passed through a sieve of which the meshes are five millimeters. The
agglomerated particles of earth are broken up by the hand. The pebbles
are also taken out and weighed. The pebbles are then treated with
hydrochloric acid until all effervescence is over. The insoluble part is
dried and again weighed. The difference in weight gives the quantity of
calcium carbonate contained on the external surface of the pebbles. The
earth which passes the sieve of five millimeters mesh is next passed
through a sieve having ten meshes to the centimeter. The masses on the
sieve are broken up with the hand or with a pestle, in such a manner as
to separate the fine agglomerated particles. The material which remains
upon the sieve after being dried at 100°, is weighed. This gives the
coarse sand. This is treated with hydrochloric acid as were the pebbles
before, washed and the residue dried and weighed. The difference in
weight gives the quantity of calcium carbonate adhering to the surface
of the coarse sand.
The mechanical analysis is continued with the matter which has passed
the sieve with ten meshes to the centimeter and which consists of the
soil, properly so-called. Ten grams of this are taken, dried at 100°
until no further loss takes place and the moisture thus determined.
Another ten grams are taken and placed in a capsule with a flat bottom,
and from nine to ten centimeters in diameter. This is moistened with a
small quantity of water in such a way as to make a paste. This paste is
rubbed with the finger in fifteen cubic centimeters of water. Ten
seconds after the stirring is completed the supernatant liquid is poured
into a precipitating jar of about 250 cubic centimeters capacity, taking
great care not to allow any particles to pass over which have been
deposited during that time. This operation is repeated in the same way
waiting about ten seconds each time before decanting, until the decanted
liquor is almost perfectly clear. In this way the particles of different
fineness are separated. The decanted portions contain the fine sand and
clay. The remaining portion contains the sand and particles of medium
fineness. This last part is dried, being kept at 100° until it has a
constant weight. It is afterwards treated with dilute nitric acid to
dissolve the calcium carbonate. When the carbonate is abundant, it is
sufficient to determine it by difference which is done by washing the
material, drying and weighing. But when the proportion of carbonate is
very small and in consequence when its exact determination acquires a
greater importance, it is better to determine the lime directly. For
this purpose the part soluble in dilute nitric acid is collected,
treated with ammonia and acetic acid and precipitated with ammonium
oxalate. Details of this operation will be given in another part of this
manual.
In regard to the matter which is insoluble in nitric acid, it is
composed chiefly of silica or silicates, and sometimes also of vegetable
débris. The vegetable matter is determined by the incineration of the
material which has been previously dried. The loss of weight gives the
proportion of vegetable or organic débris contained in the soil and of
combined water.
The portion which has been decanted, the volume of which should not
exceed 500 cubic centimeters, is treated with nitric acid until
effervescence ceases. It is then left to digest for some time, in order
to permit the whole of the carbonate to dissolve. It is next thrown upon
a smooth filter about one decimeter in diameter. After filtration it is
washed to secure the complete elimination of the soluble lime salts. The
lime is determined in the filtered liquid.
The insoluble portion contains the fine sand, the clay and humus bodies.
In order to separate the three elements the precipitate which was
received upon the filter, is rubbed with water, the filter is broken and
all its contents washed through. The volume of wash water is made up to
200 cubic centimeters; two or three cubic centimeters of ammonia are
added and the whole left to digest for two or three hours. The volume of
the liquid is then made up to one liter with distilled water, vigorously
shaking in such a way as to put all the matter in suspension. It is then
left to settle for twenty-four hours. At the end of this time the
supernatant liquid is decanted by the aid of a siphon. To the residue
are added two cubic centimeters of ammonia and one liter of water. The
matter is again brought into suspension and allowed to settle for
twenty-four hours. The supernatant liquid is again decanted with a
siphon, and added to the liquid previously removed. For ordinary soils
two decantations are generally sufficient but when the soils contain a
large quantity of clay it is convenient to decant three or four times.
By an examination of the supernatant liquid it is easy to tell if the
washings have been sufficiently prolonged. The decanted liquors contain
the organic matter and that which it is convenient to call clay, which
is constituted of very fine particles of sand and colloidal clay which
play, in arable soil, a rôle somewhat like that of cement.
These matters are estimated in the following manner: The liquor is first
treated with nitric acid and the clay and the humic matters are
precipitated together. They are collected upon a smooth filter one
decimeter in diameter and washed with water. By means of a washing
bottle all the solid matters which have stuck to the sides of the filter
are finally collected in the bottom of it. Since the last washings pass
the filter very slowly, they can be removed after the complete
deposition of the matter they contain, by means of a pipette. When all
the liquid is removed the filter is placed upon blotting paper, great
care being taken to avoid desiccation, having in view only the
elimination of the excess of humidity. The folds in the filter are then
carefully smoothed out with the finger. The matter which has collected
upon the filter is then removed completely with a washing bottle, placed
in a dish and dried at 100° and weighed. After weighing, the mass is
incinerated in a muffle in order to destroy the humic bodies. The
difference in weight before and after incineration, gives the total
weight of the humic bodies and since the diminution in weight comprises
not only the weight of the humic bodies, but also the weight of the
combined water which is lost during the process of incineration, there
should be subtracted from the total loss of weight ten per cent of the
weight of the residual mineral matter, which represents the water of
composition of the hydrated silicate.
=210. Statement of the Analysis.=—Schloesing in his original paper[144]
recommends that the analysis be commenced with 1,000 grams of soil. The
data of the analysis and the method of arrangement are illustrated by
the following example.
The physical examination of the earth having been completed as above,
the results can be tabulated as follows: taken, 1,000 grams of dry
earth, digested in water, thoroughly worked by hand, sifted, and passed
through the meshes of the sieve by a stream of water, the meshes having
a diameter of one millimeter.
Dry residue, fifty-five grams, contains Pebbles │ 21 grams.
„ Gravel │ 33 „
„ Organic débris│ 1 „
Sifted earth by difference, 1000 − 55 = │ 945 „
│————
│1000 grams.
Humidity of the homogeneous paste, twenty-seven per cent. Then 945 grams
of the dry sifted earth correspond to (945)/(1.00 − .27) = 1294.5 of
paste.
The analysis, therefore, should be carried on upon this weight or some
aliquot part say 0.01 thereof; _viz._, 12.945 grams.
12.945 grams of the │1st.—Coarse sand dry │Noncalcareous 3.05 grams.
paste after successive│giving by treatment │sand
kneadings and │with acid and │
decantations furnish │ignition. │
dry: │ │
„ │ „ │Calcareous 1.19 „
│ │sand
„ │ „ │Organic 0.08 „
│ │débris
│2nd.—Fine elements decanted with the water,
„ │their weight calculated by difference, 9.45 −
│4.32 = 5.13 grams.
_Treatment of the Fine Elements._—Treated by nitric acid until a
complete decomposition of the calcareous matter is secured, filtered,
washed, the residual matter collected upon a filter, and the liquid
received in a two-liter flask, a little ammonia added, allowed to
digest, the flask filled with distilled water, left for twenty-four
hours at repose, and decanted:
The decantation furnishes│1st.—A deposit of fine calcareous│3.14 grams.
│sand weighing dry │
„ │2nd.—Clayey liquid giving after coagulation
│by acid, filtration and drying 0.85 grams of
│clay.
│ │
Then: Total fine elements │5.13 grams.
Fine elements determined │Fine calcareous sand 3.14│3.99 „
directly. │ │
„ │Clay 0.85│ „
│ │————
Fine calcareous sand by difference │1.14 „
Calculating these results to the original quantity of 1,000 grams the
following data are obtained:
RÉSUMÉ.
One thousand grams of dry earth contain:
Pebbles 21 grams.
Gravel 33 „
Organic débris 1 gram.
Fine earth 945 grams.
————
Total 1000 „
945 grams of │Coarse sand 432 gms. │Noncalcareous sand 305 gms.
fine earth │ │
contain: │ │
„ │ „ │Calcareous sand 119 „
„ │ „ │Organic débris 8 „
„ │
„ │Fine elements 513 gms.│Fine, noncalcareous sand 314 gms.
„ │ „ │Clay 85 „
„ │ „ │Fine calcareous sand 114 „
│ │ ——
│ │Total 1000 „
There are counted as clay all the elements which have remained in
suspension in the water after a period of repose of twenty-four hours.
In fact, these elements comprise a notable proportion of very fine sand
which is not deposited during that time. In order that the liquid should
become entirely freed from this sand it would be necessary to wait
several weeks and even several months. Such a prolongation of the
analysis is evidently inadmissible. The period of twenty-four hours of
repose therefore has been adopted. This is merely conventional, in the
same way that the period of ten seconds adopted for the precipitation of
the gravel is conventional. But this convention is justified by the fact
that the substance which is called clay presents, when it has a proper
degree of humidity and cohesion, a plasticity entirely analogous to that
property of natural clay. Moreover, as has already been said, that which
is chiefly important in these analyses is the employment of processes
always comparable among themselves in their results and generally
followed.
=211. The Belgian Method.=—The method of estimating the percentage of
sand and clay practiced at the Gembloux Station[145] is essentially that
recommended by Schloesing with a few minor modifications.
With the ball of the thumb or with the finger, 100 grams of fine earth
are rubbed with water in a porcelain capsule or mortar with a capacity
of about 250 cubic centimeters. The suspended particles are poured off
with the wash water and the process repeated five or six times, using in
all about 200 cubic centimeters of water.
The water containing the sediment is rendered slightly acid
(hydrochloric acid) adding the acid in minute particles with constant
stirring for about an hour in order to dissolve all the carbonate and to
separate the organic acids from the bases with which they are combined.
The liquid is allowed to remain at rest for five or six hours and a part
of the liquor decanted to remove any supernatant particles of organic
matter which may have passed the sieve in the original preparations of
the sample. Filter through a smooth filter about twelve centimeters in
diameter, wash until the chlorin has disappeared, and throw the filtrate
away.
Break the filter paper over the vessel in which the soil was treated
with hydrochloric acid and wash all the contents of the filter into this
vessel with as little water as possible (about 100 cubic centimeters),
add five cubic centimeters of strong ammonia water, allow to stand for
three hours, shaking from time to time and with distilled water make the
volume up to 250 cubic centimeters. Stir vigorously with a glass rod or
spatula, take this out and wash any adhering particles back, leave at
rest for twenty-four hours, siphon the turbid liquid into a two-liter
vessel. Make the volume up again to 250 cubic centimeters and treat as
above described and repeat the operation until the water becomes clear
after standing for twenty-four hours. Usually eight or ten washings are
necessary. Wash the residual sand into a weighed dish, evaporate to
dryness, ignite and weigh. The weight obtained divided by the weight of
the original sample gives the per cent of sand. The sand is separated by
sieves of varying fineness into coarse, fine, and pulverulent sand.
Add to the ammoniacal liquor collected in the two-liter flask some
powdered potassium chlorid (five grams per liter) to hasten the
coagulation and rapid deposit of the clay.
After twenty-four hours siphon the clear liquor, collect the deposited
clay in a smaller vessel, allow to remain at rest and decant as much of
the clear liquor as possible. Pass through a plain tared filter about
nine centimeters in diameter, dry at 150° and weigh the clay.
=212. The Italian Method.=—Schloesing’s method as carried out by the
Italian chemists[146] is as follows:
A kilo of earth dried in the air is passed through a sieve the threads
of which are separated a distance of five millimeters; and with this the
small pebbles are separated.
With another sieve having spaces of one millimeter, the coarse sand is
separated. The pebbles and sand are dried, weighed, treated with
hydrochloric acid and again weighed in order to find the quantity of
calcareous matter contained in them. In ten grams of this fine earth the
humidity is determined by drying at 100°.
Ten grams are mixed in a capsule with fifteen to twenty cubic
centimeters of water and after eight to ten seconds the supernatant
liquid is poured into a beaker having a capacity of 250 cubic
centimeters. The same operation is repeated until there are contained in
the beaker the fine sand and the clay, while the coarser sand remains in
the capsule.
This last is then dried and weighed and the quantity of calcium
carbonate determined by treating it with diluted nitric acid. By means
of calcination the organic matter is determined. The liquid decanted in
the beaker, the volume of which must not surpass 200 to 250 cubic
centimeters, is treated with nitric acid, filtered after some time,
washed and the calcium is directly determined by precipitating the
solution with ammonium oxalate.
The part in the filter which contains the fine sand, the clay, and the
humus material is mixed with water to a volume of about 200 cubic
centimeters; there are then added to it two to three cubic centimeters
of ammonia and after two or three hours it is diluted to a liter and
strongly agitated.
After twenty-four hours of rest it is decanted and the residuum is
treated a second time with diluted ammonia, decanting after twenty-four
hours. Ordinarily these two treatments suffice, if, however, the earth
is very argillaceous, this operation should be repeated three and even
four times.
The clay which is found in the liquid suspended in colloidal form
coagulates and is precipitated by adding thirty to forty cubic
centimeters of a saturated solution of potassium chlorid, while the
humus substance, under the influence of the ammonia remains dissolved.
Sestini found that the method of Schloesing was the only one which
indicated exactly the quantity of clay in the soil. He modified this
method by reducing the time of rest from twenty-four hours, as proposed
by Schloesing, to only twelve hours, a reduction which in his opinion
does not in the least impair the exactness of the method.
Sestini also proposes twelve treatments instead of six.
SEPARATION OF THE SOIL PARTICLES BY A LIQUID IN MOTION.
=213. General Principles.=—The laws, already discussed, applying to the
subsidence of a solid particle in a liquid, are equally applicable to
the separation of the particle by imparting a motion to the liquid at a
given rate. If a solid particle subside in a given liquid at the rate of
one millimeter per second it follows that this particle will remain at
rest if the liquid be set in motion upward with a like velocity. If the
velocity be greater the particle will be carried upward and eventually
out of the containing vessel. Such a particle is said to have a
hydraulic value of one millimeter per second. If there be a perfect
separation of a soil into its constituent particles and no subsequent
flocculation, all the particles of one millimeter hydraulic value and
less will be separated by a current of the velocity mentioned.
The general principles on which the separation rests, therefore, are the
securing of the proper granulation of the sample and the maintenance of
a fixed velocity of the current until the separation is finished. The
separation must be commenced with a period of subsidence so as to remove
first of all the suspended clay or impalpable particles. The velocity
can then be increased in a certain fixed ratio to secure a separation
into particles of any required hydraulic value.
=214. Nöbel’s Apparatus.=—One of the earliest methods of separating the
soil particles by a moving liquid is that of Nöbel.[147] The apparatus
is shown in Fig. 28. The four separating vessels 1, 2, 3, 4 are of
glass, pear shaped, and have a relative capacity of 1³, 2³, 3³, 4³, or 1
: 8 : 27 : 64. No. 4 has an outlet tube leading to the beaker B, of such
a capacity as to allow the passage of just nine liters of water in forty
minutes, constant pressure being maintained by means of a Mariotte’s
bottle or of the constant level apparatus A, _a_, _b_, which is
connected with the main water supply through the tube _a_ by means of a
rubber hose. The reservoir C should hold about ten liters. The sample of
soil to be separated should be previously boiled and passed through a
sieve having circular openings one millimeter in diameter. The flask in
which the sample is boiled is allowed to stand for some time when the
muddy supernatant liquid is poured into elutriator No. 2 and the
remaining sediment washed into No. 1. No. 1 is filled with water by
connecting it with the water supply and opening the pinch-cock _p_. The
water is carefully admitted until the air is all driven out and Nos. 1
and 2 connected. The cock _p_ is then opened and the vessels all filled,
and the water allowed to run into B for forty minutes, the level being
maintained uniformly at A.
[Illustration:
FIGURE 28.
NÖBEL’S ELUTRIATOR.
]
Of the water used, four liters are found in the elutriating vessels and
nine liters in the receiving vessel No. 5. The apparatus is left
standing for an hour until the liquid in the elutriators is clear and
the portions in each vessel are received on weighed filters dried at
125°, and the weight of each portion determined.
It is recommended that the loss on ignition of each part be also
determined. The separated particles thus secured are classified as
follows:
No. 1. Débris and gravel. No. 2. Coarse sand. No. 3. Fine sand. No. 4.
Clayey sand. No. 5. Finest parts or clay.
Although the method of Nöbel has been much used, the results which it
gives are entirely misleading. The convection currents produced in the
conical vessels by the passing water and the flocculation of the soil
particles prevent any sharp separation into classes of distinct
hydraulic value. The process may be useful for a qualitative test, but
its chief claim to a place in this manual is in its historic interest
arising from its use in the first attempts at silt analysis.
=215. Method of Dietrich.=[148]—The difficulties attending the silt
separation by the Nöbel method, led Dietrich to construct an apparatus
in which the sides of the elutriating vessels were parallel, but these
vessels, with the exception of the first, were not set in an upright
position.
[Illustration:
FIGURE 29.
DIETRICH’S ELUTRIATOR.
]
The apparatus (Fig. 29) consists of a series of cylindrical vessels
connected by rubber tubing.
The elutriators are of the following dimensions:
No. 1. Seventeen centimeters long, 2.8 centimeters in diameter,
position upright.
No. 2. Thirty-four centimeters long, four centimeters in diameter,
inclined 67°.5.
No. 3. Fifty-one centimeters long, 5.2 centimeters in diameter,
inclined 45°.
No. 4. Sixty-eight centimeters long, 6.4 centimeters in diameter,
inclined 22°.5.
The rubber tubes passing from one vessel to the other are furnished with
pinch-cocks so that each one of the elutriating vessels can be shut off
from the others and independently removed from the circuit.
The stream of water is made to pass through the apparatus under a
constant pressure of one meter.
Only the fine earth, boiled with water or hydrochloric acid, is to be
placed in the apparatus. The part coming through a sieve with a mesh
0.67 millimeters is to be used and placed in No. 1. About thirty grams
of soil, are employed for each elutriation. Before adding the soil, the
air is completely removed from all parts of the apparatus by connecting
it with the water supply and allowing it to be filled with water.
The rate of flow is controlled by the orifice of the last effluent tube
and the analyst is directed to continue the operation until the effluent
water collected in the beaker glass (5) is clear. The particles then
remaining in each of the vessels are collected separately.
The author of the method claims that in respect of likeness of particles
the results are especially gratifying and that duplicate analyses give
results fully comparable. The process, however, has not commended itself
to analysts, but it marks a distinct progress toward the principles of
later investigators. Had each of the elutriating vessels been placed
upright and the rate of flow determined, the apparatus of Dietrich would
have served, to a certain extent, for the more rigid investigations of
his successors.
=216. Method of Masure.=—The sifted earth, from ten to fifteen grams, is
carefully mixed with 200 cubic centimeters of water. It is then
introduced into a doubly conical elutriator B, Fig. 30, of about 250
cubic centimeters capacity. A current of distilled water is allowed to
flow from a Mariotte’s bottle, A, which secures a regular and constant
flow. The bottle A is joined to the elutriator B by means of a rubber
tube and the vertical glass tube D, the top of which is expanded into a
funnel for the purpose of receiving the water from the Mariotte flask.
The current of water flowing upward through the elutriator B carries in
suspension the most finely divided particles of clay, and these are
collected with the emergent water in the receiver C. The sand and
coarser particles of clay remain in the elutriator. The water flows out
by the tube F, the diameter of which should be less than that of D. When
the emergent water becomes limpid the operation is terminated. After the
apparatus is disconnected, the water is decanted from the sand in the
elutriator, and the whole residue is weighed after drying for two hours
at 110°.
[Illustration:
FIGURE 30.
MASURE’S SILT APPARATUS.
]
The fine soil collected in C may also be separated and weighed, for
control, after drying as above.
The pebbles and coarse sand separated by the sieves should also be
weighed. By this process the soil is separated into four portions;
_viz._,
(1) Pebbles. (2) Coarse sand. (3) Fine sand and other materials not
carried off by the current of water. (4) Fine soil, carried into the
receiver C.
=217. Method of Schöne.=—The method of Schöne[149] is based on the
combination of a cylindrical and conical separatory tube through which
the flow of water is regulated by a piezometer.
If, in the process of silt separation, the water move perpendicularly
upward with a given velocity, _e.g._ = v the separation is dependent:
(1) On the volume of the silt particles, (2) On their specific gravity,
and, (3) On their state of disintegration.
If it be assumed that the silt particle is a sphere with a diameter = d,
then according to Newton’s law of gravity, the following formula would
be applied: d = v² ((3Z)/(4g (S − 1))).
[Illustration:
FIGURE 31.
SCHÖNE’S ELUTRIATOR.
]
In the above formula Z = a coefficient which depends on the condition of
the surface against which the hydraulic pressure or resistance works, in
this case a sphere; g = the acceleration of gravity equivalent to 9.81
meter; and S = the specific gravity of the particle.