15 
table. Soil No. 164 is almost devoid of organic matter and combined 
water and contains the least soluble alumina, while, on the other 
hand, those soils in which the volatile matter is highest contain the 
most soluble alumina, this being especially noticeable in soils Nos. 
428 and 448. 
The effect of heat upon the solubility of alumina and silica, 
especially in water, is probably referable to a number of causes. It 
is believed, however, to be primarily physical, being related to an 
alteration of the films surrounding the soil particles and to a modifica- 
tion of the colloidal forms which these elements probably assume 
under the prevailing conditions. The former effect will be discussed 
in greater detail farther on. Dehydration and certain chemical 
alterations at the higher temperatures would, on the other hand, 
tend toward increasing the solubility in acids through the action of 
heat upon the hydrated silicates. It has long been known that 
certain aluminum silicates become more soluble in acids as a direct 
effect of heat. In the early manufacture of alum advantage was 
taken of this fact. 
IRON. 
The relative amounts of iron (Fe 2 3 ) recovered by the two solvents 
appear in the following table: 
Solubility of iron in water and fifth-normal nitric acid. 
[Calculated on basis of dry soil.] 
Soil No. 
Soluble in water (parts per million). 
Soluble in fifth-normal nitric acid (per 
cent). 
Air dry. 
Dried at 
100° c. 
Dried at 
250° C 
Ignited. 
Air dry. 
Dried at 
100° c. 
Dried at 
250° C. 
Ignited. 
74 
17.6 
3.5 
3.7 
2.9 
4.8 
6.4 
2.8 
5.2 
4.6 
1.9 
1.7 
2.1 
9.7 
3.5 
5.3 
5.3 
3.2 
9.2 
2.8 
2.7 
2.7 
2.7 
1.3 
3.2 
9.2 
3.5 
3.9 
3.1 
4.4 
5.4 
2.8 
3.3 
4.0 
3.2 
1.6 
2.3 
12.5 
5.1 
4.5 
3.5 
1.8 
5.1 
2.5 
3.8 
2.1 
1.6 
1.3 
2.1 
0.003 
.005 
.007 
.194 
.069 
.324 
.032 
.026 
.515 
.024 
.037 
.051 
0.002 
.007 
.016 
.037 
.069 
.302 
.032 
.029 
.487 
. 033 
.061 
.038 
0.006 
.083 
.006 
.037 
.142 
.290 
.027 
.029 
.333 
.039 
.014 
.077 
0.055 
164 
.047 
9 
.006 
292 
.013 
290 
.046 
405 
.157 
416 
.015 
417 
.033 
406 
.158 
428 
.107 
426.... 
.082 
448 
.024 
Again, there is considerable inconsistency in the results, but an 
average shows the solubility of iron in water to be greatest in the 
air-dried soil. The solubility in dilute nitric acid is much less con- 
sistent than that in water, thereby making it impossible to advance 
any conclusions except to call attention to the fact that the alumina 
in Hawaiian soils is very much more soluble, both in water and in 
dilute nitric acid, than is the iron. But if the results from samples 
Xos. 292, 405, and 406 be disregarded, and this is plainly permissible 
since these soils are used in aquatic agriculture and the major part 
of the soluble iron is in the ferrous condition and would be oxidized 
