turned under, will produce the desired effect. 
37. Sometimes even the substances contained in a soil 
