became mixed with the lime carbonate when the rock was being formed.

_Calcium Hydroxide_ is a compound made by permitting calcium oxide to
combine with water, and is known as lime hydrate. It contains 56 parts
by weight CaO and 18 parts water, and has the formula Ca(OH)2.

_Magnesium_ is an element, and is found in magnesium carbonate, a
compound that is effective in correcting soil acidity.

_Magnesian Limestone._ Magnesium carbonate is usually found in
combination with calcium carbonate, and when about 47 per cent of the
total carbonates is magnesium carbonate, the limestone is known as
dolomite.

_Ground Limestone_ is the stone pulverized so that it can be
distributed. It is carbonate of lime (CaCO3), or a combination of
calcium and magnesium carbonate, and in a way has a right to be
designated as "lime," but such use leads to confusion.

_Fresh Burned Lime_. Calcium oxide (CaO) formerly was accurately
designated as "lime," but the words "fresh burned" are often prefixed to
prevent confusion with lime carbonate or the hydrate. It is known as
"lump lime," "caustic lime" and "stone lime."

_Ground or Pulverized Lime_. Fresh burned lime may be ground fine, so
that it can be spread on land without slaking. This product should not
be confused with ground limestone or hydrated lime. Fresh ground lime is
worth nearly twice as much per ton as ground limestone, but some of the
product on the market is far from pure. There is opportunity to grind up
unburned and waste material with the caustic lime, and this form of lime
usually contains some hydrated material.

_Hydrated Lime_ is the compound formed by the action of water or steam
on fresh burned lime.

_Air-Slaked Lime_ is a compound formed by the action of carbon dioxide
from the air on hydrated lime, and its formula is CaCO3, which is
that of pure limestone.

[Illustration: An Indiana Limestone Quarry]

[Illustration: A Limestone Plant (Courtesy of the Michigan Limestone
Company)]

CHAPTER VIII

GROUND LIMESTONE

_Variation in Quality._ Limestones vary widely in purity. They were
formed under water, and clay and sand were laid down with the lime in
such quantity in some cases that the resulting stone is not worth
handling for soil improvement. A stone that is practically all carbonate
of calcium, or a combination of calcium and magnesium, is wanted because
it is these two elements that give value to the material. If a poor
stone is used, too much