766 EXPERIMENT STATION RECORD. 



particles coiieei iicd, and ceases for the most part in (piartz powder when 

 the diameter of the grains rises to 0.2 ram., supposing all the particles 

 to he of the same size, for particles larger than 0.2 mm. diameter may- 

 enter into aggregates made up for the most part of smaller grains. 

 The number of grains which can cohere in this way varies also in ap- 

 proximately the same manner. The larger the Individual grains the 

 more easily their aggregates fall to pieces. The occurrence of sedi- 

 ments of the same hydraulic value but of different specific gravity 

 show that a higher density tends to diminish flocculation. This ten- 

 dency also decreases rapidly with an increase in temperature, being- 

 very weak in water near its boiling point.^ Colloid substances are also 

 very influential in the formation of floccules, and the free insoluble 

 humus acids produce the same result by binding the other constituents 

 together.^ The formation of aggregates is greatest when the soil con- 

 tains a medium quantity of water, i. e., about 

 40 per cent of the maximum quantity which it 

 is capable of holding, and a greater or less quan- 

 tity of water always diminishes the tendency 

 towaid flocculation.'' 



Of secondary importance is the fact that the 

 soluble salts of the soil, even when present in 

 very small quantities, exert an important influ- 

 ence on its structure.^ Caustic alkalies (potash, 

 soda, and ammonia), as well as their carbonates 

 and phosphates, cause a compact arrangement of the individual grains, 

 and hinder or prevent the formation of aggregates, while the mineral 

 acids (hydrochloric, sulphuirc, and nitric) and their salts, with the 

 exception of the neutral alkaline sulphates, which rather approach the 

 l^receding group in behavior, materially aid flocculation. In the case 

 of the chlorids and nitrates this is true only while they remain in the 

 soil liquids. When they are washed out by the atmospheric water, 

 which readily takes place, a more compact arrangement of the indi- 

 vidual atoms results. Calcium hydrate (caustic lime) and, to a less 

 extent, calcium carbonate help to form light, loose aggregates, which 

 resist external influences tending toward their disintegration for a 

 longer or shorter time, according to the quantity of the flocculating 

 substance present. 



Among external factors which influence the structure of the soil 

 frost is of first importance. By the freezing and consequent expan- 

 sion of the water the particles are torn apart. In a compact soil having 

 a tendency to form floccules frost will, as a rule, bring about a crumbly 

 structure, while in a soil already mechanically finely divided the par- 



1 E. W. Hilgard, Forsch. Geb. agr. Phya., 2, p. 441. 



2 T. Schlosing, Compt. Rend., 64, p. 1408. 



■■' H. Pncliner, P'orsch. Geb. agr. Pliys., 12, p. 239. 



"E. W. Hilgard, Forscli. Geb. agr. Phys., 2, p. 441. A. Mjiycr, Ibid., p. 251. 



