Methods of Burning. Limestone contains the calcium and magnesium that must be the chief source of supply of American soils, though marls, ashes, etc., have their place. The burning of the stone has been the leading means of bringing it to a condition of availability to the soil, excepting, of course, the vast work of disintegration carried on through all the ages by nature. Pulverization of the rock by machinery for use on land is recent.

The devices for burning are various, a modern lime plant containing immense kilns, cylindrical in form, the stone being fed into them at the top continuously, and the lime removed at the bottom. A large part of the lime that is sold for use on land is made in plants of this kind. Some is burned in kilns of cheap construction, but a traveler through a limestone country finds few such kilns now in use.

The Farm Lime Heap. A common method of producing lime for farm use has been, and continues to be, a simple and inexpensive one, involving the use only of wood, coal and limestone, with earth as a covering. Dr. Wm. Frear, chemist of the Pennsylvania station, in Bulletin 261 of the Pennsylvania department of agriculture, describes a method of burning lime on the farm as follows: "A convenient oblong piece of ground is cleared, and leveled if need be, to secure a fit platform. Upon this level is placed a layer or two of good cord wood, better well seasoned, arranged in such manner as to afford horizontal draught passages into the interior of the heap. Between the chinks in the cord wood, shavings, straw or other light kindling is placed. The stone having been reduced to the size of a double fist, sometimes not so small, is laid upon the cord wood, care being taken to leave chinks between the stones just as between the bricks in a brick kiln. It is preferred that this layer of stone should not exceed six to ten inches in thickness.

"In some cases, temporary wooden flues, filled with straw, are erected, either one at the center or, if the heap is elliptical, one near each end, and the stone 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."