current in the apparatus is just sufficient to counteract the tendency
of a given particle to sink. All particles of a smaller diameter, in
such a case, will be carried on by the current, while all of a greater
diameter would separate by sedimentation. These theoretical conditions
are not met with in practice where silt particles of all shapes and
degrees of aggregation abound. These particles, whatever their shape,
may be said to have the same hydraulic value when carried by the same
current. It is necessary, therefore, to secure some uniform standard of
expression to assume a normal form of particle and a normal specific
gravity. For the form, a sphere is evidently the normal which must be
considered and for specific gravity that of quartz is taken; _viz._,
2.65. The mean coefficient for Z may also be placed at 0.55, although
slightly different values are ascribed to it. Substituting these values
in the formula, it is reduced to the expression; d = v² × 0.0000255
millimeters. It can, therefore, be said that by this or that velocity of
the current, silt particles will be removed of this or that diameter, it
being understood that all particles of equal hydraulic value to
spherules of quartz of the given diameter are included in each class. In
order to have the theoretical formula agree with the results of analysis
it is necessary to modify it empirically to read d = v^{⁷⁄₁₁} × 0.0314
millimeters.
This formula is found to agree well with the results obtained for all
velocities between 0.1 millimeter and twelve millimeters per second, the
ordinary limits of silt separation.
_The Apparatus._—The conic-cylindrical elutriating vessel A, B, C, D, E,
F, G, Fig. 31, is of glass. The part B, C, is cylindrical, ten
centimeters in length and as nearly as possible five centimeters in
diameter.
The conical part C, D, is fifty centimeters in length. Its inner
diameter at D must not be greater than five centimeters nor smaller than
four centimeters.
The bend, D, E, F, should have the same diameter; _viz._, four to five
centimeters.
The part A, B, C, D, and D, E, F, G, may be made of separate parts and
joined by a rubber tube.
_Outflow Tube and Piezometer._—The outflow tube and piezometer, H, J, K,
L, is constructed as shown in Fig. 32. It should be made of barometer
tubing having an internal diameter of about three millimeters. The tube
is bent at J at an angle of forty to forty-five degrees. The knee J must
be as acute as possible not to interfere with the inner diameter. The
form and especially the magnitude of the outlet are of great importance.
It must be circular and nearly 1.5 millimeter in diameter. It must not
be larger than 1.67 millimeter nor smaller than 1.5 millimeter. The
opening should be so made as to direct the stream of outflowing liquid
in the direction shown by the arrow.
[Illustration:
FIGURE 32.
SCHÖNE’S ELUTRIATOR, OUTFLOW TUBE.
]
The piezometer L, K is parallel to the arm H, J, of the delivery tube.
Its graduation has its zero point in the center of the outlet K. It
commences with the one centimeter mark. From one to five centimeters it
is divided into millimeters, from five to ten centimeters into
one-fourth centimeter, from ten to fifty centimeters into one-half
centimeter, and from fifty to 100 centimeters into centimeters. The
dimensions given are those required for ordinary soils and for
velocities ranging from two-tenths millimeter to four millimeters per
second.
For greater velocities, a delivery tube with a larger outlet must be
used and the piezometer must be of greater internal diameter than
indicated.
[Illustration:
FIGURE 33.
SCHÖNE’S ELUTRIATOR, ARRANGEMENT OF APPARATUS.
]
_Arrangement of the Apparatus._—The apparatus is conveniently mounted as
shown in Fig. 33, giving front and side views of all parts of apparatus
in position ready for use. When numerous analyses are to be made much
time is saved by having a number of apparatus arranged _en batterie_.
_The Sieve._—The soil, before being subjected to elutriation, should be
passed through a sieve of which the meshes are 0.2 millimeter square.
_The Process._—To measure the diameter of the cylinder, two marks are
made with a diamond upon the glass which are distant from each other a
certain space, for instance, _h_ centimeters. The space between these
two marks is filled with water exactly measured. Suppose that a cubic
centimeters were used, then the diameter is determined by the formula:
D = √(4_a_)/(π_h_) centimeters.
In order to determine that the elutriating cylinder is strictly
comparable in all its parts this measurement should be made upon several
parts thereof.
The apparatus should now be tested in regard to the quantity of liquid
which it will deliver under a given pressure in the piezometer. By means
of the stop-cock H the flow of water is so regulated that the outflow at
_c_ can be measured at a given height of the water in the piezometer.
Suppose that _a_ cubic centimeters of water flow in _t_ seconds, then
the quantity which would flow in one second is determined by the
formula, Q = a/t cubic centimeters. Since according to the law of
hydraulic outflow the quantities are proportional to the square root of
the height of the column it is easy to compute from any given height the
quantity which will flow from any other one desired. For the retardation
due to capillary attraction, it is sufficient, in general, to take it in
a constant quantity; if this constant quantity be represented by C, the
observed height of the water in the piezometer by _h_, and the quantity
of water flowing out by Q, the data required for any given velocity can
be calculated from the following proportion:
√(_h_₁ − C) : √(_h_₂ − C) = Q₁ : Q₂.
It is necessary to compute the magnitude of this constant C which is to
be subtracted. This is accomplished by measuring the quantity of water
which flows out at two different heights of the column in the
piezometer. From the foregoing proportion, the value of C is as follows:
C = (Q₁² _h_₂ − Q₂² _h_₁)/(Q₁²) − Q₂²) centimeters.
The value of C can be the more exactly determined as _h_₁ is greater and
_h_₂ smaller. It is best to choose the lowest height from which an exact
reading can be made; that is, by which the regular rise and fall of the
level of the water in the piezometer (in consequence of the formation of
drops) just begins to disappear. This usually takes place when _h_₂ =
1.5 centimeter to 1.7 centimeter. For the higher value _h_₁ it is best
to take about 100 centimeters. Suppose, for example, the following
results are obtained:
Height of
column to be
Observed height. Observed quantity of subtracted
outflow. due to
capillary
attraction.
_h₂_ _h₁_ Q₁ cubic Q₂ cubic
centimeters. centimeters. centimeters. centimeters. centimeters.
80 1.6 5.53 0.406 1.21
100 1.6 6.13 0.484 1.17
80 1.8 5.53 0.406 1.19
100 1.8 6.13 0.484 1.19
The same quantity of water which flows out in a unit of time passes also
at the same time over a cross section of the elutriating cylinder. The
diameter of this cylinder being D the equation is derived
_v_ = Q(4)/(πD²) centimeters.
Since the velocity in the elutriating cylinder v is directly as the
quantity of water overflowing so is
_v_ : _v_ₙ = √(_h_ − C) : √(_h_ₙ − C);
then _v_ₙ = √(_h_ₙ − C) (_v_)/(√(h − C))
and _h_ₙ = _v_ₙ²((_h_ − C)/(_v_²) + C. The constant (_h_ − C)/(_v_²)
is obtained from the means of a number of estimations; for example as
illustrated in the following data:
Observed Corresponding velocity
quantity of in elutriating cylinder Constant.
Observed height, outflow, cubic of 4.489 centimeters (_h_ −
centimeters. centimeters. diameter, millimeters. C)/(2)
1.6 0.406 0.0257 621
1.8 0.484 0.0306 652
80.0 5.530 0.3490 647
100.0 6.130 0.3870 660
———
Mean 645
Then are obtained the following values of _h_ₙ and _v_ₙ:
_h_ₙ = 645(Vₙ²) + 1.19 centimeters.
and _v_ₙ = √(_h_ₙ − 1.19) × 0.0394 centimeters.
In order to be able easily and rapidly to judge under what pressure the
outflow has taken place in any particular instance, a larger number of
values are computed with the help of the formula given and placed
together in tabular form. As an example the following table may serve
which was computed for one of the apparatus used. Usually it will be
sufficient to test the apparatus for four different heights and then to
interpolate the values for all the others. The numbers marked with a
star in the table are those which were determined by experiment; the
others were calculated.
Height of column Velocity in the elutriating Corresponding diameter of
in piezometer. cylinder of 4.489 silt particles. _d_ =
_h_ centimeters diameter. _v_ v(⁷⁄₁₁)0.0314 millimeters.
centimeters. Observed, Calculated, millimeters.
millimeters. millimeters.
1.5 0.222 0.220 0.0120
1.6 0.257* 0.252 0.0131
1.7 0.284 0.281 0.0140
1.8 0.306* 0.307 0.0148
1.9 0.323 0.332 0.0155
2.0 0.346 0.355 0.0162
2.5 0.427 0.451 0.0185
3.0 0.531 0.530 0.0210
3.5 0.577 0.599 0.0227
4.0 0.650 0.660 0.0236
4.5 0.694 0.717 0.0254
5.0 0.751 0.769 0.0265
6.0 0.850 0.864 0.0286
7.0 0.942 0.950 0.0304
8.0 1.050 1.028 0.0320
9.0 1.120 1.101 0.0334
10.0 1.170 1.169 0.0347
15.0 1.490 1.460 0.0400
20.0 1.730 1.710 0.0441
25.0 1.940 1.920 0.0476
30.0 2.100 2.110 0.0506
35.0 2.310 2.290 0.0532
40.0 2.460 2.450 0.0556
45.0 2.610 2.610 0.0578
50.0 2.770 2.750 0.0598
60.0 3.030 3.020 0.0635
70.0 3.290 3.270 0.0667
80.0 3.490* 3.500 0.0697
90.0 3.710 3.710 0.0724
100.0 3.870* 3.920 0.0749
Suppose the problem is by means of the apparatus tested above, to
separate into a number of groups a mixture of silt particles, whose
hydraulic values are found between the following diameters: 0.01, 0.02,
0.03, 0.04, 0.05, 0.06, 0.07 millimeters. The table will show at once
under what pressure of water the piezometer must be placed in order to
give the values; _viz._, 1.4, 2.8, 7.0, 15.0, 29.0, 53.0, and 83.0
centimeters respectively.
The apparatus described above, is adapted for velocities in the
elutriating cylinder varying from two-tenths millimeter to four
millimeters per second. The largest silt particles which can be
separated by the velocities given above, have approximately a diameter
of 0.08 millimeter. For the separation of larger particles a sieve can
take the place of the silt apparatus. If, however, it be desired to
subject larger particles to silt analysis, the dimensions of the
elutriating cylinder and of the outlet of the delivery tube must be
changed accordingly.
_Preparation of Sample._—The conduct of silt analysis of natural soils
must, in certain cases, be preceded by a special treatment of the
sample. If the latter be rich in humus the organic substance must
previously be separated as completely as possible. With sandy soils this
can be accomplished by ignition. With clayey soils, on the contrary, it
is to be performed by boiling the soils at least one hour with water
which contains from one to two per cent of free alkali. Soils which
contain lime must also be subjected to treatment with dilute
hydrochloric acid, and the hydrochloric acid must be as carefully
removed, as possible before the sample is subjected to elutriation;
afterward follows the boiling of the sample in the ordinary way with
water. This, of course, can be omitted when it has already been treated
with boiling dilute alkali. It is also important to remove the larger
particles by a sieve before the elutriation begins. It is well to pass a
sample through a sieve after it has been boiled, by which all particles
of a larger diameter than 0.2 millimeter are removed. This will usually
require about one liter of water and this water should be allowed to
rest from one to two hours and poured off with the suspended material
which it contains. Only what subsides should be brought into the
apparatus. In rinsing the sample as much water must be used as will fill
the apparatus up to its cylindrical portion.
After the sample has been placed in the apparatus, the water is allowed
slowly to enter, being careful to avoid reaching more than the lowest
required velocity, until the outflow begins. The water then is so
regulated by the stop-cock as to bring it to the desired height in the
piezometer. This being accomplished, the different velocities which have
been decided upon for separating the particles of silt are used one
after the other, as soon as all the silt which can be removed at each
given velocity, has been secured. From three to five liters of water
will be required for the separation of each class of particles.
Sometimes the reading of the height of the water in the piezometer is
difficult; as, for instance, when foam or bubbles accumulate therein.
These bubbles can be removed by simply blowing into the tube, or
dropping into it a little ether. The outflow of water can be received in
vessels, beaker glasses, or cylinders, in which it is allowed to
subside. The finest particles which remain in suspension in the water
are best determined by difference. If it be desired to weigh them
directly, the water can be treated with ammonium bicarbonate until it
contains from one to two per cent thereof. The precipitation then takes
place in a few hours.
The collection and weighing of silt particles are accomplished in the
usual way. That which finally remains in the elutriating vessel is taken
out after the end of the operation by closing the stop-cock, removing
the stoppers with the piezometer tube, pouring the contents of the
elutriating vessel into a beaker glass and rinsing out carefully all
adhering particles. Examples of the working of the apparatus follow:
The soil was taken from the Imperial Russian Agricultural Experimental
Institute at Gorki. It was a fine clay sand and was carefully treated
with hydrochloric acid. The results of the analysis are given in the
following table:
Velocities Largest diameter of the Percentage of silt product
employed in collected particles in obtained in repeated
millimeters. millimeters. elutriations.
0.25 0.012 13.4 12.6 11.9
0.5 0.020 9.1 8.7 9.5
1 0.032 21.0 21.4 20.8
2 0.050 30.4 29.8 31.7
3 0.063 16.7 16.1 15.5
4 0.076 5.3 5.5 5.5
Residue 4.2 4.9 3.8
————— ———— ————
Total 100.0 99.0 98.7
Holthof modifies the apparatus of Schöne by putting into the lower mouth
of the elutriator a little mercury so that the particles of earth are
deposited upon its surface and are thus better agitated and washed by
the current of water.
=218. Mayer’s Modification of Schöne’s Method.=—An improvement of
Schöne’s apparatus in the direction of greater simplicity has been
tested by Mayer[150] with satisfactory results:
The apparatus, (Fig. 34), consists of a glass vessel having a glass
stop-cock at the bottom for admitting the water. For a distance of
twenty centimeters the sides of the tube are parallel and the diameter
about one centimeter. Next for a distance of fifty centimeters the tube
is conical expanding at a regular rate until the internal diameter
reaches five centimeters. For a distance of ten centimeters the vessel
is again strictly cylindrical and it is in this cylindrical portion that
the separation of the different constituents takes place. The vessel is
then rapidly narrowed until it carries the stopper A two centimeters in
diameter. This stopper carries two glass tubes, one F bent downward to
conduct the overflow into the receiving vessels, and one H for the
purpose of regulating the rate of overflow by the height of the column
of water therein. The orifice of the overflow tube F should be so
regulated that with a pressure of five centimeters water in H, one liter
shall pass over in ten minutes.
[Illustration:
FIGURE 34.
SCHÖNE’S APPARATUS
FOR SILT
ANALYSIS,
MODIFIED
BY MAYER.
]
If the separation be conducted in an apparatus thus mounted and
graduated with a pressure of two centimeters in H all that portion of
the soil which can properly be called clay will pass over. The fine
earth, that is, earth in which all coarse particles have been removed by
proper sifting, is used in ten-gram lots for each experiment. The
residue, after the separation is complete, consists of pure sand or at
least pure sand mixed with humus. Before the fine earth is placed in the
apparatus, the calcium carbonate therein is removed with hydrochloric
acid. The treatment with hydrochloric acid, however, is not to be
recommended in soil containing many undecomposed particles of calcium
carbonate or dolomite for then large additions to the silt output might
be made from these particles, which could not be regarded as coming from
the soil as it actually exists. For alluvial soil, however, previous
treatment with hydrochloric acid is recommended unconditionally.
=219. Schöne’s Method as Practiced by Osborne.=—The apparatus used by
Osborne[151] was obtained from Germany and was similar to that described
by Schöne in his original paper, except that it was furnished with a
second elutriating tube as suggested by Orth. The modification made by
Orth consists, essentially, of a second elutriating tube with straight
sides into which the bulk of the soil is introduced, only the final part
being carried over into the Schöne’s tube proper. Water is supplied to
the apparatus under constant pressure by means of a Mariotte’s bottle.
The preliminary treatment recommended by Schöne is omitted, as these
steps have been shown to be undesirable, on account of affecting the
accuracy of the results.
Twenty grams of the air-dried soil are passed, under water, through a
sieve of one-fourth millimeter mesh. That part of the soil which remains
in suspension after being sifted is placed at once in the Schöne’s tube
of the apparatus, the coarser portion being rinsed into the Orth tube.
The current is regulated so that the largest particles of quartz carried
off have an average diameter of 0.01 millimeter. When all is carried off
that can be removed at this rate the current is increased until the
largest quartz grains passing off have a diameter of 0.05 millimeter.
As noticed by Hilgard with Schulze’s apparatus, secondary currents are
formed during the process of elutriation which descend along the walls
of the conical portion of the Schöne’s tube and some distance along the
sides of the cylindrical portion. The tendency of these currents is to
produce globular aggregates of particles which fall to the bottom. They
are broken up from time to time by increasing the velocity of the
current but even this method fails to disintegrate a considerable
quantity of them.
=220. Statement Of Results.=—Two samples of soil from the garden of the
experiment station analyzed by Schöne’s method gave the following
proportions of sediment.
In the table the term clay is used to designate all that part of the
soil which has diameters less than 0.01 millimeter and which remains
suspended after twenty-four hours standing in water having a depth of
200 millimeters.
SOIL, FROM GARDEN OF THE EXPERIMENT STATION.—NOT
BOILED.
Analyses with the Schöne-Orth Elutriator.
_A._ _B._ _C._
Above 0.25 millimeter 48.82 48.82 48.82
0.25–0.05 27.36 29.94 22.37
0.05–0.01 millimeter 8.63 6.07 13.70
0.01 millimeter and less depos. 7.36 7.31 7.20
Clay (by difference) 1.00 1.03 1.08
Loss on ignition 6.83 6.83 6.83
—————— —————— ——————
100.00 100.00 100.00
The last column _C_ represents the average of three direct beaker
elutriations according to the method of Osborne.
The differences which these figures show are found to be due to
imperfect separation of the finer grades from the coarser and even when
the various fractions separated by the Schöne method are subjected to
beaker elutriation and the portions separated from them added to the
grades to which they properly belong the Schöne elutriator was found to
effect far less exact separations than the beaker method.
Samples of prairie soil from Mercer County, Ill., not boiled, were
examined by the two methods with the following results:
Schöne-Orth Beaker
elutriation. method.
Above 0.25 millimeter 0.76 0.62
0.25–0.05 millimeter 11.25 2.42
0.05–0.01 millimeter 52.65 43.58
0.01 millimeter and less deposited 14.84 31.58
Clay 4.44 5.81
Loss on ignition 14.49 14.49
————— —————
98.43 98.50
In this case it is seen that Schöne’s method varies considerably from
the beaker method and if the beaker method be regarded as correct the
Schöne method is evidently less reliable.
In the next table are given the data of the examination of brick clay
from North Haven, Conn., by the two methods.
BRICK CLAY FROM NORTH HAVEN, CONN.
Schöne-Orth elutriation. Beaker method.
Above 0.25 millimeter 1.02 1.02
0.25–0.05 millimeter 3.91 0.76
0.05–0.01 millimeter 29.63 20.95
0.01 millimeter and less 58.58 71.01
Loss on ignition 6.60 6.60
————— ——————
99.74 100.34
The failure of the Schöne method to give the results obtained by the
beaker method is ascribed to the fact that it is impossible for the
current of the strength used to disintegrate the clay and further that
the particles after they are once separated tend to coalesce by the
currents produced by the elutriating process.
=221. The Berlin-Schöne Method.=—Osborne has also made a study of the
Schöne method as modified by the Bodenlaboratorium of Berlin. The
directions for the analysis by this laboratory method are as follows:
Five hundred grams of the soil are sifted through a sieve with circular
holes two millimeters in diameter. Of the earth passing the sieve from
30 to 100 grams are boiled in water with constant stirring from one-half
to one hour or longer, according to the character of the soil. The finer
the texture of the soil the smaller the quantity taken and the longer
the time of boiling. Treatment with acids or alkalies is not practiced.
The finer portion of the soil remaining suspended in the water, after
boiling, is poured into the Schöne tube, the remaining coarse part is
rinsed into the Orth tube. The clay, together with the finest sand, is
collected in a separate vessel, the water in which it is suspended is
evaporated and the residue after drying in the air is weighed. The rest
of the operation is carried out as previously described except that the
products of elutriation are not ignited but weighed air dried, in order
that they may be further examined, chemically if desired. By proceeding
in this manner the following results were obtained:
SOIL, FROM GARDEN OF THE EXPERIMENT STATION,
BOILED FORTY-FIVE MINUTES.
Separations by the Berlin-Schöne method.
Air-dried. Ignited.
Above 0.05 millimeter 72.63 71.76
0.05–0.01 millimeter 14.17 12.53
0.01 millimeter and less 12.97 9.38
Loss on ignition 6.83
————— —————
99.77 99.50
For the sake of comparing the mechanical separation attainable by this
procedure with those yielded by other methods, the air-dried products
were ignited and again weighed and examined.
By subtracting from the ignited portion above 0.05 millimeter, 49.37 per
cent, the amount of this soil that remained on a 0.25 millimeter sieve,
the fraction between 0.25 millimeter and 0.05 millimeter is found, and
the separations in this analysis may be compared with those previously
obtained by the beaker method as follows:
SOIL FROM GARDEN OF EXPERIMENT STATION.
│
│ Beaker Method.
│ ———————————————————————————————————————
Berlin-Schöne,│ Boiled Pestled, not Not boiled
boiled │ twenty-three boiled. nor pestled,
forty-five │hours, average average of
minutes. │ of four three
│ analyses. analyses.
│
Above 0.25 49.37│ 47.77 48.82 48.82
millimeter │
0.25–0.05 21.39│ 20.75 22.44 22.37
millimeter │
0.05–0.01 12.53│ 11.18 12.55 13.70
millimeter │
<0.01 clay 9.38│ 13.47 9.36 8.28
included │
Loss on 6.83│ 6.83 6.83 6.83
ignition │
───────────────────────────┼───────────────────────────────────────────
99.50│ 100.00 100.00 100.00
Osborne concludes from the above facts that the Berlin-Schöne method,
while showing close agreement with the beaker method, does not give
results which are identical with that method. On subjecting portions
separated by the Berlin-Schöne method to the beaker analysis additional
separations were secured. In the case of heavy loams the inability of
the Berlin-Schöne method to effect even a rough or approximate
separation of the several grades becomes very conspicuous.