_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 subsoil, although containing apparently 15% more clay than the former, and 7% more than the latter. But No. 246 is a highly ferruginous clay, in which the ferric hydrate is in a very finely divided condition, and materially influences the physical qualities of the clay substance. Were it all accumulated in the “clay,” it would diminish the percentage of true clay by 11.75%, reducing the clay-percentage to 28.5% which accords more nearly with the soil’s only moderate adhesiveness, and not excessively heavy tillage. But it must be remembered that the iron oxid shown in the analysis is not nearly always in this finely diffused condition. Frequently it incrusts the sand grains; quite commonly it forms small concretions of limonite, which themselves act as sand grains; and again, it may be present in the form of “black sand” or magnetic oxid, as is commonly the case in California and on the Pacific slope generally. To take this point properly into account, therefore, it would be necessary to determine the amount of ferric hydrate actually present in the “clay” as separated by subsidence of the granular constituents. _Other substances._—This circumstance as well as the inevitable presence of other modifying substances, clearly shows the desirability of being enabled to examine the physical properties of this “clay” directly, by collecting its entire amount as obtained in analysis, instead of merely determining it by weighing fractional portions. When this is done the analysis is much more valuable as indicating the true tilling qualities of the land. The increase of bulk suffered by this substance after wetting, is a very fair index of its content of true clay, and is preferable to the chemical analysis proposed by some investigators. For it is quite impossible to distinguish the silica and alumina derived from the kaolinitic substance proper, from that which is due to the decomposition of zeolites.