100 SOILS. 



zungswarme " ), 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 penetra- 

 tion he attributes most of the differences in production, and hence re- 

 fers to the internal soil-surface as governing nearly all the other physi- 

 cal 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. Un- 

 doubtedly 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 ex- 

 tremely fine clay particles as cease altogether to obey the law 

 of gravity when once suspended in water. It is at least doubt- 

 ful 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. 



Ferric Hydrate. Probably the body which most commonly modifies 

 materially the adhesive and contractile properties of the clay substance, 

 is ferric hydrate ; the more as on account of its high density it tends 

 to exaggerate materially, in many cases, the apparent content of true 

 clay, and the estimate of the soil's plasticity based upon it. A good 

 example in point is the case of soil No. 246 (Miss.) of the above table. 

 This is a heavy clay soil, yet not excessively adhesive ; scarcely as 

 much so as No. 230 (Miss.), the heavy gray " flatwoods " soil, and 

 not nearly as " sticky" when wet as No. 173 (Miss.), the prairie sub- 

 soil, although containing apparently 15 / more clay than the former, 



