of this method; _viz._, 1, the use only of separating vessels of true
cylindrical shape and 2, the employment of a mechanical stirrer to break
up the floccules formed during the process of separation. The points in
the apparatus to be considered are uniformity of the cross section of
the elutriator at every point, exact perpendicularity of position,
careful control of the rate of flow and continuous operation of the
mechanical stirrer. According to Hilgard’s observations the stirring due
to the current of water alone is not sufficient to break up the
floccules unavoidably formed during the separation, while any
inclination of the sides of the elutriating vessel from the
perpendicular due either to a conical shape or false position favors in
the highest degree the formation of floccules due to reflex currents
formed in the body of the liquid.
In order to carry out the idea suggested by Türschmidt of substituting
for the accidental and indefinite products usually appearing in the
statements of silt analyses sediments of known and definite hydraulic
value a constant head of water is used, secured by means of a Mariotte’s
bottle connecting with the tube delivering the current through a cock
provided with an arm moving on a graduated arc.
According to Hilgard the separation of sediments by the method of
subsidence does not possess the analytical accuracy of the moving liquid
method, especially when the latter is combined with mechanical stirring.
The subsidence method requires close and continuous attention and in the
case of fine sediments tending to flocculation the difficulties of the
method are greatly increased. The views of Hilgard in respect of the
laboriousness of the subsidence method lose, however, some of their
force since the modifications of Osborne have come into use. The
simplicity and cheapness of the apparatus required for subsidence give
it at the start many advantages over the more elaborate process with a
churn elutriator. For rigid scientific investigation, however, the
method of Hilgard is commended as a standard of comparison in all cases.
[Illustration:
FIGURE 35. HILGARD’S CHURN ELUTRIATOR.
]
=223. The Elutriator.=—The instrument devised by Hilgard[152] for the
purpose of breaking up these flocculent aggregates is shown in figure
35, together with the simpler form, a Schöne’s elutriator, figure 36,
which can serve for grain sizes above eight millimeters hydraulic value.
The latter is conveniently selected so as to have half the cross-section
of the former, so that with the same position of the index lever the
velocity will be just doubled. The cylindrical glass tube, of about
forty-five millimeters inside diameter at its mouth, and 290 to 300
millimeters high, has attached to its base a rotary churn consisting of
a brass cup, shaped like an egg with point down, so as to slope rather
steeply at base, and triply perforated; _viz._, at the bottom for
connection with the relay reservoir, and at the sides for the passage of
a horizontal axis bearing four grated wings. This axis, of course,
passes through stuffing boxes, provided with good thick leather washers,
saturated with mutton tallow. These washers, if the axis runs true, will
bear a million or more revolutions without material leakage. When a
beginning is noted additional washers may be slipped on without emptying
the instrument, until the analysis is finished. For the finest
sediments, from five to six hundred revolutions per minute is a proper
velocity, which may be secured by clock work, turbine or electric power.
The driving pulley should not be directly connected with the axis, both
because it is liable to cause leakage, and because it is necessary to be
able to handle the elutriator quickly and independently. This is
accomplished by the use of “dogs” on the pulley and churn axis. For the
grain sizes of one to eight millimeters hydraulic value lower velocities
are sufficient; too low a velocity causes an indefinite duration of the
operation and may be recognized by the increase of turbidity as the
velocity is increased.
As the whirling agitation caused by the rotation of the dasher would
gradually communicate itself to the whole column of water and cause
irregularities, a wire screen of 0.8 millimeter aperture is cemented to
the lower base of the cylinder.
The relay vessel should be a thick, conical test glass with foot; its
object is to serve as a reservoir for the heavy sediments not concerned
at the velocity used in the elutriator tube, and whose presence in the
latter or in its base, the churn, would only cause abrasion of the
grains and changes of current velocity, such as occur in the apparatus
of Schöne, and compel the current measurement of the water delivered. It
is connected above with the churn by a brass tube about ten millimeters
in clear diameter, so as to facilitate the descent of the superfluous
sediments, which the operator, knowing the proportion of area between
the connecting tube and elutriator, can carry to any desired extent;
thus avoiding the disturbance of the gauged current velocities, as well
as all material abrasion.
[Illustration:
FIGURE 36.
IMPROVED SCHÖNE’S APPARATUS
WITH
RELAY.
]
A glass delivery tube should extend quite half way down the sides of the
relay vessel, to insure a full stirring up of the coarse sediments when
required. By means of a rubber hose, not less than twenty inches in
length, this delivery tube connects with the siphon carrying the water
from near the bottom of the Mariotte’s bottle, a ten-gallon acid carboy.
A stop-cock provided with a long, stiff index lever, moving on an
empirically graduated arc, regulates the delivery of water through the
siphon. Knowing the area of the cross section of the elutriator tube,
the number of cubic centimeters of water which should pass through it in
one minute, at one millimeter velocity, is easily calculated, and from
this the lever positions corresponding to other velocities are quickly
determined and marked on the graduated arc. The receiving bottle for the
sediments, also shown in the figure, must be wide and tall, so as to
allow the sediment to settle while the water flows from the top into the
waste pipe. The receiving funnel tube must dip nearly to the bottom of
the bottle. Thus arranged, the instrument works very satisfactorily, and
by its aid soils and clays may readily be separated into sediments of
any hydraulic value desired. But in order to insure correct and
concordant results, it is necessary to observe some precautions; _viz._,
(1) The tube of the instrument must be as nearly cylindrical as possible
and must be placed and maintained in a truly vertical position. A very
slight variation from the vertical at once causes the formation of
return currents, and hence of molecular aggregates on the lower side.
(2) Sunshine, or the proximity of any other source of heat, must be
carefully excluded. The currents formed when the instrument is exposed
to sunshine will vitiate the results.
(3) The Mariotte’s bottle should be frequently cleansed, and the water
used be as free from foreign matters as possible. For ordinary purposes
it is scarcely necessary to use distilled water. The quantities used are
so large as to render it difficult to maintain an adequate supply, and
the errors resulting from the use of any water fit for drinking purposes
are too slight to be perceptible, so long as no considerable development
of the animal and vegetable germs is allowed. Water containing the slimy
filaments of fungoid growths and moss protonema, algae, vorticellae,
etc., will not only cause errors by obstructing the stop-cock at low
velocities, but these organisms will cause a coalescence of sediments
that defies any ordinary churning, and completely vitiates the
operation.
(4) The amount of sediment discharged at any time must not exceed that
producing a moderate turbidity. Whenever the discharge becomes so
copious as to render the moving column opaque, the sediments assume a
mixed character, coarse grains being, apparently, upborne by the
multitude of light ones whose hydraulic value lies considerably below
the velocity used, while the churner also fails to resolve the molecular
aggregates which must be perpetually reforming where contact is so close
and frequent. This difficulty is especially apt to occur when too large
a quantity of material has been used for analysis, or when one sediment
constitutes an unusually large portion of it. Within certain limits the
smaller the quantity employed the more concordant are the results.
Between ten and fifteen grams is the proper amount for an instrument of
the dimensions given above.
=224. Preparation of the Sample.=[153]—In some cases simple sifting will
be sufficient to prepare the air-dried soil for the elutriator. In most
cases, however, some mechanical aid must be invoked to secure particles
of sufficient fineness. Nothing harder than a rubber pestle should be
used and care must be taken not to break up any calcareous or
ferruginous masses which the particles of fine soil may contain. The use
of water in this mechanical attrition should be avoided, if possible,
but in some heavy clay and adobe soils wetting becomes necessary. In
this case the parts separated by the sieve are collected separately and
the turbid mass removed by water and dried for further examination.
A sieve of 0.5 millimeter mesh is recommended as the best because that
is almost exactly the diameter of the particles passing off at the
maximum velocity of sixty-four millimeters per second to which the
elutriator is adapted. The particles passing the 0.5 millimeter mesh are
called fine earth.
=225. Preparation by Boiling.=—The method of preparation by boiling may
be applied to all samples of fine earth. The fact pointed out by
Osborne, that diffusibility of some clays is diminished by long boiling,
renders it important to restrict the time of this operation as much as
possible. With most soils from eight to fifteen hours will be long
enough, occasionally extending to even twenty-four hours. A thin
long-necked flask of about one-liter capacity should be used; filled
three-quarters full with distilled water and the sample of soil added.
The flask is supported over the lamp on a piece of wire gauze at an
angle of 45°. It carries a cork with a long condensing tube. At first
the boiling goes on smoothly, but after a time violent bumping may
supervene, endangering the flask but promoting the object in view.
The contents of the flask are transferred to a beaker and diluted with
distilled water to one and a half liters, shaken and allowed to settle
for a time necessary to allow all particles of 0.25 millimeter hydraulic
value to reach the bottom. The supernatant turbid liquid is decanted and
the process repeated with smaller quantities of water until no further
turbidity is produced. The united decantations, of which there will be
from four to eight liters, are well shaken and a proper time allowed for
the 0.25 millimeter hydraulic value sediments to fall. This last step is
necessary to remove any such sediments which may have been carried over
mechanically in the first separation. The dilution being very great, a
fairly perfect separation is thus secured and the sediments are then
ready for the elutriator.
=226. Separation of Clay and Finest Silt.=—The property which pure clay
possesses, of remaining suspended almost indefinitely in pure water,
affords a ready means of separation from the silt particles of less than
0.25 millimeter hydraulic value. But the finest silt particles subside
so slowly that this method of separation is too long to become
practically applicable to secure a perfect demarcation between the
finest silt and so-called colloidal clay.
Hilgard recommends the following procedure: The clay water from the
previous separation is placed in a cylindrical vessel of such a diameter
as to allow the column of water to be 200 millimeters high where it is
allowed to settle for twenty-four hours. When the clay is very abundant
a longer time may be allowed; _viz._, from forty to sixty hours. The
line of separation between the dark silt below and the translucent clay
above is sharply defined. Finally the clay water is decanted and the
remaining liquid poured off leaving the sediment as sharply defined as
possible. The sediment is rubbed with a rubber pestle and a few drops of
ammonia water added. Distilled water is added, the beaker well shaken or
stirred to break up the floccules that may have formed and subsidence
permitted as before. This operation is repeated from six to nine times
until the water remains quite clear after subsidence or the decanted
turbid water fails to be precipitated by brine showing the suspended
matter to be fine silt and not clay.
The diameter of the particles of silt thus obtained is from 0.001 to
0.02 millimeter, and it is impossible to obtain it quite free from any
admixture with clay.
=227. Estimation of the Colloidal Clay.=—The importance of the colloidal
constituent of the clay is such as to make its direct determination
desirable. The volume of the clay waters at this stage of the analysis
may amount to twenty liters. One method of determination consists in
evaporating an aliquot portion and this method will yield good results
if the sample be free from soluble salts and the quantity taken be not
too small. At least 500 cubic centimeters should be used for this
purpose. A better method consists in precipitating the clay by means of
a saline solution. A saturated solution of salt is recommended for this
purpose of which fifty cubic centimeters are sufficient to precipitate
the clay from one liter of the clay water. The precipitation is hastened
by heating. Each portion of the clay water should be precipitated as
soon as obtained, the total volume of the precipitate at the end of
twenty-four hours is thus reduced to a minimum. The clay water from the
succeeding separations of the same analysis can be mixed with the
precipitate which diffuses therein, thus promoting the precipitation of
the rest of the clay inasmuch as the separation takes place more readily
where more clay is present. When all the clay is thus collected it can
be gathered on a tared filter and washed with weak brine. Pure water may
not be used because of the diffusibility of clay therein. After drying
at 100° and weighing it is washed with a weak solution of ammonium
chlorid until all sodium is removed. The filtrate is evaporated to
dryness, ignited at low redness, and weighed. The weight of the sodium
chlorid thus obtained plus the weight of the filter deducted from the
total weight gives the weight of the clay precipitate. Whenever the clay
collected as above will not diffuse in water it may be washed with water
and its weight directly obtained. An excess of iron in clay will usually
allow of the above treatment.
=228. Properties of Pure Clay.=—The percentage of pure clay as obtained
by the procedure described is about seventy-five in the finest natural
clays, forty-five in heavy clay soils, and fifteen in ordinary loamy
soils. When freshly precipitated by brine it is gelatinous resembling a
mixed precipitate of iron and aluminum oxids. Its volume greatly
contracts on drying, clinging tenaciously to the filter, from which it
may be freed by moistening. On drying, it becomes hard, infriable, and
often resonant. It usually possesses a dark brown tint due to iron oxid.
Under the action of water it swells up like glue, the more slowly as the
percentage of iron is greater. In the dry state it adheres to the tongue
with great tenacity. According to Whitney the finest particles of
colloidal clay have a diameter of 0.0001 millimeter. With a magnifying
power of 350 diameters, however, Hilgard states that no particles can be
discerned.
=229. Chemical Nature of the Fine Clay.=—The fine particles separated as
above consist essentially of hydrous aluminum silicate or kaolinite. It
doubtless contains, however, other colloids or hydrogels whose
absorptive powers are similar to those of clay. It appears also to
contain sometimes free aluminum hydroxid, and colloidal ferric hydroxid,
and amorphous zeolitic compounds.
While the most careful mechanical separation can give at best only
approximately the really plastic kaolinite substance, yet it is far
closer than that attained by determination of total alumina with boiling
sulfuric acid. By the latter treatment all the lime-kaolinite particles
are decomposed and the method does not lead to even an approximate
estimate of the soil’s plasticity.
=230. Separation of the Fine Sediments.=—The sediments remaining after
the separation of the clay and fine silt are ready for separation in the
churn elutriator. The apparatus mounted, as already described, is
brought into use by beginning with a low velocity of the water in the
upright tube. The rate of flow should be set at from 0.25 millimeter to
0.50 millimeter per second, and the churn put in motion.
When the elutriating tube is partly full of water the sediments should
be poured in from a small beaker which is perfectly cleaned by means of
a washing flask. The stopper and delivery tube of the elutriator are
then put in place. The rate of flow should be so regulated that the
sediments shall have had a few seconds of subsidence before the water is
within thirty millimeters of the top. At this point the required
velocity for the first sedimentation should be turned on; _viz._, 0.25
millimeter per second. At first the sediment passes off rapidly and the
water in the elutriator is distinctly turbid. This excess of turbidity
ceases in a few hours and then some attention is necessary in order to
determine when the process is complete. In fact it never is completely
finished, but where no more than one milligram of silt comes off with
one liter of water it may be said to be practically done. The time
required for the first operation varies from fifteen to ninety hours.
Downward currents in the elutriator are likely to form in spite of all
precautions, and floccules of silt adhere to its walls. These should be
detached from time to time with a feather in order to bring them again
in contact with the churn.
Hilgard has found that, practically, 0.25 millimeter per second is about
the lowest velocity available within reasonable limits of time, and that
by successively doubling the velocities up to sixty-four millimeters a
desirable ascending series of sediments is obtained; provided always,
that a proper previous preparation has been given to the soil or clay.
It would seem that according to the prescription given above for the
preliminary sedimentation, no sediment corresponding to 0.25 millimeter
velocity should remain with the coarser portion. That such is
nevertheless always the case, often to a large percentage, emphasizes
the difficulty, or rather impossibility, of entirely preventing or
dissolving the coalescence of these fine grain sizes by hand stirring,
as in beaker elutriation. It is only by such energetic motion as is
above prescribed that this can be fully accomplished, and the delivery
of 0.25 and 0.50 millimeter hydraulic value really exhausted.
It is desirable to run off the upper third of the column at intervals of
fifteen to twenty minutes by temporarily increasing the velocity. Recent
sediments, river alluvium, etc., are more easily separated than soils of
more ancient formation. The second, third, etc., separations are
naturally accomplished in much less time than the first. The respective
velocities of the separations should be 0.25 millimeter, 0.50
millimeter, one millimeter, two millimeters, four millimeters, eight
millimeters, sixteen millimeters, thirty-two millimeters, and sixty-four
millimeters a second. Below a velocity of four millimeters a second the
mechanical stirrer is indispensable. Above this velocity the current of
water in the conical base will be sufficient to bring the desired
particles into the ascending column. At this velocity also a smaller
elutriating tube having one-half or one-quarter the cross-section of the
first may be employed to hasten the operation and diminish the quantity
of water required. The quantity of water required for a complete
separation is from 100 to 120 liters. Any soft water free of organic
matter may be used, but distilled water is best. Hard water should be
avoided.
The mean time required for the different separations is as follows: 0.25
millimeter hydraulic value, thirty-five hours; 0.50 millimeter hydraulic
value, twenty hours; one millimeter hydraulic value, seven and a half
hours; two to sixty-four millimeters hydraulic value, eight hours. With
proper arrangements for night work, an analysis may be finished in three
or four days not counting the time required for the previous separation
of the clay.
=231. Weighing the Sediments.=—The sediments should be dried at the same
temperature used for drying the soils. Hilgard dries both at 100°. Great
care should be used in weighing the exceedingly hygroscopic clay
sediments. In the case of the sediment of 0.25 millimeter hydraulic
value it is allowed to subside as much as possible and after removing
the supernatant water the residue, twenty-five to fifty cubic
centimeters, is evaporated in a platinum dish and weighed therein. The
water can be completely decanted from the other sediments, and they can
be dried and weighed without any unusual precautions.
The loss in the separation of clays and subsoils containing but little
organic matter is usually from 1.5 to 2 per cent. This loss is partly
due to the fine silt which comes off during the whole of the process and
which is lost in the decanted waters of the sediments of 0.25
millimeters hydraulic value and above. The procedures indicated above
are not strictly applicable to soils rich in humus and other organic
matters, but the destruction of these matters by ignition leaves the
residual soil in a condition wholly unfit for sedimentary separation.
=232. Classification of Results.=—A convenient method of stating the
results of an analysis may be seen from the following classification.
The percentage obtained for each of the classes is to be entered in the
column provided for that purpose.
No. Names of Silt Diameter of Velocity of
Classes. grains in current Per
millimeters. millimeters cent.
hydraulic
value.
Sieves. 1. Grits 1 –3 2.07
„ 2. Fine grits 0.5–1 „
───────────────────────────────────────────────────────────────────────
Elutriator 3. Coarse sand 0.50 64 0.55
without churn.
„ 4. Medium sand 0.30 32 „
„ 5. Fine sand 0.16 16 „
───────────────────────────────────────────────────────────────────────
Elutriator 6. Finest sand 0.12 8 0.21
with churn.
„ 7. Coarse silt 0.072 4 1.21
„ 8. Large silt 0.047 2 2.92
„ 9. Medium silt 0.036 1 7.36
„ 10. Silt 0.025 0.5 8.86
„ 11. Fine silt separated 0.016 0.25 7.85
in elutriator
───────────────────────────────────────────────────────────────────────
Beaker 12. Fine silt separated 0.010 <0.25 35.22
sedimentation. from clay water
„ 13. Clay 0.0001 <0.0023 33.16
—————
Total 99.36
The measurements of diameters in the above table is of the best formed
quartz grains in each class. Naturally the actual size of the particles
may vary in each class within the extreme limits of the diameter next
above and below. It is not easy to indicate in popular language
distinctions not popularly made but the grades of particles designated
by the names grits, sand and silt, may serve, at least, to establish
uniformity of expression. The term grits is thus applied to all grains
above one millimeter in diameter up to gravel. Below one millimeter down
to 0.1 millimeter may be called sand and below that silt may designate
the particles down to an impalpable powder.
=233. Influence of Size of Tube.=—The diameter of the elutriating tube
exerts a sensible influence on the character of the sediments. The
friction against the sides of a small tube is comparatively greater than
in a large tube. Strictly speaking, no class of sediments strictly
corresponds to the hydraulic value calculated from the cross section of
the tube and the quantity of water supplied thereto. The sediments
correspond actually to higher velocities, due to the fact that the
lateral friction causes a more rapid flow in the center of the water
column. This may be demonstrated by slightly diminishing the velocity
while a sediment is copiously discharging. The turbid column then
remains stationary while clear water is running off.