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 with a fine stone. _Air-Slaking a Slow Process._ Lime changes to an air-slaked condition slowly unless it has full exposure. Old heaps will remain in hydrate form for many years, excepting the outside coat, which excludes the air. Complete air-slaking would not reduce ability to correct soil acidity, the total amount of calcium and magnesium remaining constant, but weight would be added in the slaking, and therefore the value per ton would be reduced. The slowness with which air-slaking proceeds gives reason to expect that any bulk of old lime may contain a considerable percentage of the hydrate, and therefore have greater strength than a true carbonate like limestone. This is a consideration of value to a buyer. _Agricultural Lime._ Some manufacturers have found in the demand for lime by farmers an opportunity of disposing of much material that would not be satisfactory to manufacturers and builders. In some cases this so-called agricultural lime is sold at a price that is not beyond value, but it varies much in its content of pure lime. If the unburned cores of kilns are ground up, the material simply retains the value of unburned stone. Any air-slaked material put into it has like value. Forkings, ground up, have less value, and sometimes no value at all. Some better material may go into this mixture that is given the name "agricultural lime," and the product cannot be standardized or have a valuation given it that would be true for another lot.