THE CHEMICAL COMPOSITION OF SOIL COLLOIDS 7 
a clay containing about 21 per cent of alumina and 51 to 54 per 
cent of silica " — and the second-rate soils " yield a clay containing 
about 30 per cent of alumina and 50 per cent of silica." 
Hilgard (22) considered that the fine fractions of soils which he 
obtained by his method of mechanical analysis consisted of a " pure 
clay substance," humus, silicic acid, hydrated oxides of iron and 
aluminum, and " zeolitic hydrates." He stated that " the pure clay 
substance probably consists of silica and alumina in the proportion 
of very nearly 46 to 40, 5 the rest (14 per cent) being water of 
hydration." He did not give any direct experimental evidence for 
the existence of the different kinds of material he considered present 
in the finest soil fractions, except in the case of alminum hydroxide. 
His evidence for the existence of this compound was that by the 
acid digestion method of analysis a number of clay fractions of 
various soils from Mississippi and California showed a much larger 
proportion of alumina to silica than is present in kaolinite'. Hilgard 
considered that no combination of alumina and silica in soils could 
be more. basic than kaolinite, and therefore the excess of alumina 
over that required to form kaolinite was present as free aluminum 
hydroxide. 
Blanck (5) concludes from the analyses of the colloidal fractions 
of seven soils from near Breslau, Germany, that the colloidal frac- 
tion was mainly made up of quartz particles. 
Ramann (•?#), p. 27) gives a list of colloidal products resulting 
from the weathering of soil minerals. This list is similar to Van 
Bemmelen's, but includes in addition hydrous iron silicates and 
hydrous magnesium silicates. Eamann's list of substances and also 
that of Ehrenburg (12) are apparently based on the previous work 
of other investigators. 
Oden (SO) has stated that "the true soil colloids are the innumer- 
able fragments of both weathered and unweathered minerals, crystal 
chips, and amorphous substances which, in a state of fine subdi- 
vision, constitute the clay." 
The general ideas of ceramic chemists concerning the composition 
of the finest fractions in ceramic clays have been summarized by 
Searle (40) in a general review on the subject of clays. He states: 
The smallest particles which are obtained by elutriating the materials com- 
monly known as clays are found to correspond, more or less closely, on 
analysis, to a composition which may be represented by the formula AI2O3.- 
2Si() 2 .2H 2 0. The constancy of composition of the better qualities of white 
burning clays has led to the supposition that there is in all clays an essential 
substance — " true clay, clayite, or pelinite " — on which all clayey mixtures 
depend for their chief property. The existence of this " true clay " has been 
so often assumed that there is a wide spread impression that it really exists 
as a definite chemical compound though it has never been satisfactorily iso- 
lated. 
In summarizing the earlier work on the inorganic composition of 
soil colloids, it can be said that many investigators, especially the 
ceramists, believed the main constituent of the finest fractions of 
soils and ceramic clays to be a hydrous aluminum silicate of the 
composition of kaolinite. Others believed the colloidal matter to be 
heterogeneous and considered that it contained, in addition to the 
5 This percentage composition corresponds to that of kaolinite. In a footnote Hilgard 
states that " there is still some discussion as to the chemical identity of colloidal clay 
with kaolinite, but the objections are net convincing." 
