REVERSIBILITY OF THE COLLOIDAL CONDITION or SoILs 491 
4. Alternate drying and wetting of some soils favors some of the soil 
activities which are correlated with fertility. 
5. Previous drying increases the solubility of inorganic and organic 
constituents, such increase usually accompanying an increase in the 
temperature of drying. 
6. Soils containing considerable organic matter are affected by moisture 
changes to a greater degree than are those containing small amounts of 
organic matter. 
7. Changes in certain phe rical properties of soils, such as cohesion and 
plasticity, are, to a degree at least, reversed by moisture changes. 
8. Changes in the proper ties of soils due to moisture changes have been 
attributed by many workers to changes in the colloidal condition. 
9. Studies with more or less pure colloidal materials usually found in 
the soil in the colloidal condition indicate a possible reversibility of all 
these materials under some conditions. 
10. Most of these studies cited have been on soil properties other than 
colloidal, and with methods other than those of colloid chemistry. 
EXPERIMENTAL WORK 
Study of methods 
Snyder (1917) lists eighteen possible methods, under eight distinct 
headings, for measuring the colloidal content of soils. The relative merits 
of some of these have been discussed fully by Stremme and Aarnio (1911), 
and Snyder reviews ail of them. 
Altho most writers on the subject are of the opinion that the method 
of water-vapor adsorption, originally devised by Mitscherlich (1905), is 
the best general method for measuring colloidal content, Snyder thinks 
that the dye-adsorption meth first used by Ashley (1809), has the 
greatest possibilities. eoGe recognizes the speci icity of dyes for par- 
ticular colloids. These two methods are undoubtedly the most feasible 
and practicable at present known. 
Owing to the unsatisfactory status of methods for measuring colloid- 
ality, it was felt necessary to investigate some points most commonly 
disputed. Also, -in the attempt to use various methods, some difficulties 
were encountered which required settling. Some time was spent, there- 
fore, in preliminary work on methods before they were worked out as 
