112 GEORGE F. KAY AND J. NEWTON PEARCE 



washing, the clay becomes still more finely divided and finally 

 passes through the filter, giving a turbid non-settling suspension. 

 On adding a small amount of an electrolyte the milk-white liquid 

 coagulates and settles. Upon washing again another point is 

 reached at which the particles become infinitely fine and pass 

 through. So it is in clay soils: an excess of water percolating 

 downward removes the excess of coagulating electrolyte from the 

 leached clay. This permits first a swelling of the reversible col-, 

 loidal material and finally, to a shght extent at least, the gradual 

 re-formation of the colloidal "sol." The suspended particles are 

 thus permitted to pass slowly downward, where they are again 

 coagulated at some lower level. 



The inorganic colloids of soils and clays exhibit a marked 

 tendency to adsorb upon their surfaces the various organic emul- 

 soids formed from plant and animal debris. The humus is full 

 of these. The adsorbed emulsoidal material forms an oil-like 

 film about the suspended particles and imparts to them its own 

 reversibility and stability. Hence when a mixture of the emul- 

 soid and suspensoid materials are evaporated to dryness and the 

 dry material is again placed in water the whole mass again passes 

 into colloidal suspension. Furthermore, if emulsoidal material 

 of any sort is added to a coagulated hydrogel, such as clay, the 

 emulsoid possesses the power to peptizate or deflocculate the clay 

 hydrogel, thus rendering it capable of colloidal suspension. By 

 their reversible and protective influence humous materials hinder 

 the coagulation of clay colloids; by their deflocculating influence 

 they tend to make the hard, dry, sun-baked clays again reversible. 



The terms humus and humic acid have been mentioned. The 

 latter is a very complex substance of doubtful composition; it is an 

 acid and possesses a colloidal nature. It dissolves in 8,337 parts 

 of water at 6°. Its ammonium and magnesium salts are rather 

 easily soluble; calcium humate dissolves in 3,125 parts of water, 

 while the least soluble ferric humate dissolves in 5,000 parts. The 

 humic acids are solvents for silica. Humic acid has the property^ 



' Julien, "On the Geological Action of the Humic Acids," Proc. Amer. Assoc. Adv. 

 Sci. (1879), pp. 311-410. 



