THE PHYSICAL PROPERTIES OF THE SOIL. 763 



tation or liydrostatic method (Davy, Scliiibler, Spreugel, Beuiiiiigsen- 

 Forder, Knop, Schlosiug, KiOin, Osborne, FadejejBf). The other depends 

 on the force of upward flowing water, or hydraulic i)re8sure, and is 

 called the hydrodynainic method (Nobel, Schone, A. Mayer, Hilgard). 

 Of the modifications of the first or hydrostatic method, Kiihn's is pref- 

 erable for ordinary work on account of its simplicity; Fadejeflf's method, 

 however, gives more exact and more closely agreeing results. Of the 

 hydrodynamic methods, those using a conical washing vessel (Nobel, 

 Schone, A. Mayer) give unsatisfactory results, on account of the back- 

 ward currents along the sides of the vessel, which produce flocculent 

 aggregates of the smaller grains which vsink to the bottom and remain 

 with the coarser sand. To avoid this difficulty Hilgard uses a verti- 

 cal cylindrical tube and an agitating apparatus the object of which is 

 to destroy any fioccules that may be formed. Hiigard's apparatus may 

 be regarded as the most satisfactory, at least for particles less than 

 0.1 mm. (8 mm. hydraulic value), which has yet been devised.^ 



In carrying out mechanical soil analysis of any kind various precau- 

 tions must be observed in order to insure accuracy. Besides the 10 to 

 20 hours' brisk boiling of the sample which is essential, water as pure 

 as possible (distilled) must be used in the elutriation, because in water 

 containing salts fioccules are a])t to form which settle and remain with 

 the coarser ingredients. For the same reason the apparatus must be 

 protected from all variations of temperature and from strong light. 

 It is therefore best placed in a dark cellar which is subject to only 

 slight variations in temperature. For the accurate separation of sedi- 

 ments of definite hydraulic value the elutriation must be preceded by 

 the removal of the clay, as otherwise the heavy, clayey water, having 

 a high hydraulic value, will carry away larger particles than correspond 

 to the real velocity of the water. The other particles may be separated 

 from the clay by repeated sedimentation in a column of water 20 cm. 

 deep for i)eriods of 24 hours.^ 



The separation of soil into the above-mentioned portions furnishes 

 a basis for interpretation of those physical properties which dej^end 

 ui)on the size of the soil particles, but throws no light on the influence 

 in this respect of colloid and humus substances. Colloid clay ^ is the 

 most imj)ortant of the former class, for to it is due the property of 

 plasticity. Kaolin particles occur in the soil, but they behave like any 

 other mechanical constituent, and are no more plastic than chalk or 

 fine siliceous earth. For the determination of " active" clay the method 



1 F. Wabnschaffe, Anleitung zur wissenschaftlichen Bodenuntersuchung, 1887, p. 23. 

 T. B. Osborne, Forscli. Geb. agr. Pbys., 10, p. 196. E. W. Hilgard, Ibid., 2, p. 57, and 

 6, p. 52. A. Mayer, Ibid., 5, p. 228. 



2E. W, Hilgard, Forscli. Geb. agr. Phys., 6, p. 53. 



^M. Whitney and others have recently ascribed the properties of colloid clay and 

 other similarly constituted substances to the extreme fineness of their jiarticles, and 

 hence the very much greater attraction the 1 attor can exert compared with any sub- 

 stances made up of comparatively coarse grains. 



