22 
There is practically no doubt that colloids exist in Hawaiian soils. 
The physical properties indicate such to be the case. A chemical 
analysis of the clay shows it to be primarily a silicate of iron and 
aluminum with a probabihty of the hydrates being present also. 
Noncoagulable clay, by analysis, shows a higher percentage of iron 
and silica and less alumina than the coagulable clay. This indicates 
that part of the iron exists in the form of ferric hydrate. 
While the chemical composition may affect the nature of the colloids 
and the degree of flocculation the phenomenon itself is physical. 
The relation is between the composition and nature of the colloidal 
film surrounding the clay particle and its degree of surface tension. 
The effects of any added acid or salt is to alter the nature of the film 
probably through penetration or chemical action, thereby increas- 
ing or decreasing the surface tension, depending on the nature of the 
added substance, and increasing or decreasing the degree of floccula- 
tion. Some authors maintain that the salt or acid actually replaces 
bases within the colloid, thereby altering its composition, while others 
maintain that it only alters the film and by washing upon a filter 
with water the clay wiU revert to its original colloidal state. This 
latter contention seems to apply best to the conditions found to exist 
in Hawaiian soils. At all events the flocculation of Hawaiian clay 
is influenced as follows : 
(1) Most acids and neutral salts, especially electrolytes, increase 
the degree of flocculation. 
(2) Highly dissociated acids are the strongest coagulants, and the 
less dissociated acids act more or less in proportion to their degree of 
ionization. 
(3) Electrolytes of greater valency possess a greater degree of 
flocculation than those of lesser valency. 
(4) Most highly dissociated alkaUs are strongest deflocculants, as 
are also the alkah salts of weak acids, such as phosphoric. 
(5) Ammonium hydroxid is an exception, being only slightly 
ionized, but at the same time it is the strongest deflocculant. 
(6) The degree of flocculation depends upon the strength or valency 
of the anion as well as of that of the cation. 
COHESION. 
The film of moisture around soil particles imparts to them cohesion 
by which the particles are bound together. As the moisture content 
decreases surface tension of the film increases and the particles are 
drawn together. Hence, in a clay soil where shrinkage is greatest, 
there results the formation of cracks. There is a definite moistm-e 
content at which tenacity of the soil particles is at a minimum, the 
texture is best for culture, and the whole environment is most con- 
ducive to the best plant growth. The factors bringing about such 
