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TIIE CHEMISTRY OF SOILS AND MANURES. 
1 in water containing carbonic acid in solution, carbonate of rime dissolves to a considerable extent. 
Spring- waters generally contain free carbonic acid in solution ; and as almost all soils, and many rocks, 
in which springs find their origin, contain lime, spring-waters frequently contain this element in 
solution. When such waters are boiled for some time, they become muddy ; the volatile carbonic 
acid of the water is expelled, and the lime, now deprived of its solvents, is deposited gradually from 
the muddy water, and constitutes, along with some gypsum, likewise deposited under these circumstances, 
the greater part of the incrustations in tea-kettles and boilers. 
Salts of lime are found in all ashes of plants. Soils capable of sustaining vegetable life, therefore, 
must contain lime in some form or other. Generally, carbonate of lime is the combination which 
occurs most frequently in soils ; but the silicate, phosphate, nitrate, and sulphate of lime are found in 
many soils. The relative proportions of lime in soils varies much. While some soils contain as much 
as 20 or even 30 per cent of lime ; others contain only 2 or 3 per cent. On the whole, lime-salts are 
found in much larger quantities in soils than potash-salts or soda-salts. 
Sulphate of Lime, or Gypsum, or Plaster of Paris, is a well-known, white, often crystallized 
compound of sulphuric acid and lime, which occurs in many localities in England, forming occasionally 
mineral deposits of considerable extent. Gypsum frequently accompanies rock-salt, and is found likewise 
in sea-water, in many soils, and in springs which percolate through such soils, or which ascend from 
beds in which gypsum exists. Gypsum is slightly soluble in water ; 500 parts of water being required 
for dissolving 1 part of gypsum. Crystallized, native, or commercial gypsum, contains about 21 per 
cent, of water, which may be expelled at a temperature a little above 300° Fahrenheit. Burned or 
anhydrous gypsum possesses the property of uniting again with the water which it lost by heating ; and 
this takes place so readily, that a thin paste made of burned gypsuni and water in a few minutes 
sets or hardens into a solid mass. On this property depends the application of burned gypsum for 
plaster casts and ornamental purposes. Many ashes of plants contain an appreciable quantity of 
gypsum ; for instance, the ashes of peas, beans, lentils, and other leguminous plants : these are the 
plants wliich derive most benefit from gypsum. 
Nitrate of Lime is a white, deliquescent salt, which occurs almost always in cultivated calcareous 
soils. When animal and vegetable substances, containing nitrogen, are mixed with quick lime, and 
".1 lowed to putrify, nitrate of lime is constantly formed; hence its presence in compost heaps, made by 
the addition of quick lime to animal and vegetable refuse matters, and in highly manured soils, abounding 
in rime. 
Phosphate of Lime, the combination of phosphoric acid with lime, which occurs in mineral veins in 
Cornwall and Cumberland, under the name of Apatite, is a hard mineral. A variety of phosphate of 
lime, called phosphorite, is found in large quantities in the province of Estremadura in Spain ; but, 
generally speaking, in most cultivated soDs of England phosphate of lime rarely exists, unless it be 
incorporated with the soil purposely in the shape of farm-yard manure, bones, eoprolites, or other 
artificial manures. 
4. Magnesia. — Pure or caustic Magnesia, which is sold in the shops as calcined magnesia, is a 
compound of magnesium with oxygen, and is usually obtained by calcining the white or carbonate of 
magnesia. Like quick lime, caustic magnesia is slightly soluble in water ; while carbonate of magnesia 
is insoluble in that medium. Carbonate of magnesia, which in some places occurs native in a pure 
state, is commonly associated in nature with carbonate of lime. The magnesian limestones, wliich are 
such natural compounds of carbonates of lime and magnesia, contain from 30 to 40 per cent of carbonate 
of magnesia. It is in this form, and in all dolomitic rocks, or dolomites, that carbonate of magnesia is 
found in large quantities in nature. Existing as it does in many solid rocks, magnesia is never wanting 
in fertile soils, and it is found likewise in the ashes of plants. Soils containing much carbonate of 
magnesia, absorb moisture from the atmosphere with great avidity, and it is perhaps on this account 
that such soils are rendered cold. 
Silicate of Magnesia enters into the composition of many minerals, which are distinguished by 
appearing soapy or greasy to the touch. Serpentine rocks, meerschaum, asbestos, soap-stone, are 
examples of minerals containing a large proportion of silicate of magnesia. Limestone also frequently 
contains the same compound. 
The compounds of sulphuric acid and muriatic acid with magnesia, are found in many mineral 
waters; the latter more particularly in sea-water. The former, sulphate of magnesia, exists in large 
quantities in several salt-springs near Epsom ; hence the derivation of the familiar name of Epsom salts, 
given to sulphate of magnesia. This salt exists almost in all soils wliich are formed from the decompo- | 
sitiou of dolomitic rocks ; and its presence sometimes is indicated by the white efflorescence wliich, 
in warm and dry weather, appears on the surface of soils, in which sulphate of magnesia abounds. 
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