_Kind of lime to be used._ The first consideration in procuring lime for manuring land, is to select that which contains but little, if any _magnesia_. Nearly all stone lime contains more or less of this, but some kinds contain more than others. When magnesia is applied to the soil, in too large quantities, it is positively injurious to plants, and great care is necessary in making selection. As a general rule, it may be stated, that the best plastering lime makes the best manure. Such kinds only should be used as are known from experiment not to be injurious. _Shell lime_ is undoubtedly the best of all, for it contains no magnesia, and it does contain a small quantity of _phosphate of lime_. In the vicinity of the sea-coast, and near the lines of railroads, oyster shells, clam shells, etc., can be cheaply procured. These may be prepared for use in the same manner as stone lime.[AG] _The preparation of the lime_ is done by first burning and then slaking, or by putting it directly on the land, in an unslaked condition, after its having been burned. Shells are sometimes _ground_, and used without burning; this is hardly advisable, as they cannot be made so fine as by burning and slaking. As was stated in the first section of this book, lime usually exists in nature, in the form of carbonate of lime, as limestone, chalk, or marble (being lime and carbonic acid combined), and when this is burned, the carbonic acid is thrown off, leaving the lime in a pure or caustic form. This is called burned lime, quick-lime, lime shells, hot lime, etc. If the proper quantity of water be poured on it, it is immediately taken up by the lime, which falls into a dry powder, called _slaked lime_. If _quick-lime_ were left exposed to the weather, it would absorb moisture from the atmosphere, and become what is termed _air slaked_. [What is air slaking? If slaked lime be exposed to the air, what change does it undergo? What is the object of slaking lime? How much carbonic acid is contained in a ton of carbonate of lime? How much lime does a ton of slaked lime contain? What is the most economical form for transportation?] When _slaked lime_ (consisting of lime and water) is exposed to the atmosphere, it absorbs carbonic acid, and becomes carbonate of lime again; but it is now in the form of a very fine powder, and is much more useful than when in the stone. If quick-lime is applied directly to the soil, it absorbs first moisture, and then carbonic acid, becoming finally a powdered carbonate of lime. One ton of _carbonate of lime_ contains 11-1/4 cwt. of lime; the remainder is carbonic acid. One ton of _slaked lime_ contains about 15 cwt. of lime; the remainder is water.