The Soil. 
21 
ignition, shows the sand grains to be coated to a greater or less 
extent with this substance, oxid of iron. It does not appear as red 
oxid of iron on the grains before ignition, but becomes evident 
after. This is well seen in the color of the mass before and after 
ignition. This suggests the existence of hydrates of iron or organic 
compounds, but the loss on ignition is so small and irregular, bear¬ 
ing in its quantity no relation to that of the iron, that the organic 
compounds, if any, are very small in amount. There seems to be, 
as we shall see later, but a small amount of iron present in the form 
of a silicate insoluble in hydrochloric acid, certainly not more than 
may be present in a perfectly fresh feldspar. This would still be 
the case if the quartz sand were all deducted or considered as carry¬ 
ing no iron whatever. The portion soluble in hydrochloric acid, on 
the other hand, is rich in iron. The total loss on ignition is not 
unusually large, and a portion of this is water retained at the 
temperature of 110° C, probably in the form of hydrated oxid of 
iron. 
SOLVENT ACTION OF HYDROCHLORIC ACID ON THE SOIL. 
§ 42. The portion of these soils dissolved or decomposed by 
hydrochloric acid, sp. gr. 1.115, upon digestion for five days, varied 
from 30 to 50 per cent. The amount dissolved certainly depends 
upon the character of the substances present in the samples, but in 
this respect our samples are very uniform, containing essentially the 
same mineralogical components. The variation in the amount dis¬ 
solved depends upon other conditions, principally upon the fineness 
of the soil particles, and the particular degree of their decomposi¬ 
tion. The former condition is the more important, as the latter one 
is largely dependent upon it. This is quite clearly seen upon a 
comparison of the mechanical analyses of these samples, taking the 
particles having a diameter greater than 0.05 millimeter as one 
group, and those of less diameter as another group. The amount 
dissolved under the convential conditions, HC1., sp. gr. 1.115, and 
five days digestion on water bath, varies, but not closely, with the 
ratio of these two groups, into which every soil can be divided. In 
other words, it is the very fine particles of the soil which yield to the 
hydrochloric acid the elements of plant food found in the solution. 
This is unquestionably the case in the soil, as well as in the analy¬ 
sis. The decomposition of the minute particles of the minerals will 
take palce more slowly in the soil solutions than in the hydrochloric 
acid, but the order in which they will be attacked is the same, the 
finest first. 
RESULTS OF CHEMICAL ANALYSES NOT COMPETENT CRITERIA. 
§ 43. The chemical analysis does not give us a good criterion 
by which to judge of the virtues of a soil. It may not follow the 
same course of decomposition that takes place in the soil, and it cer- 
