_Lime._--Lime is by far the most important of the mineral manures, and an almost indispensable agent of agricultural improvement. It has been used as chalk, marl, shell and coral sand, ground limestone, and as quick and slaked lime, and its action varies according as it is applied in any of its natural forms, or after being burnt. In all of its native forms the lime is combined with carbonic acid in the proportion of fifty-six parts of lime to forty-four of carbonic acid, and the carbonate is generally mixed with variable quantities of earthy ingredients, which in some instances are important additions to it, and affect its utility as a manure. _Chalk_ is a very pure form of carbonate of lime, and where it abounds has been largely employed as an application on the soil. It is dug out of pits and exposed to the action of the winter's frost, by which it is thoroughly disintegrated, and in spring it is applied in quantities, which, in many instances, are only limited by the question of cost. _Marl_ is a name given to a mixture of finely-divided carbonate of lime, with variable proportions of clay and siliceous matters, which is found at the bottom of valleys and in hollow places in beds often of considerable extent and thickness, where it is deposited from the waters of lakes holding lime in solution, fed by streams passing over limestone, or rocks rich in lime. The composition of marls differs greatly in different districts, and they have been divided into true marls, and clay marls, according as the carbonate of lime or clay is the preponderating ingredient. The following table illustrates the composition of different varieties:-- +---------------------------+------------+-----------+-----------+------------- | | Barbadoes. | Luneburg. | Ayrshire. | Wesermarsh. +---------------------------+------------+-----------+-----------+------------- | Carbonate of lime | 93.2 | 85.4 | 8.4 | 8.2 | Carbonate of magnesia | ... | 1.3 | ... | 3.0 | Sulphate of lime | ... | 0.1 | ... | 0.5 | Phosphate of lime | 0.1 | 2.3 | ... | 1.2 | Alumina and oxide of iron | 1.6 | 4.6 | 2.2 | 7.2 | Alkaline salts | ... | 0.1 | ... | 1.0 | Silica and clay | 4.6 | 5.6 | 84.9 | 78.9 | Organic matter | 0.5 | 0.6 | 2.8 | ... | Water | ... | ... | 1.4 | ... +---------------------------+------------+-----------+-----------+------------- | | 100.00 | 100.00 | 99.7 | 100.00 +---------------------------+------------+-----------+-----------+-------------