stone-lime (CaO).

_Pulverized lime_, often called ground lime, is stone-lime after being
pulverized to permit even distribution. When it is fully exposed to the
air or moisture, it slakes and doubles in volume.

_Hydrated lime_, often called slaked lime, is a combination of
stone-lime and water. The water causes an increase in weight of 32 per
cent, 56 pounds of stone-lime becoming 74 pounds of the hydrate.

_Pulverized limestone_, often called carbonate of lime, is the unburned
limestone made fine so that good distribution may be possible.

_Air-slaked lime_, often called carbonate of lime, is stone-lime or
hydrated lime combined with carbonic acid from the air, and thereby
increased in weight. Fifty-six pounds of stone-lime, or 74 pounds of
hydrated lime, become 100 pounds of air-slaked lime.

_Agricultural lime_, or land-lime, may embrace anything that the
manufacturer of lime chooses to market. It may be reasonably pure
unslaked lime, or it may have less value than a finely pulverized pure
limestone. There is a custom of grinding the core, or partially burned
limestone of the kiln, together with impurities removed from builders'
lime, and with this may be put some air-slaked lime. Some manufacturers
market under this name a lime of excellent value. There is no standard,
and one should not pay more than a finely pulverized pure limestone
would cost unless he knows that the content of fresh burned lime is
high.

The element with which we are concerned in any of these forms of lime
is calcium. It is the base whose union with the acids destroys the
latter. It should be obvious that the addition of water to stone-lime,
which adds weight and causes 56 pounds of the stone-lime to become 74
pounds of hydrated lime, adds no calcium. Likewise the change to the
air-slaked condition adds no calcium, but again adds weight.

The Kind to Apply.--If a soil contains free acid, the amount of calcium
needed is definite. The form of lime that can supply the need in that
particular field at least expenditure of money and trouble is the one
to be selected. A ton of stone-lime, or pulverized lime, can correct as
much acid as 2640 pounds of hydrated lime or 3570 pounds of pulverized
limestone, if all the original material was pure.

In other words, if the value of a given weight of pulverized limestone
is placed at 100, the value of the same weight of hydrated lime would
be 132 and the value of stone-lime would be 180, when each