"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 to distribute the lime in small piles in the field, placing the piles at such convenient distance apart that the lime, after slaking, could be spread easily with a shovel. The water for slaking comes from rains, or from moisture in the air and earth. The method is wasteful and can be justified, if ever, only where farm-burned lime costs little per ton, and the nature of the soil is such that a relatively heavy application can be safely made. The distribution is necessarily uneven, and if the required amount goes upon all the surface, a great excess is sure to go upon a portion of it. Very often an excess of water puddles much of the lime in the pile, and lumps may be seen lying in ineffective form in the soil for years. The practice is responsible for much of the excessive application that brought the use of caustic lime into disrepute.