REVERSIBILITY OF THE COLLOIDAL CONDITION OF SOILS 497 
other methods. For example, a colloid in the dry state can perhaps best 
be measured by an adsorption of water vapor, since the adsorption of a 
dye from solution would introduce the possible effect of free water on the 
condition. In the moist condition, on the other hand, the dye-adsorption 
method would not have that disadvantage at least. It was also thought 
wise to attack the problem with as many methods as were reasonably 
practicable. Results obtained during the course of the experimentation 
strengthened these opinions. 
Three methods were used: (1) a slight modification of Mitscherlich’s 
water-vapor-adsorption method; (2) a modification of Ashley’s dye- 
adsorption method; and (8) the suspension method. The first and the 
second method were used the most extensively. The details of each 
method as finally worked out and used are as follows: 
The water-vapor-adsorption method.— Five-grany. portions of oven-dried 
soil or the equivalent were spread out as uniformly as possible in petri 
dishes having a diameter of approximately 10 centimeters. This gave in 
each dish a layer of soil with an average thickness of less than 1 millimeter. 
The dishes with the soil were placed in desiccators containing a little 
10-per-cent sulfuric acid in the lower part. By setting the dishes one on 
another, from five to seven dishes could easily be placed in the small 
desiccators and twice that number in the large desiceators. Mitscherlich 
used only one dish to a desiccator, which made the method exceedingly 
slow. 
The dishes were separated from one another by mats of cotton cloth 
which had been saturated with the acid solution, the excess being removed 
by wringing. Each mat was kept from touching the soil in the dish below 
by means of a support made from a piece of cardboard and dipped into 
hot paraffin. The mats served to hasten the vaporization in the immediate 
vicinity of the soil. On the inner sides of the desiccators (humidifiers, 
properly speaking) were broad strips of cotton cloth, which were held 
in place by air pressure and adhesion and which dipped into the acid 
solution beneath. These served to bring up the liquid and hasten 
vaporization. 
Hilgard (1911) used a wooden box, the sides of which were lined with 
blotting paper moistened with water to insure saturation of the air. 
Mitscherlich (1905) used 10-per-cent sulfuric acid in the bottom of the 
desiccator, but no strips of cloth or paper to hasten vaporization. 
