REVERSIBILITY OF THE COLLOIDAL CONDITION OF SOILS 499 
having been ascertained that this amount of shaking gave a maximum, 
or very nearly maximum, adsorption. 
After removal from the shaker, the bottle and its contents were allowed 
to stand for a few minutes, until the bulk of the soil had settled. A por- 
tion of the supernatant suspension, about 50 mils, was poured into a 
centrifuge tube (together- with a few mils of 1-per-cent alum solution 
when methylene blue was used as the dye), and was centrifuged until 
clear. With diamine sky-blue a flocculant could not be used because of 
its effect on the dye. 
After clarification the supernatant dye solution was compared with a 
standard dye solution in a colorimeter, and from these readings and the 
strengths of dye used the amount of dye adsorbed was computed. 
This method differs from Ashley’s cr:'gimal method particularly in regard 
to the kind of dye used and the addition of a flocculant to clarify the 
suspension. It was found impossible to clarify some suspensions without 
a flocculant, even by centrifuging at high speed for several hours or by 
letting the suspension stand for several days. 
The suspension method.— Suspensions were made by shaking the soil in 
the medium used for thirty minutes, allowing it to stand for thirty minutes, 
evaporating an aliquot portion of the suspension, drying, and weighing. 
Both distilled water and 4-per-cent ammonium hydroxide were used as 
the dispersion media. 
Experiments with artificially prepared colloids 
Colloidal silica, alumina, and ferric oxide were prepared by the ordinary 
precipitation methods and were washed free of electrolytes with distilled 
water. Humus was prepared from muck by extracting the bases with 
hydrochloric acid, washing out the excess acid, and deflocculating with 
ammonia. The greater part of the excess ammonia was washed out with 
the water. 
The object of the experiment was to ascertain the effect of drying upon 
the adsorption of dyes by the colloidal materials most commonly found 
in soils. - 
Each of the materials was divided into three equal lots. Each lot was 
subjected to different treatment and was divided into four replicates. 
