Wollny (Forsch. Agr. Phys. Vol. 12, 1889, p. 385), discusses the influence of color on soils in relation to moisture and content of carbonic acid. The results show in general simply the effects due to increase of temperature when the soils are either darker-colored throughout, or made so superficially. _“Red” Soils._—Next to a black soil, a “red” one will usually command the instinctive approval of farmers. The cause of this preference is not as obvious as in the case of the black tints; but the general consensus of opinion requires an examination of its claims. It is of course easy enough to adduce examples of very poor “red” soils, derived from ferruginous sandstones that supply little else than quartz and ferric hydrate; the Cotton States supply cogent examples in point, as do also the lower Foothills of the Sierra Nevada of California. It is not, therefore, the iron rust or ferric hydrate that renders the land productive; but its presence is a sign of some favorable conditions. First among these is, that ferric hydrate cannot continue to exist in badly drained soils; a “red” soil is therefore a well-drained one, and this is probably one of the chief causes of the popular preference. The “white land” sometimes seem in tracts otherwise colored with iron, is distinctly inferior in production to the red lands; and examination will generally show that from some cause, such white lands have been subjected to the watery maceration which proves so injurious (see chap. 3, p. 46, chap. 12, p. 231). That finely-diffused ferric hydrate has a very high power of absorbing moisture as well as other gases of the atmosphere, has been shown in the preceding chapter; it stands in this respect next to humus itself, and hence highly ferruginous soils need not contain as much humus as “white” soils from this point of view. Like humus, also, it renders heavy clay soils more easily tillable. _Origin of Red Tints._—Where crystalline rocks prevail, the red tint usually indicates the derivation from the weathering of hornblende; implying also, outside of the tropics, the presence of sufficient lime in the land. Such lands are naturally preferred to those of lighter tints derived from purely feldspathic rocks (see chap. 3, p. 32), although they may be poorer in potash than the latter. But the red tint has also its intrinsic advantages in the more ready absorption of the sun’s heat by the colored than by a white surface. This is probably the chief cause of the higher quality of wines grown on red hillsides in the middle and northern vine districts of Europe, where everything that aids earlier maturity is of the greatest importance. The function of ferric oxid as a carrier of oxygen (chap. 4, p. 45) probably also aids nitrification. “Yellow” lands owe their tint, of course, to smaller amounts of ferric hydrate, but share more or less in the advantages of the “red.”