This removal arises from two causes. The rain-water that descends upon the land holds in solution carbonic acid which it has absorbed from the air. But water charged with carbonic acid is capable of dissolving carbonate of lime, and thus year after year the rains slowly remove as they sink to the drains, or run over the surface, a portion of the lime which the soil contains. Acid substances are also formed naturally in the land, by which another portion of the lime is rendered easily soluble in water, and, therefore, readily removable by every shower that falls. The _chemical_ effects of lime upon the soil are chiefly the following:— 1. When laid upon the land in the _caustic_ state, the first action of the lime is to combine immediately with every portion of acid matter it may contain, and thus to sweeten the soil. Some of the compounds it thus forms being soluble in water, either enter into the roots and feed the plant,—supplying it at once with lime and with organic matter,—or are washed out by the springs and rains, while other compounds, which are insoluble, remain more permanently in the soil. 2. Another portion decomposes certain saline compounds of iron, manganese, and alumina, which naturally form themselves in the soil, and thus renders them unhurtful to vegetation. A similar action is exerted upon certain compounds of potash and soda, and of ammonia,—if any such are present,—by which these substances are set and placed within the reach of the plant. 3. Its presence in the caustic state further disposes the organic matter of the soil to undergo more rapid decomposition—it being observed, that where lime is present in readiness to combine with the substances produced during the decay of organic matter, that decay, if other circumstances be favourable, will proceed with much greater rapidity. The reader will not fail to recollect, that during this decay many compounds are formed which are of importance in promoting vegetation. 4. Further, quicklime has the advantage of being soluble in cold water, and thus the complete diffusion of it through the soil is aided by the power of water to carry it in solution in every direction. 5. When it has absorbed carbonic acid, and become reconverted into carbonate, the original caustic lime has no _chemical_ virtue over chalk, rich shell-sand or marl, or crushed limestone. It has, however, the important _mechanical_ advantage of being in the form of a far finer powder, than any to which we could reduce the limestone by art—in consequence of which it can be more uniformly diffused through the soil, and placed within the reach of every root, and of almost every particle, of vegetable matter that is undergoing decay. I shall mention only three of the important purposes which, in this state of _carbonate_, lime serves upon the land.