75

3. The quantity of lime adequate to the first purpose, that of
giving, per se, food to the plant—is very small. I have repeatedly
examined soils with as little as one-tenth of one per cent, of lime,
capable of producing ten barrels of Indian corn (maize) to the acre.

The minerals most generally present, which require the pre-
sence of quick lime for their degradation, and for their solubility,
are mica, (isinglass,) felspar and hornblende—they are those which
are present in all granite soils. The red sub-soil clays, so abund-
ant in many parts of our state, are formed from those rocks, and
to them, when unproductive, quick or water slaked lime will al-
ways be beneficial.

The agency of lime in decomposing vegetable matter, is also,
not an unimportant function, Some limestones contain small
quantities of sulphate of lime, (gypsum,) and phosphate of lime,
(bone-earth,) and also potash. The value of a limestone is mate-
rially increased to a soil not containing felspar, mica or horn-
blendes, when it contains mica as one of its constituents, as these
soils are most generally deficient in potash, which these minerals
supply.

Lime existing in shells and limestones, naturally, in the form of
carbonate, is deprived of its carbonic acid, by being burnt in kilns.
When exposed to the atmosphere it again receives its carbonic acid,
and thus becomes air-slaked, and crumbles into fine powder.—
When water is added to quick lime it generates a large quantity
of heat, unites to a definite quantity of water, and becomes water-
slaked, still retaining, however, some of its caustic properties.
These are entirely destroyed when it is air slaked.

The limestones of our state vary very much in their constitu-
ents, even when of the same formation, and lying very close to
each other.

There will be seen from what has been said above, that many
limestones are Dolomites—that is, composed of a large quantity of
magnesia.

Magnesia, like lime, is the oxyd or rust of a metal, called mag-
nesium, which never exists in its pure state. Magnesia exists as
carbonate—that is, as air-slaked magnesia, in rocks, and when ex-
posed to a sufficient degree of heat, in kilns, loses its carbonic
acid, and then becomes quick, caustic, or what it is most usually
called, calcined magnesia. This, on exposure to the air, absorbs
carbonic acid from it, and becomes again mild, or carbonate of mag-
nesia: it however does this very slowly, and remains caustic a
much longer time than lime. It also unites to water, in definite
proportions, but not with the same intensity of action as lime, and
gives out but little heat, in consequence of the slowness with which
the union is formed between it and water.

In Dolomites, or magnesian limestones, the proportions of lime
and magnesia are very variable, as "isomorphous substances crys-