73

and marl. In this condition, it forms a large extent of the solid
structure of our globe as limestones of different degrees of purity;
it is sometimes associated with magnesia, and this is then called
Dolomite, from the name of Dolomieu, who first described it;
sometimes we also have with it, in a visible form, mica; and
here I must caution those who are but slightly acquainted with
chemistry, not to condemn limestones as worthless, which have a
large residue after being treated with muriatic or any strong acid.
This residue is frequently mica, which contains potash, more val-
uable by far, than the part soluble in acid. Pure, quick unslaked
lime is not an element, but a compound formed of oxygen, a gas,
and calcium, a metal; it is therefore the oxyd or rust of a metal;
with carbonic acid it becomes the common air-slaked lime; with
water it is the hydrate of lime, or water-slaked lime. The quan-
tity of pure lime (oxyd of calcium) in air-slaked lime, is 56.29 per
cent.; in water-slaked lime, the quantity of pure lime is about 76
per cent. As lime is that which we wish to buy in lime, its value
depends on the quantity of pure lime, or oxyd of calcium in the arti-
cle which we purchase, and any one can, by the Rule of Three,
calculate the quantity of pure lime in air or water-slaked lime,
from the above statement. Lime has many appliances, but we
shall alone consider it in its references to agriculture.

The history of the use of lime is interesting in many particulars,
in none more so than as it affords a convincing proof in itself of the
incompetency of experience alone to determine the conditions on
which a manure should be used. If the world could, by long ac-
quaintance with and frequent use of a substance, understand the
principles which should dictate its employment, lime would have
been the agent by which it would have been taught; for no other
manure has been used so long, none so extensively, and none with
such diverse effects.

Even before the Christian era, Pliny tells us that "Edui et Pic-
toues agros calce uberrimos fecit"—that the Edui and Picts made
their fields very rich by means of lime. Marl was used in great
abundance, before the discovery of our country. In the reign of
Edward I. and II. of England, the leases have conditions that the
tenant should use a particular quantity of marl. In the Forest Char-
ters, as early as the year 1200 A. D., persons were forbidden, un-
less with certain precautions, to dig marl pits near to the king's
highway.

These facts show that the use of burnt lime and marl was well
known and practiced long before this time, and well would it have
been for succeeding generations, had the composition of the soil,
the exact composition of the lime, and marl been known. Had
there been recorded facts attendant upon its use—had science
raised the veil which covered its mode of action and the philoso-
phy of its use, there would have been no proverb in Scotland, that