first few times shall contain nothing larger than the desired diameter.
Then decant into another vessel, timing the subsidence so that the
sediment shall contain nothing smaller than the chosen diameter. This
can not be done without decanting much that is larger than the chosen
diameter, but the greater part of the particles greater and less than
the chosen diameter can be removed and an intermediate product obtained,
the diameters of whose particles are not very far from that desired.
If this intermediate portion be again subjected to the same process, two
fractions may be separated from it, one containing particles larger than
the chosen diameter and another containing particles smaller than this
diameter, while a new intermediate product will remain which is less in
amount than that resulting from the first operation. By frequent
repetitions of this process this intermediate product can be reduced to
a very small amount of substance the particles of which have diameters
lying close to the chosen limit and may then be divided between the two
fractions.
The principles of the separation described by Osborne set forth with
sufficient clearness the purposes to be achieved by the analysis. The
chief methods of manipulation practiced will be found below.
=199. Kühn’s Silt Cylinder.=—A simple form of apparatus for the
determination of silt by the sedimentation process is the one described
by Kühn.[134]
The cylinder should be about twenty-eight centimeters high with a
diameter of 8.5 centimeters. At the lower end of the cylinder five
centimeters from the bottom it carries a tube 1.5 centimeter in diameter
furnished with a pinch-cock and held in position by a rubber stopper.
In carrying out the process thirty grams of sifted soil (two millimeters
mesh sieve) are boiled with water for an hour and after cooling the soil
and water are washed into the separating cylinder. The cylinder is then
filled with water with constant shaking.
After standing for ten minutes the stop-cock is opened and the water
with its suspended matter allowed to flow into a porcelain dish.
The cylinder is then again filled with water and the process is
continued until the water drawn off is practically clear.
The fine particles having been separated in this way the next coarser
grade of particles is separated by repeating the process at intervals of
five minutes.
By these two operations it is considered that the clay is entirely
removed. The residue remaining in the cylinder is dried and weighed. The
relative proportions of clay and residue in the sample are thus
determined.
The residue is then separated into two portions by sieves of one
millimeter and 0.5 millimeter mesh.
The soil is thus separated into the following parts:
1. By the first sifting coarse quartz larger than two millimeters
diameter.
2. Fine quartz two millimeters, to one millimeter diameter.
3. Coarse sand one millimeter, to 0.5 millimeter diameter.
4. Fine sand finer than 0.5 millimeter diameter.
5. Silt, clay, humus, etc., separated by the water.
=200. Knop’s Silt Cylinder.=—The cylinder recommended by Knop[135] is
essentially that of Kühn being furnished with four lateral tubes instead
of one (Fig. 25).
[Illustration:
FIGURE 25.
KNOP’S SILT CYLINDER.
]
The sample of soil, twenty-five to thirty grams, after passing a two
millimeters mesh sieve, and long boiling, is washed through a series of
sieves of the following diameters of mesh respectively; _viz._, one
millimeter, 0.5 millimeter, 0.25 millimeter, and 0.1 millimeter. The
part which passes the finest sieve is placed in a Knop’s cylinder, the
cylinder filled with water one decimeter above upper tube and well
shaken. The cylinder is allowed to rest for five minutes when the upper
cock is opened and the water drawn off. After five minutes more the next
tube is opened and so on with equal intervals for the three upper tubes.
The operation is repeated with fresh water until the water drawn off is
clear. Finally the lowest tube is opened and all the water poured off of
the sandy residue. The space between each tube is one decimeter. The
dust remaining is dried and weighed and the weight of material carried
over as silt determined by difference.
=201. Siphon Silt Cylinder.=—Instead of the tubulated cylinder one
furnished with a siphon can be employed[136] (Fig. 26).
It should be about forty centimeters high and six centimeters internal
diameter. The cylinder first receives twenty-five to thirty grams of the
well boiled fine earth and then water until there is but a small space
between it and the stopper when the latter is inserted.
The cylinder is marked exactly 200 millimeters below the surface of the
water with a narrow strip of paper at _a_, stoppered, inverted and well
shaken. The cylinder being again placed in normal position the soil
particles under the influence of gravity tend to sink with greater or
less rapidity according to their size. The siphon _a b c_ is filled with
water, the cock at _c_ closed and the opened end _a_ placed in the
cylinder A just at the mark 200 millimeters below the surface of the
water, and the water thus transferred to B when desired. If the
suspended matter is allowed to stand for 100 seconds the particles of
more than two millimeters hydraulic value will have fallen below the
open end of the siphon. If allowed to stand 1,000 seconds the silt value
of the particles will be 0.2 millimeter per second. Whatever the number
of seconds may be, the operation is continued until the water removed is
practically clear. The open end of the siphon _a_ should be bent upwards
so that no disturbing current may bring the particles below the line
into the liquid discharged into B.
[Illustration:
FIGURE 26.
SIPHON CYLINDER FOR SILT ANALYSIS.
]
While the results obtained by this method are satisfactory as compared
with other similar processes it cannot be highly recommended because of
the time and trouble required to get a complete separation and by reason
of the difficulty of collecting the separated silt.
=202. Wolff’s Method.=[137]—As modified by Wolff the Knop process is
conducted as follows: Fifty grams of fine earth are boiled with water
and then the entire mixture is passed through three sieves with openings
of one millimeter, 0.5 millimeter, and 0.25 millimeter in diameter,
respectively. The finest part is mixed with water to a height of
eighteen centimeters in a bottle twenty centimeters high and having a
capacity of one liter and thoroughly agitated, after which it is left to
rest, and finally the turbid liquid is drawn off with a siphon, the
bottle refilled with water, agitated, and left to rest, and the process
repeated as long as the water carries any suspended matter after a
definite time.
Wolff proposes for the first three periods of rest one hour, for the
second three, a half an hour, for the third three, a quarter of an hour,
and for the fourth three, five minutes.
=203. Moore’s Modification of Knop’s Method.=[138]—The sample of soil is
first passed through a sieve having round perforations three millimeters
in diameter. The weight of the particles remaining on the sieve is then
determined, and likewise that of the portion passing through, which is
known as fine earth. The last named portion constitutes the material for
all subsequent operations of mechanical and chemical analysis.
Thirty grams of the fine earth are boiled rapidly with water until the
lumps are disintegrated and clayey portions separated from the sand. The
material is then successively washed through perforated metal sieves,
the holes of which are respectively 1, 0.5, and 0.25 millimeter in
diameter. The portions retained on the sieves are severally dried,
ignited and weighed, and the finest portion, or that passing through the
0.25 millimeter sieve, is then submitted to the following process of
separation:
The sediment and water passing through the 0.25 millimeter sieve are
placed in a glass cylinder fifty centimeters long and thirty-seven
millimeters in internal diameter. The cylinder is closed at the bottom
and is provided with a lateral tube inserted six centimeters above the
bottom. Three other lateral tubes are inserted at intervals of ten
centimeters above the first tube, and a ring is etched into the cylinder
ten centimeters above the uppermost tube. The lateral tubes are closed
with rubber tubes compressed by spring clips. The sediment being placed
in the cylinder, water is added to the mark or ring, the cylinder closed
with a rubber stopper, and vigorously shaken until the contents are
thoroughly mixed. It is then placed upright, the stopper removed, and
after standing undisturbed for five minutes the clip on the uppermost
tube is opened and the water allowed to flow into a beaker. After five
minutes further standing, the second clip is opened and the water drawn
off into the same beaker; in the same manner the water is drawn off from
the other tubes at intervals of five minutes until the level of the
lowest tube is reached. The cylinder is then refilled with water to the
mark, thoroughly shaken after inserting the stopper and the water again
drawn off at intervals of five minutes, as before; the operation being
repeated until the water drawn off is almost free from turbidity. The
sediment remaining in the cylinder from this process of washing by
subsidence is termed by Knop, fine sand, the material flowing off in
suspension in the wash waters, dust, and the process of separation by
Knop’s original method ends here.
In order to remedy the imperfect separation into definite particles
secured by the above method, Moore proposes the following device:
The fine sand from the first series of subsidences is placed in a
separate vessel, the washings are allowed to remain undisturbed for
twelve hours, the turbid liquid decanted and the sediment returned to
the cylinder. Water is then added to the mark, the whole shaken, and the
liquid drawn off at intervals of five minutes, as in the first series.
The sediment from this operation is placed in a separate beaker, the
washings returned to the cylinder, and again allowed to subside as
before; the sediment from this second subsidence is added to that from
the preceding operation, and the washings again returned to the
cylinder, the operation being repeated as long as any sediment can be
obtained from renewed treatment of the washings; the final washings are
then placed in a separate vessel for subsequent microscopic
measurements.
The collective sediments from the last series of operations are then
returned to the cylinder and allowed to subside with fresh additions of
water, as in the case of the first series; the fine sand thus obtained
being added to that from the first series, and the washings being
collected in a large beaker. The latter are left at rest for twelve
hours, and the sediment returned to the cylinder and treated as before
until no further separation can be effected. The fine sand resulting
from all of these operations is then dried, ignited and weighed; the
weight of the portion removed by the washing being determined by
difference, as it is, owing to its excessively slow rate of subsidence,
found impracticable to collect it for direct weighing. The size of the
particles of fine sand is then determined by micrometric measurement.
Similar measurements are made on the material obtained by long
subsidence from the washings from the foregoing operations. The average
diameter of the largest particles should not exceed 0.01 millimeter.
=204. Statement of Results.=—The results of the analyses on three soils
from the localities indicated in the table, and the method of stating
them, are given in the following table:
New Milford, Clarksville, Granville,
Conn., per Tenn., per N. C., per
cent. cent. cent.
Particles larger in diameter 8.55 0.32 0.23
than 3.0 millimeters
Particles of diameter from 3.0 4.96 0.45 15.04
millimeters to 1.0 millimeter
Particles of diameter from 1.0 4.43 0.96 33.43
millimeter to 0.5 millimeter
Particles of diameter from 0.5 11.86 1.25 18.82
millimeter to 0.25 millimeter
Particles of diameter from 0.25 60.54 61.58 23.59
millimeter to 0.01 millimeter
Particles smaller in diameter 9.66 35.44 8.89
than 0.01 millimeter
───────────────────────────────────────────────────────────────────────
Total 100.00 100.00 100.00
=205. Method of Bennigsen.=—The silt flasks recommended by
Bennigsen[139] are shown in Fig. 27.
The glass flask _b_ carries a long cylindrical neck _a_ the upper part
of which is graduated in cubic centimeters. Ten grams of the fine soil
are shaken with water in the flask, the neck of which is closed with a
rubber stopper. The flask is then inverted bringing the soil and water
into the neck. The flask is hung up and sedimentation is assisted by
imparting a pendulous motion to the neck for ten minutes. After an hour
the soil particles have separated into a coarse layer below and a fine
layer above. The relative volumes of the two layers are then read off in
cubic centimeters. While this method may be useful in helping to form a
speedy judgment concerning the character of a soil it can lay no claim
to being an accurate method of silt separation.
[Illustration:
FIGURE 27.
BENNIGSEN’S SILT FLASKS.
]
=206. Method of Gasparin.=—The method of Gasparin only gives a very
primitive separation of the various components of the earth according to
their fineness. It is conducted as follows:
Ten grams of sifted earth are put into a beaker, water is added and
strongly agitated; after five minutes the water is decanted into another
vessel, the first vessel is filled anew with water, agitated, decanted,
and this process is repeated until the liquid remains perfectly clear.
Only two portions are weighed, _i. e._, the pebbles which remain in the
sieve and the coarse sand which remains in the beaker; while the
argillaceous portion drawn off with the water is determined by the
difference.
=207. The Italian Method.=—The following modification of Gasparin’s
process is practiced by the Italian chemists:[140]
Twenty grams of earth are passed through a sieve having openings of one
millimeter in diameter, then the sifted part is mixed with 100 cubic
centimeters of water in a 200 cubic centimeters beaker and left to rest
for some hours, then strongly agitated and after ten seconds the turbid
liquid is poured into another vessel of half a liter capacity. This
manipulation is repeated until the liquid is clear.
The decanted liquid is thoroughly agitated, then left to stand until the
movement shall be completely arrested, after which the supernatant
liquid is poured into another vessel holding two liters. To the residuum
is added more water; it is agitated, decanted, and this process is
repeated until the water is no longer turbid.
=208. Method of Osborne.=—In the foregoing paragraphs the methods of
silt separation by subsidence as practiced in different countries have
been outlined. The good points of the various methods are combined in
the process as carried out by Osborne.[141] The details of this method
will be given with sufficient minuteness to make its practice possible
by all analysts.
_Selecting the Sample._—Several pounds of air-dried, fine earth are
secured by passing the soil through a sieve, the holes of which are
three millimeters in diameter.
_Sifting._—Thirty grams of the above fine earth are stirred with 300 to
400 cubic centimeters of water and then thrown successively upon sieves
with circular holes of 1, 0.5, and 0.25 millimeter diameter
respectively. By means of successive additions of water and the use of a
camel’s hair brush, all the fine material is made to pass through the
sieves and these at the last are agitated under water in a shallow dish
in such a way that the soil is immersed. The finest sieve should be well
wet with water on its lower surface just before using. The finest
particles which render the water turbid are easily washed through. The
turbid water is kept separated from the clear water which comes off with
the last portions that pass the sieves. The turbid water usually does
not amount to more than one liter.
_Elutriation._—The elutriation should be carried on so as to secure
three grades of silt; the diameters of the particles ranging in the
first grades from 0.25 to 0.05 millimeter, in the second grade from 0.05
to 0.01 millimeter, and in the third grade from 0.01 millimeter to the
impalpable powder. The term sand is applied to the first grade, silt to
the second, and dust to the third. After the turbid liquid from the
sifting has stood a short time it is decanted from the sediment and
after standing until a slight deposit is formed, is again decanted and
the sediment examined with a microscope. If sand be present, the
subsidence of the turbid liquid is continued until no more sand is
deposited. As the sand subsides rapidly there is no difficulty in
altogether freeing the liquid first decanted from this grade of
particles. The sediment thus obtained contains all the sand, a part of
the dust and much silt. As only dust and the finest silt render the
water turbid the sediment is stirred a few times with a fresh quantity
of water and decanted after standing long enough to let all the sand
settle. When the water decanted is free from turbidity, the last
portions of the soil passing through the sieve with clear water are
added to the sediment and the decantations continued so as to remove
most of the silt. When no more silt can be easily removed from the
sediment without decanting sand, the decantations are made into a
different vessel and the subsidences so timed as to remove as much of
the silt as possible. By using a little care, at least three-quarters of
the sand are thus obtained free from silt. The rest of the sand is mixed
with the greater part of the silt which has been decanted into the
second vessel. The size of the smallest particles in this vessel is
determined with the microscope, to make sure that its contents are free
from dust as they usually will be if, after settling for a few moments,
they leave the water free from turbidity.
The soil is thus separated into three portions, one containing sand, one
sand and silt, and the other silt, dust, and clay. The sand and silt are
separated from each other by repeating the subsidences and decantations
in the manner just described.
In this way there is removed from the sediment, on the one hand, a
portion of silt free from sand and dust, and on the other hand a portion
of sand free from silt. Thus is obtained a second intermediate portion
consisting of sand and silt, but less in amount than the first and
containing particles of diameters much more nearly approaching 0.05
millimeter. By repeating this process a few times, this intermediate
portion will be reduced to particles whose diameters are very near 0.05
millimeter and which may be divided between sand and silt, according to
judgment. The amount of this is usually very small. As soon as portions
are separated, which the microscope shows to be pure sand or pure silt,
they are added to the chief portions of these grades already obtained.
The same process is applied to the separation of silt from dust. When
all the silt has been removed from the dust and clay, the turbid water
containing the dust and clay is set aside and allowed to settle in a
cylindrical vessel for twenty-four hours. The vessel is filled to a
height of 200 millimeters. According to Hilgard, the separation of the
dust from clay during a subsidence of twenty-four hours, will give
results of sufficient accuracy, although the clay then remaining
suspended will not be entirely free from measurable fine particles up to
0.001 or 0.002 millimeter diameter.
Small beakers and small quantities of distilled water are used at first
for the decantations, as thus the duration of subsidence is less and
more decantations can be made in a given time than when larger
quantities of water are employed. Beakers of about 100 cubic centimeters
capacity are convenient for the coarser grades, but it is necessary to
use larger vessels for the fine sediments from which turbid water
accumulates that cannot be thrown away, as may be done with the clear
water, from which the coarse sediments settle out completely in a short
time.
It is best to keep the amount of water as small as possible in working
out the dust since loss is incurred in using too large quantities.
It is also necessary in most cases to subject the various fractions
obtained during elutriation, to careful kneading with a soft rubber
pestle so that the fine lumps of clay may be broken up and caused to
remain suspended in the water. This treatment with the pestle should be
done in such a way as to avoid as far as possible all grinding of the
particles, the object being merely to pulverize the minute aggregations
of clay and extremely fine particles which always form on drying a
sample of soil after removing it from the ground.
_Measurement of the Particles._—To determine the size of particles in
suspension, a small glass tube is applied to the surface of the liquid
in such a way as to take up a single drop which is transferred to a
glass slide. This drop will contain the smallest particles in the
liquid.
To obtain a sample of the coarsest particles the liquid is allowed to
stand long enough to form a very slight sediment and a portion of this
sediment is collected with a glass tube.
To determine the diameter of the particles in a sediment it is stirred
vigorously with a little water and the pipette at once applied to the
surface of the water. On decanting the greater part of the sediment, the
large particles remain at the bottom of the beaker and may be easily
examined.
_Time._—The time required to make the separations, above described, is
about two hours for each, so that an analysis including the sittings, is
made in five or six hours, exclusive of the time necessary for
collecting the dust and separating the clay, for which a subsidence of
twenty-four hours is allowed.
_Weighing the Sediments._—The sediments are prepared for weighing by
allowing them to subside completely, decanting the clear water as far as
possible, rinsing them into a weighed platinum dish and igniting. The
dish is cooled in a desiccator.
_Effect of Boiling._—The analyses show a very decided increase in the
particles smaller than 0.01 millimeter diameter at the expense of
coarser particles as the result of boiling. The surfaces of the coarser
particles are seen to be polished and of a lighter color than those not
boiled. The surfaces of the unboiled particles are coated with a film of
fine material probably cemented to them by clay. When these coarse
particles which have not been boiled, are violently stirred with water
for a short time, no fine particles are detached from them; and a
careful examination under the microscope fails to reveal in any of the
sediments more than an occasional grain exceeding the 0.05 millimeter
limit by so much as 0.01 millimeter, or the 0.01 limit by as much as
0.005 millimeter. It would, therefore, appear that these small particles
thus set free by long boiling are really a part of the larger ones and
should be treated as such in a mechanical analysis of these soils.
=209. The French Method.=—The Schloesing method[142] as practiced by the
French agricultural chemists[143] differs essentially from those already
described in attempting to first free the silt from carbonates and
organic matter. It is conducted as follows: