4. In drying under the influence of the sun, soils contract and diminish in bulk in proportion to the quantity of clay or of peaty matter they contain. Sand does not at all diminish in bulk in drying, but peat shrinks in one-fifth, and agricultural clay nearly as much. The roots are thus compressed, and air is excluded, especially from the hardened clays, and thus the plant is placed in a condition unfavourable to its growth. Hence the value of proper admixtures of sand and clay. By the latter (the clay), a sufficient quantity of moisture is retained, and for a sufficient length of time; while, by the former, the roots are preserved from compression, and a free access of air is permitted. 5. In the hottest and most drying weather, the soil has seasons of respite from the scorching influence of the sun. During the cooler season of the night, even when no perceptible dew falls, it has the power of again extracting from the air a portion of the moisture it had lost during the day. Perfectly pure sand possesses this power in the least degree; it absorbs little or no moisture from the air. A stiff clay, on the other hand, will in a single night absorb sometimes as much as a 30th part of its own weight, and a dry peat as much as a 12th of its weight; and, generally, the quantity thus drunk in by soils of various qualities, is dependent upon the proportions of clay and vegetable matter they severally contain. We cannot fail to perceive from these facts, how much of the productive capabilities of a soil is dependent upon the proportions in which its different earthy and vegetable constituents are mixed together.