t is always a reasonable assumption that hydrated
lime has been made from stone of a good degree of purity. A local stone,
burned on the farm, may be of low grade, but no man of business judgment
would erect a costly plant for burning and hydrating lime where the
purity of the stone would not afford a good advertisement in itself.

On the other hand, we find very little hydrated lime on the market that
has not had sufficient exposure to the air to become changed in some
part to an air-slaked condition, or has had refuse mixed with it.
Air-slaked lime is not worth as much per ton as the hydrate because it
cannot correct as much soil acidity, and the percentage of the former
cannot be determined by the buyer. Its presence may not be due to any
wrong-doing of the manufacturer, and, on the other hand, the increase in
weight that attends air-slaking may be welcomed in some degree by a
dishonest manufacturer before the goods are shipped. The difficulty in
preventing hydrated lime from adding to its weight by becoming
air-slaked is a point to be taken into consideration.

The percentages of air-slaked material in hydrated limes are widely
variable, and no manufacturer can standardize his product on the market
surely for the benefit of the farmer. In some instances the product is
adulterated with refuse material in finely pulverized condition.

CHAPTER XIII

OTHER FORMS OF LIME

_Air-Slaked Lime._ A pure limestone is a carbonate, and the chemical
formula is CaCO_3. When it is burned, the carbon dioxide (CO_2) is
driven off, leaving CaO, which is calcium oxide, called fresh burned
lime. In this process 44 pounds of a stone weighing 100 pounds passes
into the air, and there remain 56 pounds of lime. When it air-slakes, it
takes back the carbon dioxide from the air, and the new product becomes
CaCO3, or carbonate of lime, and regains its original weight of 100
pounds. This is what would happen if the process were complete, and it
is nearly so when the exposure to the air is as perfect as possible.

Fifty-six pounds of valuable material are in the 100 pounds of
air-slaked lime, just as is the case with limestone, and there is no
difference in effectiveness except in so far as the air-slaked material
is absolutely fine and available, while most pulverized limestone is
less so. In making purchase for use of land the buyer cannot afford to
make any appreciable difference in price in favor of air-slaked lime,
as compared