XV

The factors II, IV, V, VI, VII, to which must also be added the
specific gravity of the cement, VIII,) disclose all the peculiarities
of the cement as far as they refer to its action as a constituent of
soils, and if compared with the weight and character of the quartz-
sand particles and their associates (undecomposed rocks,) as deter-
mined by I and III, will result in a perfect understanding of the
physical state of the soil. These factors have to be noticed in a
tabula arranged for the purpose; and each new analysis of soil,
noticed in the form of these factors, will extend this tabula, and
thus enlarge the knowledge of the chemist so far, that he may soon
acquire the principles of a general and thoroughly practical classi-
fication of soils.

Though these factors give an entirely satisfactory account of the
action of the different substances which constitute the bulk of soils,
it is notwithstanding often desirable to know the exact quantity of
humus which is present.

To determine the quantity of humus, about 150 grains of the soil
are to be boiled for some time in dilute potash liquor, additions of
water being made to it from time to time as evaporation takes place.
By these means all the humus is dissolved, forming a dark brown
liquor, which is to be filtered from the insoluble residue, and treated
with dilute hydrochloric acid, until it becomes slightly acid. The
humus is then precipitated, and must be collected upon a weighed
filter, washed, dried at 266 F., and weighed; the resulting weight
is that of the humus of the soil minus the weight of the ashes which
are left after burning it. The average quantity of ammonia which
humus can produce is one-seventh of its weight, and can be recog-
nized by its smell either when the humus is heated by itself, or still
better when heated with soda lime.

Examination of the directly nourishing properties of

SOILS.

We have already explained that the directly nourishing proper-
ties of soils depend on the presence and respective quantities of
phosphoric acid, sulphuric acid, chlorine, silicic acid (silica in its
soluble modification,) lime, magnesia, potash and soda, which are
diffused through the bulk of soils in form of alkaline, or earthy
salts. All of them are more or less easily soluble in water, and
therefore uniformly diffused through the soil.

For the purpose of extracting them from soils, chemists are very
apt to treat soils with water, and to think that the result of analysis
of the so procured extract would represent that quantity of nour-
ishing substances in soils, which will benefit the plants for the
present. This is however not the case; we cannot imitate all the
influences which nature during a whole season exercises upon these
substances, by treating them for some hours with cold or hot water