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Magnesian Limestone# 
Magnesian Limestone# 
There is no little controversy going on at the pres¬ 
ent moment among our Pennsylvania farmers, as to 
the effect of Magnesia, when it occurs in connexion 
with Limestone in the state of carbonate, and with it 
is deprived of its carbonic acid, and in this condition 
is applied to land. To elucidate this matter, and give 
some general principles on this subject, we subjoin a 
valuable extract from Professor Johnston. 
1°. Carbonate of Magnesia is a tasteless 
earthy compound, which in some parts of the 
world forms rocky masses and veins of consid¬ 
erable height and thickness. It occurs more 
largely however, in connection with carbonate 
of lime in the magnesian limestones, so well 
known in the eastern and northern parts of 
England,—and in similar rocks, distinguished 
by the name of dolomites or of dolomitic lime¬ 
stones, in various countries of Europe. The 
pure, exceedingly light, white magnesia of the 
shops, is partly extracted from the magnesian 
limestone, and partly from the mother liquor of 
the salt pans, which generally contains much 
magnesia. 
When pure and dry, carbonate of magnesia 
consists of 43^- of magnesia, and 51-f of car¬ 
bonic acid. It dissolves readily in diluted acids 
(sulphuric, muriatic, or acetic), the carbonic 
acid at the same time escaping with efferves¬ 
cence. 
Existing as it does in many solid rocks, this 
carbonate of magnesia may be expected to be 
present in the soil, and is found in the ashes of 
many plants. Of the ashes of some parts 
of plants it constitutes one-sixth of the entire 
weight. 
When exposed to the air in a finely divided 
state, it gradually absorbs a quantity of mois¬ 
ture from the atmosphere, equal to two-thirds of 
its own weight. In this state, it dissolves in 48 
times its weight of water, though, when dry, it 
is nearly insoluble. Like carbonate of lime it 
is also soluble in water impregnated with car¬ 
bonic acid, but in a somewhat greater degree. 
In this state of solution, it may be readily car¬ 
ried into the roots, and be the means of supply¬ 
ing to the parts of living vegetables, a portion of 
that magnesia which is necessary to their per¬ 
fect growth. 
Soils containing much of this carbonate of 
magnesia, are said to be highly absorbent of 
moisture, and to this cause is ascribed the cold¬ 
ness of such soils. This opinion is, however, 
open to doubt. 
2°. Magnesia or Caustic Magnesia , the cal¬ 
cined magnesia of the shops. —When the car¬ 
bonate of magnesia is heated to redness in the 
open air, it parts with its carbonic acid much 
more readily than lime does, and is brought into 
the state of pure or caustic magnesia. In this 
state it does not eccur in nature but is occasion¬ 
ally met in combination with about 30 per cent of 
water. When magnesian limestones or dolomites 
are burned, the quick-lime obtained often contains 
caustic magnesia also in considerable quantity. 
This mixture is frequently applied to the land, 
and, as is well known in many parts of Eng¬ 
land, with injurious effects, if laid on in too 
large quantities. The cause of this hot or 
burning nature, as it is called, of magnesian 
lime, is not very satisfactorily ascertained. I 
shall, however, state two or three facts, which 
may assist in conducting us to the true cause. 
1°. Quick-lime dissolves in 750 times its 
weight of water, at the ordinary temperature of 
the atmosphere, while pure magnesia requires 
5142 times its weight. The magnesia there¬ 
fore, is not likely to injure living plants direct¬ 
ly by entering into their roots in its caustic 
state, since lime which is 7 times more soluble 
produces no injurious effect. 
2°. It seems to be the result of experience, 
that magnesia in the state of carbonate is but 
slightly injurious to the land ; some deny that 
in this state it has any injurious effect at all. 
This I fear is doubtful; we may infer however, 
with some degree of probability, that it is from 
some property possessed by magnesia in the 
caustic state, and not possessed, or at least in an 
equal degree, either by quick-lime or by carbo¬ 
nate of magnesia, that its evil influence is chief¬ 
ly to be ascribed. 
3°. When exposed to the air, quick-lime 
speedily absorbs water and carbonic acid from 
the air, forming first a hydrate* in fine powder, 
and then a carbonate. Caustic magnesia ab¬ 
sorbs both of these more slowly than lime does, 
and in the presence of the latter, or when mixed 
with it, must absorb them more slowly still, 
since the lime will seize on the greater portion 
of the moisture and carbonic acid which exists 
in the air, immediately surrounding both. 
When slaked in the air also, the lime may be 
transformed in great part into carbonate while 
the magnesia still remains in the state of hy¬ 
drate, and it is a property of this hydrate to 
attract carbonic acid more feebly and slowly, 
even than the newly burned magnesia as it 
comes from the kiln. Hence when buried in 
the soil, after the lime has become nearly all 
transformed into carbonate, the magnesia may 
still be all either in the dry caustic state, or in 
that of a hydrate only. 
4°. Now there exist in the soil, and probably 
are exuded from the living roots, various acid 
* Compounds of substances with water are called 
hydrates (from the Greek word for water). Thus sla¬ 
ked lime, a compound of lime with water, is called 
hydrate of lime —and the native compound of magne¬ 
sia with'water, alluded to in the text, is called hydrate 
of magnesia . 
