From actual experiments made with the flow of air through various soils, King calculates that while in ordinary loam soils the total surface is about an acre per cubic foot, in fine clay soils it rises to as much as four acres. If we imagine this large surface to be covered with even a very thin film of water, it is readily seen how large an amount may be present in a cubic foot of moist soil. E. A. Mitscherlich (Bodenkunds für Land-und-Forstwirthe; Berlin, 1905) attributes to the surface offered by the soil particles supreme importance in determining the productiveness of soils. According to him the internal soil-surface determines directly the ease with which roots can penetrate the soil; and he proposes the determination of this factor by means of the heat produced in wetting the soil (“Benetzungswärme”), measured in a calorimeter, as a substitute for all methods of physical soil analysis, which are vitiated by the varying shapes and densities of the particles; while his method gives directly the actual surface. To the consumption of energy required by difficult penetration he attributes most of the differences in production, and hence refers to the internal soil-surface as governing nearly all the other physical factors. The introduction of many arbitrary assumptions, and the failure to show that the admitted inaccuracy of the ordinary mechanical soil analyses are of any practical importance, greatly detract from the cogency of the rigorous mathematical discussion carried through his work by Mitscherlich. _Influence of the several grain-sizes on soil texture._—Undoubtedly the most potent of all the sediments appearing in the above table in influencing soil texture, is the “clay.” That the materials included under this empirical designation may vary considerably in different soils, has already been sufficiently insisted on; and it is doubtful that in the present imperfect state of our knowledge of the functions of the several physical grain-sizes, we would be much wiser were we to go to the extreme advocated by Williams (Forsch. Agr. Phys., vol. 18, p. 225, ff.), of determining with precision the actual amount of such extremely fine clay particles as cease altogether to obey the law of gravity when once suspended in water. It is at least doubtful that the essential property of adhesive plasticity belongs only to these, for this property doubtless increases gradually as the size diminishes, although unquestionably not a mere function of the latter, since it belongs only to the hydrated silicate of alumina.