REVERSIBILITY OF THE COLLOIDAL CONDITION OF SOILS A487 
Christensen (1917) states as a result of his studies that the ability of 
soils to free acid from calcitum-acetate solution is from two to four times 
as great in the air-dry as in the moist condition, a result just the opposite 
of what was expected. With litmus solution and litmus paper, however, 
practically no difference was noticeable as a result of drying. 
The classical and fundamental researches of Van Bemmelen (1909) on 
the reversibility of the hydrogels of various oxides have supplied the 
greater part of the knowledge on this subject. He worked particularly 
with the hydrogel of silica. By placing the hydrogel in desiccators, each 
with a different vapor pressure, he obtained data which showed that the 
final dehydration depends on the preparation and previous history of the 
gel and on the vapor pressure at a given temperature. It was found 
that by lowering and raising the vapor pressure, within certain limits, 
dehydration and rehydration are possible repeatedly, altho the reversi- 
bility is not always along the same path or complete. In other words, 
unless dehydrated too far the dried gel eagerly took up water again, but 
did not take up as much as it previously had nor did the volume return 
to its original size. 
_ According to Cushman (1904), if the hydrogel of silica is heated to a 
temperature of about 1000° C. it loses its power to take up moisture to 
any great extent. However, a small amount of moisture is taken up. 
Van Bemmelen worked also with colloidal oxides of alumina, iron, and 
other metals. For ferric oxide and alumina he found results similar to 
those for silica but with minor differences in regard to transition points. 
Zsigmondy (Zsigmondy and Spear, 1917) points out that irreversible 
colloids may become reversible by the addition of reversible colloids. 
This is probably an adsorption phenomenon, and opens up interesting 
possibilities because of the lack of uniformity in the composition of 
soils. 
Muller (1907) prepared a reversible colloidal alumina by peptizing a 
freshly precipitated aluminum hydroxide with hydrochloric acid. The 
hydrosol thus obtained is fairly stable, and leaves a precipitate which is 
soluble in water. 
Ehrenberg (1915) reports Schloesing as saying that an artificial ciay 
may be made by mixing 1 per cent of glue with finely pulverized sand. 
This mixture exhibits reversibility of properties, cohesion, and plasticity, 
on being dried and wetted. 
