_Influence of the Nature of the Soil-Material._—Aside from the surface condition, the nature of the material itself exerts a certain influence, not only upon the rate of introduction of heat, but also upon the amount taken up. Thus quartz sand having the highest density (greatest weight per cubic foot) and also the highest capacity for heat among the usual mineral soil-ingredients, will, mass for mass, experience a smaller rise of temperature than would clay or loam soil, of less density or volume-weight, and also of lower heat-capacity. While this holds good theoretically, differences corresponding to this consideration rarely occur in nature, for the reason that the much greater influence of the mechanical condition of the soil mostly overbalances these effects. Thus Wollny has shown that while quartz is a better heat-conductor than clay, quartz cobbles or gravel will materially increase the temperature of the soil in which they are imbedded. Yet compact clay is a better conductor of heat than loose sand; hence the latter, when exposed to the intense heat of the summer sun in the desert, becomes intensely hot on the surface, yet allows of the existence of abundant moisture at a depth of ten or twelve inches; while clay in the same region, being usually in a compacted condition, will show a lower surface-temperature and will be warmer and drier at a depth at which sand will still retain abundant moisture, and be comparatively cool (See chap. 13, p. 257.) So much indeed depends upon the state of mechanical division and flocculation in which the several soils may happen to be, that a hard-and-fast statement in regard to their relations to heat cannot and should not be given, as it would only lead to disappointments and practical mistakes; the more as in all cases the moisture-condition exerts an influence predominating by far over that of the dry material itself, and this moisture-condition is subject to rapid changes, owing to intrinsic differences in the several classes of soils. Wollny states as the result of his experiments, that in summer sandy soils are warmest; then humous, lime and loam soils; while in winter humous soils are warmest, then loams; and sand coldest. _Influence of Evaporation._—In treating of the Conservation of Soil Moisture (chapter 13), the effects, conditions and control of evaporation from the soil have already been discussed from several points of view; so that a summary review of the subject must suffice in this place.