one and coal are built up around them; thus, when they
are burned out, a chimney or two is secured, which may be damped by
pieces of stone or sod. Upon this first layer of stone is spread a layer
of coal, and upon that a thicker layer of stone (12 inches), and so on,
coal and stone alternating until the heap is topped with smaller stone.
The largest stones should be placed near the top of the heap, but not
near the outside, so that they may be exposed to the highest heat. The
proportion of coal is diminished in the upper layers, the effort being
to distribute one-half of the total coal employed in the two lower
layers, so as to secure the highest economy possible in the use of the
fuel.

"Fire is then kindled in the straw or shavings; when the flames have
communicated themselves to the cord wood and lowermost layer of coal,
and tongues of flame shoot out from the crevices in the sides of the
heap, earth, previously loosened by a few turns of the plow about the
heap, is rapidly spread over the entire heap, thus damping the drafts
and retarding the combustion. Steam and smoke slowly escape during the
first hours, but later the entire heap, including the outer covering of
earth, is heated to a dull red glow. The burning goes on slowly for
several days, the interior often being hot for several weeks. When the
lower portion of the heap has reached an advanced stage of calcination,
a portion of the outer layer of lime sometimes slips down; if so, a
fresh covering of earth must promptly be applied at the exposed point;
otherwise it will serve as a vent for the heat, and the top and other
sides will fail of proper calcination."

[Illustration: Effect of Excessive Use of Burned Lime Without Manure at
the Pennsylvania Experiment Station]

[Illustration: A Hydrated Lime Plant

(Courtesy of the Palmer Lime and Cement Company, York, Pa.)]

CHAPTER XII

LIME HYDRATE

_Slaking Lime._ The usual means of reducing fresh burned stone lime to a
condition that makes even distribution upon land possible is by slaking.
A few years ago considerable effort was made to create a market for lime
pulverized by machinery, but the difficulty in excluding the moisture of
the air so that packages would not burst has been in the way of
developing a market. Slaking, by the addition of water to the fresh
burned lime, is the common method of getting the required physical
condition. When the slaking is done on the farm, the custom has been t