THE CHEMICAL COMPOSITION OF SOIL COLLOIDS 31 
It can be seen from the table that the oxalie-acid treatment dis- 
solved a large part of the iron present. Somewhat more iron was 
dissolved from the reddish or yellowish colloids than from the black 
or cream-colored colloids. The red or yellow color of the colloidal 
material was removed by the acid treatment. 22 The digested 
residue, which had lost the color characteristic of the colloid, still 
contained some iron, from 2.5 to 6.6 per cent, based on the original 
weight of the colloid. These results indicate the presence of two 
forms of iron in the colloid. Some form of free hydrous ferric 
oxide appears to be present in the red and } T ellow colloidal materials ; 
a colorless form of iron, probably a silicate, appears to be present 
in all the colloids. 
Oxalic acid, on the average, dissolved somewhat smaller quantities 
of silica and larger quantities of alumina than of iron. However, 
the silica and alumina are not dissolved in any constant proportion 
one to another. Therefore, the presence of a definite compound 
soluble in oxalic acid without change is not indicated. It can also 
be seen that the constituents of the colloid were not dissolved in the 
proportions in which they were present : about two-thirds of the iron, 
one-third of the alumina, and one-tenth of the silica having been 
brought into solution. 23 
Hydro chloric acid and sodium hydroxide. — Oxalic acid was used 
mainly for the purpose of detecting any form of free iron oxide that 
might be present in the colloidal material. Other solvents might 
yield suggestive results. If all the alumina and silica were present 
in the colloid as uncombined hydrous oxides, one would expect the 
alumina to be completely soluble in hydrochloric acid and the silica 
and alumina to be completely soluble in sodium hydroxide. 24 The 
action of these solvents on six soil colloids and on a colloidal suspen- 
sion of kaolinite was tried. 25 The kaolinite suspension was used for 
purposes of comparison, since it has been generally assumed that a • 
compound of this nature is present in soil colloids. 
A quantity of the air-dried colloid equivalent to 1 gram on the 
moisture-free basis was shaken with water in an Erlenmeyer flask 
until an intimate mixture was obtained, and the mixture was al- 
lowed to stand over night. In the morning 2 grams of HC1 or 
XaOH were added in solution and the volume made up to 200 c. c. 
After digesting on the steam bath for one hour and shaking every 
few minutes the solution was filtered through a hardened filter 
paper and the residue washed with very dilute HC1 or XaOH. The 
filtrate was analyzed. The results given in Table 9 are averages of 
analyses made of several digestions. The results for silica, alumina, 
22 Further qualitative tests showed that all reddish colloids in the entix-e list of 45 
lost their characteristic color when digested with oxalic acid. 
23 The partial solubility of the colloids in oxalic acid is not in harmony with O. Tamm's 
suggestion (J,2) that a quantitative extraction of the " gel complexes " in the soil can he 
made with oxalic acid or acid ammonium oxalate. In our experieuce the soil colloids sep- 
arated by the centrifuge are only partially soluble in oxalic acid or acid ammonium 
•oxalate, even after long digestion ; further, the constituents of the colloidal material ap- 
pear to be differentially dissolved. It seems doubtful that Tamm's proposed method 
would have quantitative significance. 
24 According to Mellor (27), colloidal silica can be quantitatively extracted from ceramic 
clays by a sodium carbonate solution, or by a mixture of sodium carbonate and sodium 
hydroxide. 
25 According to Dana (10, p. 686). kaolinite is insoluble in hydrochloric acid. This 
statement undoubtedly refers to comparatively coarse fragments. It is not certain, how- 
ever, that kaolinite would be insoluble if the particles were of colloidal dimensions. The 
largest particles in the kaolinite suspension we used were about 1 micron in diameter, 
hence somewhat coarser than the soil colloid particles. 
