30 AGRICULTURAL £SSAt3. 



by their union their common virtues are heightened, while their 

 defects are rectified and subdued." It is very obvious there- 

 fore, that a mixture of those two kinds of earth will greatly in- 

 crease the value of the soil. "Lime, called calcareous earth, 

 enters into the composition of soils. This is never found in its 

 pure state, but in combination with carbonic acid, for which it 

 has so strong an affinity, that it attracts it from the atmosphere. 

 It is closer- than sand, and much less adhesive than clay. It 

 occVipies therefore ft. middle region between the two, free 

 from their imperfections, and blending their common qualities. 

 It is a powerful promoter of putrefaction, and helps to decom- 

 pose the anini&l aud vegetable matter lying in the soil. To 

 this circumstance is owing i^ a great measure its efficacy, as a 

 manure. It has the power of fixing and retaining a very great 

 quantity of carbonic acid, and although it combines chemically 

 with a certain portion, which can only be expelled by red heat, 

 yet the excess can easily be disengaged in a low temperature, 

 and thus tends to nourish the growing crops. This earth ex- 

 ists in abundance among the solid strata of our globe, mostly 

 without any foreign mixture, except the acids with which it 

 combines ; but is occasionally blended also, v/ith the other prim- 

 itive masses. With carbonic acid it forms the most frequent 

 compounds; denominated carbonates of lim.e,"^ and assumes a 

 variety of appearances, and even possesses distinct properties. 

 Spar, marble, stalactites, limestone and chalk, are all Jiarieties 

 of this combination. With the sulphuric acid, (which is com- 

 posed of sulphur and oxigen,) it forms plaster of paris. It is al- 

 so found ml.-'.ed with clay, and sometimes with sand, and then 

 receives the appellation of marble, and which is valuable pre- 

 cisely in proportion to the quantity of this earth." Magnesia, 

 the last earth v/hich has been found in soils, and that too, in a 

 much smaller proportion than the other tliree, is a substance 

 with which every farmer must be acquainted, since it is com- 

 monly used as a medicine of the shops. Its properties are 

 nearly analogous to those of lime, and were long supposed to 

 operate in the same way. It has also a strong, though less, af- 

 finity for carbonic acid, and- often forms a constituent principle 

 in lime-stone rock. Agricultiirali^s are divided at present, 

 with respect to its usefulness as a manure. Some of them 

 holding it to be poisonous to plants, while others support it by 

 an appeal to experiments. But it is thought to be uncertain 



* Carbonate of lime is not soluble in water, unless the wa • 

 ter itself be charged with carbonic acid. Carbonate of lime is * 

 compound of lime 86, carbonic acid 44. 



