In a soil which is naturally dry or has been drained, the superfluous moisture escapes by the drains, and only that comparatively small quantity which is retained by capillary attraction is evaporated, and hence the soil is more frequently and for a longer period in a condition to take advantage of the heating effect of the sun's rays, and in this way the period of germination, and, by consequence also, that of ripening is advanced. The extent of this influence is necessarily variable, but it is generally considerable, and in some districts of Scotland the extensive introduction of draining has made the harvest, on the average of years, from ten to fourteen days earlier than it was before. It is unnecessary to insist on the importance of such a change, which in upland districts may make cultivation successful when it was previously almost impossible. The removal of moisture by drainage affects the physical characters of the soil in another manner; it makes it lighter, more friable, and more easily worked; and this change is occasioned by the downward flow of the water carrying with it to the lower part of the soil the finer argillaceous particles, leaving the coarser and sandy matters above, and in this way a marked improvement is produced on heavy and retentive clays. The access of air to the soil is also greatly promoted by draining. In wet soils the pores are filled with water, and hence the air, which is so important an agent in their amelioration, is excluded; but so soon as this is removed, the air is enabled to reach and act upon the organic matters and other decomposable constituents present. In this way also provision is made for the frequent change of the air which permeates the soil; for every shower that falls expels from it a quantity of that which it contains, and as the moisture flows off by the drains, a new supply enters to take its place, and thus the important changes which the atmospheric oxygen produces on the soil are promoted in a high degree. The air which thus enters acts on the organic matters of the soil, producing carbonic acid, which we have already seen is so intimately connected with many of its chemical changes. In its absence the organic matters undergo different decompositions, and pass into states in which they are slowly acted on, and are incapable of supplying a sufficient quantity of carbonic acid to the soil; and they thus exercise an action on the peroxide of iron, contained in all soils, reduce it to the state of protoxide, or, with the simultaneous reduction of the sulphuric acid, they produce sulphuret of iron, forms of combination which are well known to be most injurious to vegetation.