mix. In other words, the tendency here is to reduce surfaces of

separation to the least possible value, large particles growing in mass and bulk mechanically at the expense of smaller particles; in other words, exhibiting the phenomenon of flocculation.

=189. Physical Explanation of Subsidence.=—Whitney[128] thinks that the phenomena of the suspension of clay in water may be explained on purely physical principles, and that neither the partial solution nor hydration hypotheses are necessary, or will explain the suspension of clay in water, for the solution, or hydrated substance, would still have a higher specific gravity than the surrounding liquid. He calls attention in the first place to the fact, that in a turbid liquid, which has been standing for weeks and which is only faintly opalescent, the grains in suspension are still of measurable size, if properly stained as in bacteriological examinations and viewed through an oil emersion objective. He gives a value of 0.0001 millimeter, as the lower limit of the diameters of these particles of “clay,” which are usually met with in agricultural soils. He refers to the fact that fine dust and ashes, and even filings of metals, may remain in suspension in the air for days and even months in very apparent clouds, or haze, although they may be a thousand times heavier than the surrounding air. Particles of clay, no smaller than the limits which have been assigned, should remain in suspension in the much heavier fluid, water, for an indefinite time, for the volume or weight of the particles (⁴⁄₃)(πr³) decreases so much more rapidly in proportion than the surface (4πr²), that there is, relatively, a larger amount of surface area in these fine clay particles, and a great deal of surface friction in their movement through a medium, and they would settle very slowly. Under ordinary conditions, however, the mean daily range of temperature is about twenty degrees, the mean monthly range is fifty degrees, and the yearly range 100° F., and the ordinary convection currents, induced by the normal change of temperature, would be sufficient of itself to keep these fine particles in suspension in the liquid for an indefinite time, as it is known that currents of air keep fine particles of dust and ashes in suspension. If the volume or weight of a fine gravel, having a diameter of one and five-tenth millimeters, be taken as unity, then for a particle, having a diameter of 0.00255 millimeter, which is the mean diameter for Whitney’s clay group, the volume decreases in the ratio 1:0.000000004853, and the surface decreases only in the ratio 1:0.000286.

=190. Practical Applications.=—The action of mineral substances in promoting flocculence has been taken advantage of in later times in the construction of filters for purifying waters holding silt in solution. In these filters the introduction of a small quantity of alum, or some similar substance, into the water usually precedes the mechanical separation of the flocculent material. In the same way the action of iron and other salts on sewage waters has been made use of in their purification and in the collection of the sewage material for fertilizing purposes.

=191. Separation of the Soil Into Particles of Standard Size.=—The agronomic value of a soil depends largely on the relative size of the particles composing it. The finer the particles, within a certain limit, the better the soil. The size of the particles may be estimated in three ways: (1) by passing through sieves of different degrees of fineness; (2) by allowing them to subside for a given time in water at rest; (3) by separating them in water moving at a given rate of speed. The first method is a crude one and is used to prepare in a rough way, the material for the second and third processes.

=192. Separation in a Sieve.=—The soil should be dry enough to avoid sticking to the fingers or to prevent agglutination into masses when subjected to pressure. It should not, however, be too dry to prevent the easy separation of any agglutinated particles under the pressure of the thumb or of a rubber pestle.

The sieve should have circular holes punched in a sheet of metal of convenient thickness to give it the requisite degree of strength. Sieves made of wire gauze are not so desirable but it is difficult to get the finer meshes as circular perforations. Such sieves cannot give a uniform product on account of the greater diagonal diameter of the meshes and the ease with which the separating wires can be displaced. It is convenient to have the sieves arranged _en batterie_; say in sets of three. Such a set should have the holes in the three sieves of the following dimensions; _viz._,

1st sieve 2 millimeters diameter. 2nd „ 1 millimeter „ 3rd „ 0.5 „ „

Coarser single sieves may be used to separate the fragments above two millimeters diameter if such a further classification be desired. Each sieve fits into the next finer one and the separation of a sample into three classes of particles may be effected by a single operation. In most cases, however, it is better to conduct each operation separately in order to promote the passage of agglutinated particles by gentle pressure with the thumb or with a rubber pestle. In no case should a hard pestle be used and the pressure should never be violent enough to disintegrate mineral particles.

There is much difference of opinion concerning the smallest size of particles which should be obtained by the sieve.

Most analytical processes prescribe particles passing a sieve of one millimeter mesh (¹⁄₂₅ inch). There is little doubt, however, of the fact that a finer particle would be better fitted for subsequent analysis by the hydraulic method.

For this purpose a sieve of 0.5 millimeter circular mesh is preferred.

=193. Sifting with Water.=—In soils where the particles adhere firmly the sifting should be done with the help of water. In such cases the soil is gently rubbed with a soft steple or the finger in water. It is then transferred to the sieve or battery of sieves which are held in the water, and rubbed through each of the sieves successively until the separation is complete. After the filtrate has stood for a few minutes the supernatant muddy liquor is poured off, the part remaining on the sieve is added to it and the process repeated until only clean particles larger than 0.5 millimeter are left on the sieve. These particles can be dried and weighed and entered on the note book as sand. The filtrate should be evaporated to dryness at a gentle temperature and when sufficiently dry be rubbed up into a homogeneous mass by a rubber pestle.

The sieve recommended by the Association of Official Agricultural Chemists[129] for the preparation of fine earth for chemical analysis has circular openings ¹⁄₂₅ inch (one millimeter) in diameter.

Wahnschaffe[130] directs that a sieve of two millimeters mesh be used in preparing the sample for silt analysis and that the residue after the silt analysis is finished, which has not been carried over by a velocity of twenty-five millimeters per second, be separated in sieves of one millimeter and 0.5 millimeter meshes respectively.

Hilgard objects to leaving this coarse material in the sample during the process of churn elutriation on account of the attrition which it exerts and therefore directs that it be separated by sieve analysis before the elutriation begins.

=194. Method of the German Experiment Stations.=[131]—In the method recommended for the German Agricultural Stations an attempt is made to secure even a finer sieve separation than that already mentioned.

Sieves having the following dimensions are employed; sieve No. 1, square meshes 0.09 millimeter in size, diagonal measure 0.11 millimeter; sieve No. 2, square meshes 0.14 to 0.17 millimeter in diameter, diagonal measure 0.22 to 0.24 millimeter; sieve No. 3, square meshes 0.35 to 0.39 millimeter in diameter, diagonal measure 0.45 to 0.50 millimeter; finally a series of sieves one, two and three millimeters circular perforations.

Five hundred grams of the soil (in the Halle Station only 250) are placed in a porcelain dish with about one liter of water and allowed to stand for some time with frequent stirring, on a water bath. After about two hours, when the soil is sufficiently softened so that with the help of a pestle it can be washed through the sieves, the process of sifting is undertaken in the following manner: Sieve No. 3 is placed over a dish containing water, the moistened soil placed therein and the sieve depressed a few centimeters under the water and the soil stirred by means of a pestle until particles no longer pass through. After the operation is ended the residue in the sieve is washed with pure water and dried. The part passing the sieve is thoroughly stirred and then washed with water into sieve No. 2 and treated as before. The product obtained in this way is brought into sieve No. 1 and carefully washed. All the products remaining on each of the sieves are dried at 100° and weighed. The portion passing sieve No. 1 is either dried with its wash water or estimated by loss by deducting from the total weight taken, the sum of the other weights obtained. If a more perfect separation of the first sieve residue be desired it can be obtained by passing it through sieves of the last series which may have meshes varying in size, _viz._: one, two, or three millimeters in diameter. Each sieve of the same class should have holes uniformly of the same size.

The sieve products are characterized as follows: The part passing a three millimeter sieve is called fine earth, while the part remaining is called gravel. The fine earth is separated into the following products: The part that passes through the three millimeters opening and is left by the two millimeters opening is called _steinkies_. The product from the two millimeters opening and the residue from the one millimeter opening is called _grobkies_. The product from the one millimeter opening and the residue on the sieve No. 3 is called _feinkies_. The product from the sieve No. 3 and the residue from the sieve No. 2 is called coarse sand. The product from sieve No. 2 and the residue from sieve No. 1 is called fine sand. The product from sieve No. 1 is called dust. The dust can be further separated into sand, dust, and clay. For the examination of the clay the Kühn silt cylinder as modified by Wagner, is recommended. The cylinder has a diameter of eight centimeters and a height of thirty centimeters, and is furnished with a movable exit tube reaching to its bottom.

=195. General Classification of the Soil by Sieve Analysis.=—The classification recommended by the German chemists is satisfactory but the following one is more simple. All pebbles, pieces of rock, etc., should first be separated by a two millimeters circular mesh sieve, dried at 105° and weighed. The result should be entered as pebbles and coarse sand.

The finer sand may be separated with a sieve of one millimeter circular openings.

The still finer sand is next separated with the sieve of 0.5 millimeter circular openings as indicated above.

The sample may now be classified as follows:

1. Coarse pebbles, sticks, roots, etc., separated by hand.

2. Pebbles and coarse sand not passing a two millimeters sieve.

3. Sand not passing a one millimeter sieve.

4. Fine sand not passing a 0.5 millimeter sieve.

5. Fine earth passing a 0.5 millimeter sieve.

=196. Classification of Orth.=[132]—As fine silt are reckoned those particles which range from 0.02 to 0.05 millimeter; as fine sand the groups from 0.05 to 0.2 millimeter; as medium sized sand those ranging from 0.2 to 0.5 millimeter and for large grained sand those particles ranging from 0.5 to 2 millimeters in diameter. Particles over two millimeters form the last classification.

SEPARATION OF THE EARTH PARTICLES BY A LIQUID.

=197. Methods of Silt Analysis.=—The further classification of the particles of a soil passing a fine sieve can best be effected by separation in water. The velocity with which the current moves or with which the particles subside will cause a separation of the particles into varying sizes. The slower the velocity the smaller the particles which are separated. There is, however, a large and important constituent of a soil which remains suspended in water, or in a state of seeming solution. This suspended matter would still be carried over by a current of water moving at a rate so slow as to make a subclassification of it impossible. This suspended matter passing off at a given velocity may be classed as clay, and it consists in fact chiefly of the hydrated silicate of alumina, or other particles of equal fineness. The laws which govern its deposit have already been discussed.

The apparatus which have been used for silt analysis may be grouped into four classes.

(1) Apparatus depending on the rate of descent of the particles of a soil through water at rest. The apparatus for decanting from a cylinder or a beaker belong to this class.

(2) Apparatus which determine the rate of flow by passing the liquid through a vessel of conical shape. The system of Nöbel is a good illustration of this kind of apparatus.

(3) Apparatus in which the elutriating vessel is cylindrical and the rate of flow determined by a stop-cock or pressure feed apparatus. The system of Schöne represents this type.

(4) Apparatus in which the above system is combined with a device for mechanically separating the particles and bringing them in a free state into the elutriating current. The system of Hilgard is the type of this kind of apparatus.

In practice the use of cylindrical apparatus with or without mechanical stirring and the method by decantation have proved to be the most reliable and satisfactory procedures. Between the beaker and churn methods, of separation there is little choice in regard to accuracy. Which is the superior method, is a question on which the opinions of experienced analysts are divided. The various processes will be described in the order already mentioned.

=198. Methods Depending on Subsidence of Soil Particles.=—The simplest method of effecting the further separation of the soil particles is without doubt that process which permits them to fall freely in a liquid sensibly at rest. The practical difficulties of this method consist in the trouble of securing a perfect separation of the particles, in preventing flocculation after division and in avoiding currents in the liquid of separation.

For the separation of the soil particles for this method boiling and wet pestling are the only means employed. The flocculation of the separated particles may be partially prevented by adding a little ammonia to the water employed. The author has also tried dilute alcohol as the separating liquid but the results of this method are not yet sufficiently definite to find a place in this manual. Evidently the practical impossibility of avoiding convection currents prevents the use of water at a high temperature for this separation, although the tendency to flocculation almost disappears as the temperature approaches 100°. The general method of avoiding the errors due to flocculation in the subsidence method consists in repestling the deposited particles and thus subjecting them as often as may be necessary to resedimentation. These principles are well set forth by Osborne,[133] who states that when a soil is completely suspended in water by vigorous agitation, particles of all the sizes present are to be found throughout the entire mass of liquid. When subsidence takes place, the larger particles will go down more rapidly than the smaller ones, but some of the small particles that are near the bottom will be deposited sooner than some of the larger ones which have a much greater distance to travel. Thus, independently of the fact that the larger particles in their descent are somewhat impeded by the smaller, the smaller being at the same time somewhat hastened by the larger, the sediment that reaches the bottom at any moment is a more or less complex mixture of all the mechanical elements of the soil. The liquid, however, above this sediment at the same moment will have completely deposited all particles exceeding certain dimensions of hydraulic value, determined mainly by the time of subsidence.

If now the aforesaid first sediment be suspended in pure water, and allowed to subside for the same time as before, the larger part of it will be again deposited, but some will remain in suspension, consisting of a considerable part of the finer matter of the first sediment. By pouring off these suspended particles with the water and agitating the sediment again with clear water as before, another portion of fine particles will be suspended and may be decanted from it. On continuing this process of repeated decantations it will soon be found that the soil has been separated into two grades.

It is evident that in this way a separation can be made, but it is perhaps not so clear that such a separation would be sharp enough for the purposes of a mechanical soil analysis. If, for instance, the separation is to be made at 0.05 millimeter diameter, it is evident that by repeated decantations all below 0.01 millimeter can be washed out of that above 0.05 millimeter, but it may not appear so probable that all below 0.045 millimeter can be removed without removing some above 0.055 millimeter.

Such a result may be easily attained, however, if the following principle be adhered to:

Make the duration of the subsidence such that the liquid decanted the 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).

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.

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.

=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:

One kilogram of the soil previously dried in the air, is taken and 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.

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.

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°.

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))).

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.

This expression signifies that in a given case, the velocity of the 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.

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.

_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.

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.

=222. Method of Hilgard.=—Two important principles lie at the foundation 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.

=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.

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.

=234. Statement of Results.=—A complete silt analysis of a soil, conducted by the method of Hilgard, depends largely for its practical value on an intelligible tabulation. The method of collating results is illustrated in the table of analyses of Mississippi soils shown on page 237.

The character of the soils entering into the given analyses is as follows:

Nos. 248, 206, 209, 397, 219, belong to the end of the drift period.

No. 230 is one of the two chief varieties of soils occurring in what is known as the flat-woods, a level surface bordering on the cretaceous area, having lower tertiary clays near the surface.

No. 165 is a light soil which occurs in the former in irregular strips and patches, is easily tilled, absorbs rain water readily, but is subject to drought and does not hold manure.

SILT ANALYSES OF MISSISSIPPI SOILS AND SUBSOILS.

──┬─────────────┬────────────┬───────────┬────────── │ Designation │ Diameter. │ Velocity │ DRIFT │of Materials.│Millimeters.│(Hydraulic │ │ │ │ value). │ │ │ │Millimeters│ │ │ │per second.│ ──┼─────────────┼────────────┼───────────┼────────── │ „ │ „ │ „ │ „ ──┼─────────────┼────────────┼───────────┼────────── │ „ │ „ │ „ │ „ ──┼─────────────┼────────────┼───────────┼────────── │ „ │ „ │ „ │ 238 │ │ │ │ White │ │ │ │pipeclay. │ │ │ │Tishomingo │ │ │ │ Co. │ │ │ │ ──┼─────────────┼────────────┼───────────┼────────── 1│Coarse grits │ 1.0 to 3.0│ │ 2│Fine „ │ 0.5 to 1.0│ │ 3│Coarse sand │ 0.40│ 6│ 0.06 4│Medium „ │ 0.30│ 32│ „ 5│Fine „ │ 0.16│ 16│ „ 6│Finest „ │ 0.12│ 8│ 0.08 7│Dust „ │ 0.072│ 4│ 0.02 8│Coarsest silt│ 0.047│ 2│ 0.04 9│Coarse „ │ 0.036│ 1│ 0.08 10│Medium „ │ 0.025│ 0.5│ 0.08 11│Fine „ │ 0.015│ 0.25│ 2.00 12│Finest „ │ 0.008│ <0.25│ 21.15 13│Clay │ 0.0001│ <0.0023│ 74.65 ──┴─────────────┴────────────┴───────────┼────────── │ 98.16 Compactness (resistance to tillage) │ 97.80 Porosity │ 0.36 Hygroscopic Moisture (+7° to +21°) │ 9.09 Ferric Oxide │ 0.13 ─────────────────────────────────────────┴──────────

──┬─────────────┬───────────────────────────────────────────────────── │ Designation │ UPLAND. │of Materials.│ │ │ │ │ │ │ ──┼─────────────┼───────────────────────────────────────────────────── │ „ │ YELLOW LOAM. ──┼─────────────┼─────────────────────────┬─────────────────────────── │ „ │ SANDY. │ LOAM. ──┼─────────────┼─────────┬─────────┬─────┼────────┬─────────┬──────── │ „ │ 248 │ 165 │ 206 │ 209 │ 397 │ 219 │ │Tallahoma│ Lt. │Pine │ Pine │ Oxford │ Table │ │ subsoil │Flatwoods│Hill │ Hill │subsoil. │ Land │ │ Jasper │ soil. │soil.│subsoil.│Lafayette│subsoil. │ │ Co. │Chickasaw│Smith│ Smith │ Co. │ Benton │ │ │ Co. │ Co. │ Co. │ │ Co. ──┼─────────────┼─────────┼─────────┼─────┼────────┼─────────┼──────── 1│Coarse grits │ 6.94│ 2.90│ 0.36│ 0.36│ │ 0.23 2│Fine „ │ 17.65│ 6.96│ 2.98│ 0.83│ │ „ 3│Coarse sand │ 18.81│ 2.81│ 6.62│ 6.21│ 0.79│ 1.47 4│Medium „ │ 10.16│ 4.41│ 7.75│ 3.38│ „ │ 2.33 5│Fine „ │ 2.66│ 3.13│ 3.01│ 3.85│ „ │ 1.17 6│Finest „ │ 1.66│ 2.02│ 1.59│ 1.49│ 0.18│ 0.78 7│Dust „ │ 1.02│ 2.23│ 1.19│ 0.64│ 0.78│ 0.76 8│Coarsest silt│ 0.88│ 5.06│ 3.56│ 2.63│ 3.56│ 9.79 9│Coarse „ │ 1.96│ 9.67│ 6.50│ 5.40│ 13.12│ 7.26 10│Medium „ │ 7.89│ 14.18│13.97│ 7.77│ 16.64│ 13.14 11│Fine „ │ 8.40│ 22.03│14.20│ 16.65│ 27.28│ 15.07 12│Finest „ │ 15.53│ 15.62│29.36│ 37.75│ 18.87│ 26.50 13│Clay │ 8.63│ 7.86│ 4.58│ 10.70│ 17.23│ 19.19 ──┴─────────────┼─────────┼─────────┼─────┼────────┼─────────┼──────── │ 99.28│ 98.68│95.67│ 97.77│ 98.35│ 97.65 Compactness (res│ 32.56│ 45.33│48.14│ 45.10│ 63.38│ 60.82 Porosity │ 59.55│ 40.40│37.89│ 47.13│ 20.23│ 26.04 Hygroscopic Mois│ 1.80│ 3.36│ 2.48│ 7.69│ 8.79│ 7.21 Ferric Oxide │ 1.10│ 1.45[G]│ 1.25│ 4.15│ 2.53│ 5.11 ────────────────┴─────────┴─────────┴─────┴────────┴─────────┴────────

──┬─────────────┬──────────────────────────────────── │ Designation │ UPLAND. │of Materials.│ │ │ │ │ │ │ ──┼─────────────┼────────┬─────────────────────────── │ „ │ YELLOW │ TERTIARY. │ │ LOAM. │ ──┼─────────────┼────────┴─────────────────────────── │ „ │ CLAY. ──┼─────────────┼────────┬─────────┬────────┬──────── │ „ │ 173 │ 230 │ 246 │ 196 │ │Prairie │ High │ Red │ Hog │ │subsoil.│Flatwoods│ Hills │ Wallow │ │ Monroe │ soil. │subsoil.│subsoil. │ │ Co. │Pontotoc │ Attala │ Jasper │ │ │ Co. │ Co. │ Co. ──┼─────────────┼────────┼─────────┼────────┼──────── 1│Coarse grits │ 2.10│ 0.33│ 1.97│ 0.83 2│Fine „ │ „ │ 0.35│ „ │ 1.19 3│Coarse sand │ 0.62│ │ 0.72│ 1.96 4│Medium „ │ „ │ │ 2.32│ 1.64 5│Fine „ │ „ │ │ 2.09│ 0.88 6│Finest „ │ 0.20│ 0.23│ 0.70│ 0.26 7│Dust „ │ 1.26│ 0.18│ 1.29│ 0.19 8│Coarsest silt│ 2.92│ 1.61│ 1.81│ 2.49 9│Coarse „ │ 7.36│ 2.66│ 3.60│ 3.67 10│Medium „ │ 8.81│ 9.13│ 2.73│ 5.39 11│Fine „ │ 7.85│ 26.64│ 13.30│ 10.31 12│Finest „ │ 35.22│ 32.35│ 25.33│ 24.18 13│Clay │ 33.16│ 25.48│ 40.25│ 47.03 ──┴─────────────┼────────┼─────────┼────────┼──────── │ 99.50│ 97.87│ 96.11│ 100.00 Compactness (res│ 69.77│ 84.47│ 78.88│ 81.52 Porosity │ 17.04│ 6.40│ 39.18│ 10.12 Hygroscopic Mois│ 11.35│ 9.33│ 18.60│ 14.48 Ferric Oxide │ 5.42│ 5.90[G]│ 10.50│ 4.00 ────────────────┴────────┴─────────┴────────┴────────

──┬─────────────┬──────────────────────────────────────────────────────────────────────── │ Designation │ MISSISSIPPI BOTTOM. │of Materials.│ │ │ │ │ │ │ ──┼─────────────┼───────────────────┬──────────────────────────────────────────────────── │ „ │ Champlain. │ MODERN. ──┼─────────────┼───────────────────┼──────────────────────────────┬───────────────────── │ „ │ Swamp River. │ RIVER DEPOSIT. │ DELTA. ──┼─────────────┼─────────┬─────────┼────────────┬─────────┬───────┼──────────┬────────── │ „ │ 390 │ 237 │ 365 │ 377 │ 395 │ Southwest│ Southwest │ │Buckshot │ Loess. │Tallahatchie│Frontland│Dogwood│ Pass.│ mudlump. │ │ soil. │Claiborne│soil. Panola│subsoil. │ ridge │Plaquemine│Plaquemine │ │Issaquena│ Co. │ Co. │Sunflower│ soil. │ Par.│ Par. │ │ Co. │ │ │ Co. │Coahoma│ │ │ │ │ │ │ │ Co. │ │ ──┼─────────────┼─────────┼─────────┼────────────┼─────────┼───────┼──────────┼────────── 1│Coarse grits │ 0.09│ 0.24│ 0.09│ │ │ │ 2│Fine „ │ 0.05│ „ │ „ │ │ │ │ 3│Coarse sand │ │ 0.37│ 0.04│ 0.32│ 0.15│ 0.18│ 0.10 4│Medium „ │ 0.36│ 0.61│ 0.05│ „ │ │ „ │ „ 5│Fine „ │ │ 0.93│ 0.21│ 2.97│ │ 0.47│ 5.02 6│Finest „ │ 0.31│ 1.65│ 1.30│ 2.41│ 3.74│ 7.03│ 3.68 7│Dust „ │ 0.27│ 1.95│ 2.68│ 16.90│ 21.49│ 12.38│ 5.34 8│Coarsest silt│ 1.56│ 14.25│ 9.38│ 19.79│ 21.83│ 13.27│ 10.09 9│Coarse „ │ 2.23│ 16.20│ 9.88│ 13.90│ 14.01│ 15.87│ 5.58 10│Medium „ │ 3.68│ 20.08│ 20.37│ 4.27│ 9.93│ 8.25│ 9.54 11│Fine „ │ 8.97│ 5.59│ 19.79│ 1.89│ 9.58│ 7.26│ 8.01 12│Finest „ │ 38.19│ 33.38│ 25.30│ 30.08│ 8.65│ 19.67│ 34.46 13│Clay │ 44.30│ 2.51│ 9.64│ 5.51│ 10.35│ 12.20│ 18.18 ──┴─────────────┼─────────┼─────────┼────────────┼─────────┼───────┼──────────┼────────── │ 100.01│ 97.74│ 98.73│ 98.04│ 99.72│ 96.58│ 100.00 Compactness (res│ 89.46│ 41.48│ 54.63│ 37.48│ 28.57│ 39.13│ 60.65 Porosity │ 4.87│ 38.44│ 23.63│ 58.25│ 61.50│ 49.20│ 28.81 Hygroscopic Mois│ 14.31│ 4.18│ 6.12│ 5.68│ 3.95│ │ Ferric Oxide │ 5.82[G]│ 3.27│ 2.58│ 2.31│ 2.69│ │ ────────────────┴─────────┴─────────┴────────────┴─────────┴───────┴──────────┴────────── Footnote G:

Bog ore.

No. 248 is from a soil stratum three feet thick. The soil is so light that the finer particles of it are carried away by high winds.

Nos. 206 and 209 are typical of the soils producing the long-leaf pine. This soil is much improved by an admixture of the subsoil No. 209, which enables it to hold manure.

No. 219 is a cotton upland soil of the best quality, found in Western Mississippi and Tennessee.

No. 397 is the same soil of a second rate quality. These lands are easily washed into gullies on account of their lack of perviousness to water. They also easily swell up in contact with water, and become thereby readily diffused. The denudations produced by heavy rains are rapidly destroying the lands covered by these soils.

No. 173 is a sedimentary or residual subsoil of the cretaceous prairies of Northeastern Mississippi, forming a stratum from three to seven feet thick.

No. 230 is a residual soil which is formed by the disintegration of the old tertiary clays. It yields good crops only in very favorable years, and is easily injured both by wet and dry seasons.

No. 246 is a soil of the same origin, but is more easily tilled than the foregoing, does not crack, but becomes very hard when dried slowly. Its superiority to the former soil as regards tillage consists in the presence of the large amount of iron and lime.

No. 196 is a typical heavy clay soil; is better suited for the potter than the farmer. It cracks on drying, whence its popular name. On the accession of rain the edges of these cracks crumble and fall, until finally the lumpy surface is produced which is locally known as hog wallows.

No. 390, the richest soil of the Yazoo Bottom, seems to have a physical composition like the preceding one. Its superiority is due not only to the increased quantity of plant food which it contains, but to its property of crumbling on rapid drying. Even when plowed wet, on drying each clod crumbles into a loose pile resembling buck-shot; whence its name. It is strongly calcareous.

As comparative data, are added the soils 365, 377, and 395, representing alluvial deposits, and two deposits from the Delta of the Mississippi.

=235. Comparison of Osborne’s Method with Hilgard’s Method.=[154]—The comparative results obtained by Osborne’s method, beaker elutriation, and Hilgard’s method, churn elutriation, are given in the following tables:

SOIL FROM EXPERIMENT STATION GARDEN, NEW HAVEN, CONN. SURFACE SOIL, BOILED TWENTY-THREE HOURS.

Beaker elutriation. Average of four Churn elutriation. analyses. Diameter in millimeters. per cent. per cent. per cent. Removed by sieves 47.77 47.77 47.77 0.25–0.05 22.06 21.95 20.75 0.05–0.01 11.20 11.62 11.18 <0.01 9.82 9.14 10.72 Clay (difference) 2.32 2.69 2.75 Loss on ignition 6.83 6.83 6.83 —————— —————— —————— 100.00 100.00 100.00

SUBSOIL, BOILED TWENTY-THREE HOURS.

Churn elutriation. Beaker elutriation. Diameter in millimeters. per cent. per cent. per cent. per cent. Removed by sieve 39.33 39.33 39.33 39.33 0.25–0.05 33.61 30.83 32.35 32.95 0.05–0.01 10.91 12.25 10.32 10.37 <0.01 7.05 8.11 8.29 7.64 Clay 5.02 5.40 5.63 5.63 Loss on ignition 4.08 4.08 4.08 4.08 —————— —————— —————— —————— 100.00 100.00 100.00 100.00

These analyses agree quite as well as could be expected from two such different methods.

_Elutriation of Clayey Soils._—Hilgard found that by churn elutriation no satisfactory results could be obtained on clay without long boiling and subsequent kneading of the finer sediments. Osborne examined a sample of clay by his method after previous boiling for twenty-three hours. When the sediments were examined by the microscope they were found to contain many aggregations of particles which broke into dust under the pressure of the thin glass slide-cover. These sediments were then gently crushed in the beaker with the help of a soft rubber stopper with a glass rod for a handle, the grinding together of the particles being, as much as possible, avoided. This pestling was continued with clear water as long as it occasioned turbidity. Comparison of the analyses shows that practically identical results were obtained on this soil whether it was boiled or not and indicates that the sediments are reduced to their elements by gentle pestling alone. For such soils, therefore, it is demonstrated that pestling is a much safer treatment than boiling. The same remark may be applied to the fertile prairie soil of Mercer County, Illinois, where boiling proved quite insufficient and in which the pestling process proved completely successful. The general conclusions arrived at from the results obtained by Osborne are as follows:

1. On sands and silts of pure quartz or similar resistant material Hilgard’s method and beaker elutriation give practically identical results.

2. With coarse sands and silts upon whose grains finer matter has been cemented by silicates, etc., and with soils containing soft slaty detritus, the churn elutriator with preliminary boiling may give results too low for the coarse and too high for the finer grades. In these cases beaker elutriation with pestling yields more correct figures.

3. Some loamy soils containing no large amount of clay or of extremely fine silt, as well as prairie soils rich in humus, cannot be suitably disintegrated by twenty-four hours’ boiling, but are readily reduced by pestling.

4. Beaker elutriation preceded by sifting, gives results in five or six hours with use of two to three gallons of pure water, which, in churn elutriation, require several days and consume eight to ten gallons of pure water.

5. Hilgard found that practically 0.25 millimeter is about the lowest velocity of water current per second available within reasonable limits of time in his elutriator. Such a current carries over particles up to 0.015 millimeter diameter and hence the silts of less dimensions cannot be conveniently separated by churn elutriation. In beaker elutriation there is no difficulty in making good separations at 0.01 millimeter and at 0.005 millimeter.

6. Beaker elutriation requires no tedious boiling or preliminary treatment and with careful pestling of the sediments gives, we believe, as nearly as possible, a good separation of adhering particles and at every stage of the process carries with it, in the constant use of the microscope, the means of testing the accuracy of its work and of observing every visible peculiarity of the soil. It is not claimed that pestling may not easily go too far, but in any case a good judgment may be formed of its effects and of the extent to which it is desirable to carry it.

7. In beaker elutriation the flocculation of particles occasions little inconvenience and does not impair the accuracy of the results.

=236. Comparison of the Osborne with the Schloesing Method.=—Schloesing’s method has been compared by Osborne[155] with the beaker method of elutriation with the following results:

SCHLOESING’S METHOD.

Per cent. Calcium carbonate 4.20 Sand 64.91 Clay 22.65 Humus none Loss on ignition 6.60

BY THE DIRECT BEAKER METHOD.

Per cent. Above 0.25 millimeter diameter 1.02 0.25–0.05 millimeter diameter 0.76 0.05–0.01 millimeter diameter 20.95 Below 0.01 millimeter diameter 71.01 Loss on ignition 6.60

It is seen by the above that there is little agreement between the results of the two methods.

With the prairie soil from Mercer County, Ill., the following results were obtained working on the original sample and the sand separated by the Schloesing process:

SCHLOESING’S METHOD.

Per cent. Calcium carbonate 0.88 Humus 1.57 Loss at 150° C. 4.42 Sand 82.86 Clay 7.86 ————— 97.59

BEAKER METHOD.

Original Schloesing’s sand. soil. Dried at 150°C., Ignited, Ignited, per cent. per cent. per cent. Above 0.25 millimeter diameter 0.12 0.10 0.92 0.25–0.05 millimeter diameter 3.58 3.55 2.89 0.05–0.01 millimeter diameter 42.69 41.87 42.86 0.01–0 millimeter diameter 23.66 20.47 } 31.44 Clay 12.81 10.14 } Clay 7.40 Loss on ignition 6.73 14.49 ————— ————— —————— 82.86 82.86 100.00

Osborne says the above figures indicate that the treatment with acid has disintegrated the particles of less than 0.01 millimeter diameter so that one-third of this portion appears as clay, according to the Hilgard method of estimating clay, which is the one employed.

As to the humus it may be noted that loss in the analysis by Schloesing’s method; _viz._, 2.41 per cent, plus loss at 150° = 4.42 per cent, plus humus found = 1.57 per cent, plus carbon dioxid (⁴⁴⁄₅₆ of 0.88 =) 0.69 per cent amounts to 9.09 per cent, while the loss on ignition which represents humus, carbon dioxid and water is 14.49 Per cent. The 5.40 per cent difference must evidently be, for the most part, humus which has escaped estimation by the Schloesing method, having been distributed among the sand and clay.

=237. The Mechanical Determination of Clay.=—Schloesing’s method for the separation of the clay as stated by Osborne[156] is essentially one of subsidence for twenty hours from a volume of from 200 to 250 cubic centimeters of water, but of no specified height. Hilgard’s conventional method requires the same time and a height of solution of 200 millimeters.

Such methods of separation assume, first, that most of the sand and, second, that little of the clay shall settle within the fixed time. That both of these assumptions are fallacious, the following experiments show. The clay obtained by twenty hours subsidence from thirty grams of brick clay is suspended in four liters of distilled water and allowed to settle out completely, which requires several days. The water is then decanted so as to remove all soluble matters, the jar again filled with distilled water, and the clay and fine sand allowed to settle again for several days. The upper three-quarters of the liquid are then decanted and made up to a volume of four liters, and this is allowed to stand several days, when a considerable sediment forms. A decantation is again made as before. The operations are repeated until the clay water has been so far freed from the clay as to become opalescent; then it first ceases to deposit any appreciable sediment. A microscopic examination of the several sediments thus collected shows them all to contain particles of sand. It appears, therefore, that only after the liquid containing the clay has become opalescent does it cease to deposit fine particles of sand as well as of clay.

Furthermore, the character of the true clay itself is so changed under certain conditions that it loses the property of remaining in prolonged suspension in water.

A sample of clay which has been freed from particles of sand exceeding 0.005 millimeter diameter is suspended in water and precipitated from it by freezing. It is then washed by decantation with alcohol and dried in the air. A portion of this clay is shaken with water and allowed to stand a few hours, during which time the greater part of it has settled. After decanting the water and suspended clay and repeating this process a few times, a very considerable part of the clay is left which will subside completely through 100 millimeters in a few hours. After standing under water for several months, only a small part of the clay has regained the quality of prolonged suspension. It has been found, however, that if this clay be pestled, this quality of prolonged suspension is restored to it to a very considerable degree.

It is evident, therefore, that conventional methods depending on simple subsidence can give no accurate results because the ever varying amounts of finest sand and clay in different soils yield variable mixtures of the two when subjected to any simple course of treatment by elutriation and subsidence.

The method of persistent pestling and repeated subsidences and decantations continued until no further separation can be effected, although extremely tedious, is the only one which has so far yielded even approximately good separations on any of the clayey soils examined by Osborne.

A single subsidence of the clay water for twenty-four hours will free it from all particles of sand having a diameter greater than 0.005 millimeter, but in many cases a considerable amount of finer sand will remain in suspension for many hours or days.

On the other hand, the sediment formed during the twenty-four hours subsidence will not be free from clay, as may be easily seen by suspending it in water a second time and allowing it to stand again for twenty-four hours. Both Hilgard and Schloesing direct attention to these defects, but assume that they do not usually influence the results to a sufficient extent to deprive them of value. In many cases this is undoubtedly true, as, for example, in such soils as that from the garden of the Experiment Station, at New Haven, in which there is but little clay and fine sand; but in soils of the opposite character, as in the North Haven brick clay where exact separations are most desirable, a very considerable error is thus inevitably encountered.

=238. Effect of Boiling on the Texture of Clayey Soils.=—Most investigators who have worked upon mechanical soil analysis advise boiling with water in order to detach clay and sand from each other and make a good separation of the several mechanical elements practicable or possible. In general, however, the instructions as to the time and manner of boiling are rather indefinite, and no definite research as to the effects of this treatment has been undertaken.

The practice of Hilgard, to boil twenty-four hours or even longer in case of adhesive clays, according to Osborne[157] appears to be objectionable in view of the dehydration and change of physical properties known to occur in case of many hydroxids, especially those of iron and aluminum, which may be present in the soil. It is a familiar fact that the hydroxids above named and many other amorphous substances when precipitated from cold solutions are more bulky and less easily washed upon a filter than when thrown down hot. It is also well known that their properties are considerably changed by warming or boiling with water. Heating with water to boiling for some hours or days gradually converts the bulky brown-red ferric hydroxid, which when precipitated cold and air-dried for eighteen days, contains thirty-eight per cent of water, into a much denser, bright red substance containing but two per cent of water. St. Gilles has also observed the partial dehydration of aluminum hydroxid from Al₂O₃.5H₂O to Al₂O₃.2H₂O by prolonged boiling.

The hydrate of silica and the highly hydrated silicates are most probably affected in a similar manner, and if such be the case, boiling would evidently change the constitution of clay in a very essential degree.

The following experiments throw light on this subject:

Ten grams of North Haven brick clay were boiled continuously for nine days with about 700 cubic centimeters of distilled water, in a glass flask of one liter capacity and furnished with a reflux condenser. Fifteen grams were boiled in the same manner for eight and one-half days. When the boiling was concluded, the soil was found to have assumed a granular condition, the clay and fine sand being collected into a mass of small grains resembling coarse sand and settling rapidly. One portion thus boiled was elutriated by the beaker method, the other by Hilgard’s. The pestle was not used on either of those portions as it was desired to determine simply the effect of prolonged boiling. The separations thus accomplished are here compared with the elutriations of the same soil boiled twenty-three hours and of the pestled but unboiled soil.

Hilgard elutriation. Beaker elutriation. Not pestled. Not pestled. Not Pestled. Pestled. Boiled Boiled eight twenty-three Boiled nine and a half hours, days, days. Not boiled. Diameter of particles. per cent. per cent. per cent. per cent. By sieves 3.36 3.24 3.63 3.49 0.25–0.05 1.21 1.11 1.91 1.29 0.05–0.01 28.27 33.04 33.61 27.02 0.01–0 56.29 48.85 54.78 52.21 Clay 4.92 3.05 1.97 10.15 Loss on ignition 5.95 5.95 5.95 5.95 —————— —————— —————— —————— 100.00 95.24 101.85 100.11

Here it is observed that the eight to nine days boiling diminished the clay as determined by Hilgard’s conventional method by seven to eight per cent, increasing the dust by two to three per cent, and the silt by about six per cent.

Under the microscope small, rounded, opaque, brown granules were seen in large numbers, which when pressed under the cover glass, broke up into a multitude of very fine particles.

From these experiments it would appear to be conclusively proved that too long boiling precipitates clay and thereby defeats the very object of the operation.

In these experiments the time of boiling was prolonged in order to bring out unmistakably the effects of this operation. If ebullition for eight or nine days reduces clay from ten to two per cent, increasing the 0.05–0.01 millimeter diameter grades by six per cent, it is evident that boiling for one day or a shorter time becomes a questionable treatment.

Further experiments[158] made by boiling clay in a platinum vessel with a platinum condenser showed that this precipitation of the clay was largely if not wholly due to the salts extracted from the soil.

When the clay has once been converted into the granular condition, considerable difficulty is experienced in restoring it to the state in which it is capable of prolonged suspension in water.

The results of the studies herewith reported may be summed up as follows:

1. The Berlin-Schöne method of elutriation gives fairly correct separations with sandy soils containing little clay or matters finer than 0.01 millimeter diameter, but on soils of fine texture, as loams rich in humus and clays, it gives results which are grossly inaccurate, the error on single grades amounting to from eight to fourteen per cent.

2. In respect of rapidity, economy of time, and ease of operation, the Schöne elutriation has no advantage over the beaker method.

3. Schloesing’s method on its mechanical side makes no satisfactory separations, and the chemical treatment it employs is liable to alter seriously the texture of the soil.

4. The determination of clay from a single subsidence from any conventional depth or volume of water, or for any conventional time, is not a process certain to effect even a roughly approximate separation of the finest quartz grains from true clay.

5. Boiling with water must be rejected as a treatment preliminary to mechanical analysis, because it not only abrades and reduces the coarser sediments, but may dehydrate and coagulate the true clay and thus alter essentially the texture and grain of the soil.

=239. General Conclusions.=—The methods of Hilgard and Osborne have been given in detail and largely in the descriptive language used by the authors. The other methods of elutriation in use in other countries have also been described. For practical use the methods of Hilgard and Osborne are to be preferred to all others. For simplicity and speed the Osborne method has the preference over the Hilgard. For rigid control of the work the Hilgard method is to be preferred. The effect of long boiling on clay pointed out by Osborne would suggest that the boiling process preliminary to the Hilgard method be made as short as possible. It would seem that the churn attrition in the Hilgard method might well be regarded as a substitute for the soft pestling of the Osborne process, and any prolonged boiling in the former method might be safely omitted. When carefully carried out, the results of the Hilgard and Osborne method are fairly comparable.

=240. Distribution of Soil Ingredients.=—The determination of the distribution of the soil ingredients in the sediments obtained in silt analysis is illustrated by the following table:[159]

────────────╥────────────╥───────────╥────────────╥──────────── Hydraulic ║ ║ ║ ║ value ║ Clay ║ <0.25mm ║ 0.25mm. ║ 0.5mm. Per cent in ║ ║ ║ ║ soil ║ 21.64 ║ 23.56 ║ 12.54 ║ 13.67 ════════════╬══════╤═════╬═════╤═════╬══════╤═════╬══════╤═════ ║ │ ║ │ ║ │ ║ │ ║ =A= │ =B= ║ =A= │ =B= ║ =A= │ =B= ║ =A= │ =B= ║ │ ║ │ ║ │ ║ │ Insoluble ║ │ ║ │ ║ │ ║ │ residue ║ 15.96│ 4.35║73.17│17.29║ 87.96│11.03║ 94.13│12.72 Soluble ║ │ ║ │ ║ │ ║ │ silica ║ 33.10│ 7.17║ 9.95│ 2.34║ 4.27│ 0.53║ 2.35│ 0.32 Potash ║ 1.47│ 0.32║ 0.53│ 0.12║ 0.29│ 0.04║ 0.12│ 0.01 Soda ║(1.70)│ ║ 0.24│ 0.06║ 0.28│ 0.04║ 0.21│ 0.02 Lime ║ 0.09│ 0.03║ 0.13│ 0.03║ 0.18│ 0.02║ 0.09│ 0.01 Magnesia ║ 1.33│ 0.29║ 0.46│ 0.11║ 0.26│ 0.03║ 0.10│ 0.01 Manganese ║ 0.30│ 0.06║ 0.00│ 0.00║ 0.00│ 0.00║ 0.00│ 0.00 Ferric oxid ║ 18.76│ 4.06║ 4.76│ 1.11║ 2.34│ 0.29║ 1.03│ 0.14 Alumina ║ 18.19│ 3.97║ 4.32│ 1.04║ 2.64│ 0.33║ 1.21│ 0.17 Phosphoric ║ │ ║ │ ║ │ ║ │ acid ║ 0.18│ 0.04║ 0.11│ 0.02║ 0.03│ 0.00║ 0.02│ 0.00 Sulfuric ║ │ ║ │ ║ │ ║ │ acid ║ 0.06│ 0.01║ 0.02│ 0.01║ 0.03│ 0.00║ 0.03│ 0.00 Volatile ║ │ ║ │ ║ │ ║ │ matter ║ 9.00│ 1.33║ 5.61│ 1.43║ 1.72│ 0.23║ 0.92│ 0.29 ────────────╫──────┼─────╫─────┼─────╫──────┼─────╫──────┼───── Total ║100.14│21.64║99.30│23.56║100.00│12.54║100.21│13.67 Total ║ │ ║ │ ║ │ ║ │ soluble ║ │ ║ │ ║ │ ║ │ matter. ║ 75.18│ ║20.52│ ║ 10.32│ ║ 5.16│ „ „ ║ │ ║ │ ║ │ ║ │ bases ║ 41.84│ ║10.44│ ║ 5.99│ ║ 2.76│ Soluble ║ │ ║ │ ║ │ ║ │ silica in ║ │ ║ │ ║ │ ║ │ crude ║ │ ║ │ ║ │ ║ │ substance.║ 0.38│ 0.01║ │ ║ │ ║ │ ────────────╨──────┴─────╨─────┴─────╨──────┴─────╨──────┴─────

────────────╥───────────╥──────────┬──────────┬──────── Hydraulic ║ ║ │ │ value ║ 1.0mm. ║ │ │ Per cent in ║ ║ Other │ Total │Original soil ║ 13.11 ║sediments.│sediments.│ soil. ════════════╬═════╤═════╬══════════╪══════════╪════════ ║ │ ║ │ │ ║ =A= │ =B= ║ │ │ ║ │ ║ │ │ Insoluble ║ │ ║ │ │ residue ║96.52│12.74║ 13.76│ 71.89│ 70.53 Soluble ║ │ ║ │ │ silica ║ │ 0.36║ │ 10.36│ 12.30 Potash ║ │ „ ║ │ 0.49│ 0.63 Soda ║ │ „ ║ │ 0.12│ 0.09 Lime ║ │ „ ║ │ 0.09│ 0.27 Magnesia ║ │ „ ║ │ 0.44│ 0.45 Manganese ║ │ „ ║ │ 0.06│ 0.06 Ferric oxid ║ │ „ ║ │ 5.60│ 5.11 Alumina ║ │ „ ║ │ 5.51│ 8.09 Phosphoric ║ │ ║ │ │ acid ║ │ „ ║ │ 0.06│ 0.21 Sulfuric ║ │ ║ │ │ acid ║ │ „ ║ │ 0.02│ 0.02 Volatile ║ │ ║ │ │ matter ║ │ „ ║ │ 3.64│ 3.14 ────────────╫─────┼─────╫──────────┼──────────┼──────── Total ║ │13.10║ │ 98.28│ 100.63 Total ║ │ ║ │ │ soluble ║ │ ║ │ │ matter. ║ │ ║ │ │ „ „ ║ │ ║ │ │ bases ║ │ ║ │ │ Soluble ║ │ ║ │ │ silica in ║ │ ║ │ │ crude ║ │ ║ │ │ substance.║ │ ║ │ │ 0.19 ────────────╨─────┴─────╨──────────┴──────────┴────────

=A.= Calculated on the amount of sediment. =B.= Calculated on the amount of soil.

It is seen from the above analyses that the clay is by far the richest in mineral constituents, of all the ingredients separated in silt analysis, the amount in the clay being more than twice that of all the others combined. Its volatile matter is also the largest. The large amount of soda, however, is probably in part due to the sodium chlorid used in the precipitation of the diffused clay. The following points in regard to the distribution of the different ingredients are instructive:

1. The iron and alumina exist in almost identical relative proportions in each sediment, making it probable that they are in some way definitely correlated.

2. Potash and magnesia also exist in almost the same quantities, and their ratio to each other in all the sediments being almost constant seems to indicate that they occur combined, perhaps in some zeolitic silicate which may be a source of supply to plants.

3. Manganese exists only in the clay, a mere trace being found in the next sediment.

4. The lime appears to have disappeared in the clay, having probably been largely dissolved in the form of carbonate by the large quantity of water used in elutriation. Its increase in the coarser portions may be owing to its existence in a crystallized form not so readily soluble.

5. In a summary of the ingredients, it is seen that there is a loss in potash, magnesia and lime in the sediments as compared with the original soil; and this loss is doubtless due to the solution of these bodies in the water of elutriation.

A noteworthy fact shown in this table is the rapid decrease of acid-soluble matter in the coarser sediments; even what is dissolved from so fine a sediment as 1.0 millimeter hydraulic value, equal to a diameter of 0.04 millimeter, is in this case a negligible quantity. This suggests forcibly the inutility of introducing into chemical soil analysis, grains of as large a size as will pass a sieve of one millimeter aperture. The hydraulic value of these grains would be somewhere between 150 and 200 millimeters per second. While the exact results of the above analysis may not be applicable to all soils, yet the range is so wide that the systematic exclusion from chemical analysis of inert material, by means of preliminary mechanical separation, seems likely to lead to important improvements in the interpretation of the results.

=241. Percentage of Silt Classes in Different Soils.=—The adaptation of a soil to different crops depends largely on the sizes of the particles composing it and consequently on the relative percentages of the silt classes.

The following table gives the mechanical analysis of some markedly different types of subsoils:[160]

Diameter, Conventional Truck millimeters. names. and Grass Early small and truck. fruit. Tobacco. Wheat. wheat. Limestone. 1 2 3 4 5 6 2–1 Fine gravel 0.49 0.04 1.53 0.00 0.00 1.34 1–0.5 Coarse sand 4.96 1.97 5.67 0.40 0.23 0.33 0.5–0.25 Medium sand 40.19 28.64 13.25 0.57 1.29 1.08 0.25–0.1 Fine sand 27.59 39.68 8.39 22.64 4.03 1.02 0.1–0.05 Very fine sand 12.10 11.43 14.95 30.55 11.57 6.94 0.05–0.01 Silt 7.74 4.95 28.86 13.98 38.97 29.05 0.01–0.005 Fine silt 2.23 2.02 7.84 4.08 8.84 11.03 0.005–0.0001 Clay 4.40 8.79 14.55 21.98 32.70 43.44 ————— ————— ————— ————— ————— ————— 99.70 97.52 95.04 94.20 97.63 94.23 Org. matter, water and loss 0.30 2.48 4.96 5.80 2.37 5.77

=242. Description of the Soils.=—Number one represents the very early truck lands of southern Maryland. It is a light yellow sand, belonging to the Columbia terrace formation. Under an intense system of cultivation and heavy manuring with organic matter, good crops of garden vegetables are produced which mature very early, at least ten days or two weeks before the crops from any other part of the state. Under the prevailing meteorological and cultural conditions this soil maintains about five or six per cent of moisture, while a heavier wheat and grass soil maintains from twelve to twenty per cent. The truck soil is so loose and open in texture that the rain-fall passes through it very readily, and it is undoubtedly owing to this drier soil that the plant is forced to the early maturity which secures it from competition from other parts of the State and insures a good market price.

Number two represents the later truck and fruit lands of southern Maryland. These lands contain rather more clay than those just described; they are somewhat heavier and closer in texture, and are rather more retentive of moisture. This land gives a larger yield per acre than the one just described, and in every way crops make a more vigorous growth and development, but the crop is about a week or ten days later in maturing, and for this reason it brings a lower price in the market. It is much better land than number one for small fruit and peaches. These lands are altogether too light in texture for the profitable production of wheat, and it would cost altogether too much to improve them so that even a moderate yield of wheat could be obtained.

Number three is a tobacco land of southern Maryland. The finest tobacco lands of this locality come between the truck and wheat lands in texture, and contain from ten to twenty per cent of clay. The lighter the texture of the soil and the less clay it contains, the less tobacco it will yield per acre, but the finer the texture of the leaf. The tobacco yields more per acre on the heavier wheat soils, but the leaf is coarse and sappy and cures green and does not take on color. It brings a very low price in the market and does not pay for cultivation. The crop on the lighter lands is of much finer quality; there is a smaller yield per acre but the leaf takes on a fine color in curing, and brings a much better price per pound. Wheat is commonly raised on these tobacco lands to get advantage of the high manuring, and because the rotation is better for the land than where tobacco is grown continuously on the same soil. The finest tobacco lands are, however, too light in texture for the profitable production of wheat. These lands belong to the neocene formation.

Number four is a type of the wheat lands of southern Maryland. These lands represent soil of about the lightest texture upon which wheat can be economically produced under the climatic conditions which there prevail. They contain from eighteen to twenty-five per cent of clay, and are much more retentive of moisture than the best tobacco lands. This type is about the limit of profitable wheat production. These soils will maintain about twelve per cent of water during the dry season. Garden truck is so late in maturing on these lands that there is often a glut in the market when the crop matures, and the crops often do not pay the cost of transportation. The lands are too light in texture for a permanent grass sod. They belong to the neocene formation.

Number five represents the heavier wheat lands of southern Maryland, belonging probably to a different horizon of the neocene formation and containing about thirty per cent of clay. This soil is much more retentive of moisture and produces very much larger crops of wheat than the last sample. It is strong enough and sufficiently retentive of moisture to make good grass lands. It is too close in texture and too retentive of moisture for the production of a high grade of tobacco, or to be profitable for market truck.

Number six is from a heavy limestone soil of lower Helderberg formation. It is a strong and fertile wheat and grass land.

=243. Interpretation of Silt Analysis.=—The primary conceptions upon which the interpretation of the mechanical analysis is based may be briefly stated as follows:[161] The circulation of water in the soil is due to gravity, or the weight of water, acting with a constant force to pull the water downward, and also to surface tension, or the contracting power of the free surface of water (water-air surface), which tends to move the water either up or down, or in any direction, according to circumstances. There is a large amount of space between the grains in all soils in which water may be held, ranging from about thirty per cent in light sandy lands to sixty-five or seventy per cent in stiff clay soils. The relative rate of movement of water through a given depth of soil will depend upon how much space there is in the soil; upon how much this space is divided up, _i. e._, upon how many grains there are per unit volume of soil; upon the arrangement of the grains of sand and clay; and upon how this skeleton structure is filled in and modified with organic matter. It also appears that the ordinary manures and fertilizers change this surface tension, or pulling power of water; that they also change the arrangement of the grains, and consequently the texture or structure of the soil may be changed and the relation of the soil to water, through the effect of the ordinary manures and fertilizers in causing flocculation or the reverse.

=244. Number of Particles in a Given Weight of Soil.=—The approximate number of particles in the soil can be calculated from the results of the mechanical analysis by the following formula:[162]

(_a_/((π(_d_)³ω)/6)) ÷ A

Where _a_ is the weight of each group of particles, _d_ the mean diameter of the particles in the several groups in centimeters, ω is the specific gravity of the soil, and A is the total weight of soil. For the specific gravity of ordinary soils, the constant 2.65 may be used.

In using the formula the per cents are expressed as grams. Thus, if there were twenty per cent of silt, this would be taken as twenty grams, and if the results of the analysis added up ninety-seven per cent the whole weight of soil would be taken as ninety-seven grams. The diameter _d_ is taken as the mean for the extreme diameters taken for any group, for instance, for the silt this would be 0.003 centimeter, which is assumed to be the diameter of the particles in that group. This formula can only give approximate values, as the number of separations in a silt analysis must necessarily be small, amounting usually to not more than eight or ten grades, on account of the time and labor required for closer separations. There is relatively rather a wide range in the diameters of grains within any one of these grades, and absolute values could not be expected without a vast number of separations, so that all the grains in each group would be almost exactly of the same size.

The clay group has relatively the widest limits, which is unfortunate, as this is the most important of all the groups on account of the exceedingly small size of the particles. The figure 0.0001 millimeter is taken as the lowest limit of the diameter of the clay particles. These particles have been heretofore assumed to be ultra-microscopic, but by the use of a microscope of high power with oil-immersion objective and staining fluids, it has been possible to define the clay particles in a turbid liquid which has stood so long as to be only faintly opalescent.

Pending more exact measurements, the figure 0.00255 millimeter has been used as the diameter of the average sized particle in the clay group.

The following table gives the approximate number of grains per gram in the different types of subsoils calculated from the mechanical analysis of the typical soils already given:

NUMBER OF PARTICLES OF EACH CLASS IN ONE GRAM OF SOIL. ─────────────────┬─────────────────┬─────────────────┬───────────────── Silt classes. │ No. 1 │ No. 2│ No. 3 Diameter (_d_) in│ Early truck. │ Truck and small│ Tobacco. centimeters. │ │ fruit.│ ─────────────────┼─────────────────┼─────────────────┼───────────────── 0.15 │ 0│ 0│ 3 0.075 │ 85│ 34│ 102 0.0375 │ 5,511│ 4,011│ 1,900 0.0175 │ 37,230│ 54,610│ 11,890 0.0075 │ 207,500│ 199,700│ 267,900 0.003 │ 2,073,000│ 1,355,000│ 8,092,000 0.00075 │ 38,210,000│ 35,360,000│ 140,900,000 0.000255 │ 1,915,000,000│ 3,918,000,000│ 6,637,000,000 │ —————————————│ —————————————│ ————————————— │ 1,955,000,000│ 3,954,973,355│ 6,786,273,795 ─────────────────┼─────────────────┼─────────────────┼───────────────── Silt classes. │ No. 4 │ No. 5 │ No. 6 Diameter (_d_) in│ Wheat. │Grass and wheat. │ Limestone. centimeters. │ │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── 0.15 │ 0│ 0│ 12 0.075 │ 726│ 4│ 60 0.0375 │ 8,273│ 181│ 157 0.0175 │ 32,340│ 5,556│ 1,456 0.0075 │ 554,100│ 202,600│ 125,900 0.003 │ 3,962,000│ 10,670,000│ 8,231,000 0.00075 │ 73,990,000│ 154,900,000│ 199,900,000 0.000255 │ 10,150,000,000│ 14,570,000,000│ 19,430,000,000 │ ——————————————│ ——————————————│ —————————————— │ 10,228,547,439│ 14,735,778,341│ 19,638,258,585 ─────────────────┴─────────────────┴─────────────────┴─────────────────

=245. Estimation of the Surface Area of Soil Particles.=—The approximate extent of surface area of the soil grains in one gram of soil can be calculated from the foregoing by the following formula:[163]

π(_d_)²_n_

in which _d_ is the mean of the diameters of any group in centimeters, and _n_ is the number of particles in the group.

The following table gives the approximate extent of surface area of the particles in one gram of soil calculated from the preceding table:

APPROXIMATE EXTENT IN SQUARE CENTIMETERS, OF SURFACE AREA IN ONE GRAM OF SOIL.

Soil number. ————— —————— —————— —————— —————— —————— Diameter, millimeters. 1 2 3 4 5 6 1.5 0.0 0.0 0.4 0.0 0.0 0.1 0.75 1.8 0.6 1.8 12.8 0.1 0.1 0.375 24.3 17.7 8.4 36.5 31.0 0.7 0.175 35.8 52.6 11.4 31.1 5.3 1.4 0.075 21.3 35.3 47.3 97.9 35.8 22.2 0.03 218.8 38.3 228.9 112.0 301.4 232.7 0.0075 67.4 62.5 248.9 130.8 273.5 353.4 0.00255 390.8 800.5 1355.0 2072.0 2976.0 3965.0 ————— —————— —————— —————— —————— —————— Total 760.2 1007.5 1902.1 2493.1 3593.1 4575.3

=246. Logarithmic Constants.=—The following logarithmic constants have been used in the calculation of the approximate number of grains per gram and of the surface area, using 2.65 in all cases as the specific gravity of the soil.

Diameter. (_d_) Approximate number of grains. Surface area. log.(π(_d_)³_w_)/(6) log.(_d_)²π

0.15 centimeters \̅3.6703 \̅2.8493 0.075 „ \̅4.7674 \̅2.2473 0.0375 „ \̅5.8641 \̅3.6451 0.0175 „ \̅6.8711 \̅4.9831 0.0075 „ \̅7.7674 \̅4.2473 0.003 „ \̅8.5734 \̅5.4513 0.00075 „ \̅1̅0.7674 \̅6.2473 0.000255 „ \̅1̅1.3616 \̅7.3101

=247. Mineralogical Examination of the Particles of Soil Obtained by Mechanical Analysis.=—The principal object of the mechanical analysis of soils as has already been set forth is the separation of the soil into portions, the particles of which have the same hydraulic value. It is evident without illustration that particles of the same hydraulic value do not necessarily have the same size. The rate of flow of a liquid carrying certain definite particles does not imply that these particles are of the same dimensions. Of two particles of the same size and shape, that one which has the lower specific gravity, will be carried off at the lower rate of flow. At the end of the operation, therefore, the several portions of the soil obtained will be found composed of particles of sizes varying within certain limits, and of these particles the larger ones will tend to be composed of minerals of lower specific gravity, and the smaller ones of minerals of higher specific gravity. Of the same mineral substance, the particles which are most irregular, exposing for a given weight the largest surface will be found to pass over at a lower velocity than those of a more nearly spherical shape. The same law holds good for particles falling through a liquid at rest, _i. e._, the heavier and more spherical particles, weight for weight, will sooner reach the bottom of the containing vessel. To complete the value of a mechanical analysis, it becomes necessary to submit the several portions of soil obtained not only to a chemical but also to a mineralogical examination. Only the outlines of the methods of examining silt separates for mineral constituents can be given here and special works in petrography must be consulted for greater details.[164]

It is evident that the methods of separation and examination from a mineralogical point of view about to be described can only be applied to silts of the largest size. The finer silts can not be separated into portions of different specific gravities by separating liquids of varying densities on account of the slowness with which they subside, thus tending to adhere to the sides of the separating vessels and to form floccules which are not all composed of the same kind of mineral particles. While, therefore, these processes are more appropriately described in connection with the silts obtained by hydraulic elutriation, they can be applied with greater success to the fine particles passing the different sieves used in the preparation of the soil for analysis or to the finely pulverized soil as a whole.

The minerals which have contributed to soil formation, moreover, are better preserved in the larger silt particles and therefore more easily identified. While the desirability of securing like determinations in the finer silts is not to be denied, in the present state of the art the analyst must be content with the examination of the larger particles.

=248. Methods of Investigation.=—The chief points to be observed in the examination of the fine particles of soil are the following: (1) the size and shape of the particles; (2) measurement of crystal angles; (3) separation into classes of approximately the same specific gravity; (4) separation by means of the magnet; (5) determination of color and transparency; (6) determination of refractive index; (7) examination with polarized light; (8) examination after coloring; (9) chemical separation. For many of the optical studies above noted, it is first necessary to prepare thin laminae of the mineral particles and properly mount them for examination. For the purposes of this manual only those processes will be described which are essentially connected with a proper understanding of the nature of the soil particles. For the more elaborate methods of research the analyst will consult the standard works on mineralogy and petrography.

=249. Microscopical Examination.=—The direct examination of the silt particles with the microscope should attend the progress of separation. Unless the particles obtained have the same general appearance, the separation is not properly carried on. Especially is the microscope useful to determine that the value of the silt separation is not impaired by flocculation. Unless flocculation be practically prevented during the separation of the finest particles, many of these will be left as aggregates to be brought over subsequently with particles of far different properties. No special directions are necessary in the use of the microscope. The silt particles are removed with a few drops of water by means of a pipette, a drop of the liquid with the suspended particles is placed on the glass, covered and examined with a convenient magnification. A micrometer scale should be employed in order that the approximate sizes of the particles may be determined. A _camera lucida_ may also be conveniently used for the purpose of delineating the form of particles of peculiar interest.

=250. Petrographic Microscope.=—Any good microscope furnished with polarizing apparatus may be used for the examination of the silt particles and sections. For directions in manipulating microscopes the reader is referred to works on that subject. A special form of microscope for petrographic work is made by Bausch and Lomb of Rochester. The stand of this instrument is shown in Fig. 37. The base, upright pillars and arm are made of japanned iron. The stage is made in two forms, first, plain revolving, having silvered graduates at right angles and second, a mechanical stage with silvered graduations on the edge with vernier and graduations for the rectangular movements. The mirror bar is adjustable and graduated and the mirror is of large size, plane and concave. The double chambered box in the main tube carries the upper Nicol prism (analyzer). The lower Nicol prism (polarizer) is mounted in a cylindrical box beneath the stage to which it is held by a swinging arm. It is adjustable also up or down and is provided with a compound lens for securing converged polarized light. In revolving the prism a distinct click shows the position of the crossed Nicols.

=251. Form and Dimensions of the Particles.=—In order to study the contour of the fine silt particles, it is well to suspend them in a liquid whose refractive index is markedly lower than that of the particles themselves, and for this purpose pure water is commonly used. Care must be taken that not too many particles are found in the drop of water which is to be placed on the object holder and protected with a thin, even glass. The tendency to flocculation in these fine particles will make the study of their form difficult if they are allowed to come too close together. The size of the particles, or linear diameter, is to be determined by means of an eye-micrometer. This consists of a glass plate on which a millimeter scale is engraved with a diamond, or photographed. The millimeter scale is the one usually employed, each millimeter being divided into tenths. On microscopes designed especially for photographic work the micrometer is fastened to the eyepiece, and so adjusted as to read from left to right, or at right angles thereto. Sometimes an eyepiece-micrometer has two scales at right angles so that dimensions may be read in two directions without change. With an eyepiece-micrometer, not the dimensions of the object, but those of its magnified image are read, and the degree of magnification being known, the actual size of the object is easily calculated. The actual measurements may also be obtained by placing in the field of vision, a stage-micrometer and determining directly the relation between that and the eyepiece-scale. If, for example, the stage-micrometer is ruled to 0.01 millimeter, and the eye-micrometer to 0.1 millimeter, and one division of the stage-rule should cover three divisions of the eye-rule, then the one division of the eye-micrometer would correspond to an actual linear distance of 0.0033 millimeter in the object. If the two lines of division in the two micrometers do not fall absolutely together, the calculation may be made as follows: suppose that six divisions, 0.6 millimeter, in the eyepiece correspond to nearly five divisions, 0.25 millimeter, in the stage piece. To get at the exact comparison, take ninety-six divisions of the eye-scale and they will be found to be somewhat longer than eighty-one and somewhat shorter than eighty-two divisions of the stage-scale. It follows therefore that

one division of the eye-scale >0.008438 millimeter, and „ „ „ „ „ „ <0.008541 „ ;

and, hence, one division of the eye-scale corresponds almost exactly to 0.008489 linear measure.

=252. Illustrations of Silt Classes.=—In figure 38 are shown the relative sizes and usual forms of a series of silt separates made by the Osborne beaker method. The photomicrographs were made by Dr. G. L. Spencer from specimens furnished by Prof. M. Whitney.

The soil represented by the separates is from a truck farm near Norfolk, Virginia.

The particles represented in each class are not all strictly within the limits of size described. For instance, in the largest size (No. 1) are two particles at least which show a diameter of more than one millimeter. The particles in general, however, are within the limits of the class; _viz._, one-half to one millimeter, and this general observation is true of all the classes. In the case of the finer particles, especially of clay, the tendency to flocculation could not be overcome in the preparation of the slides for the photographic apparatus. The clay particles are so fine as to present but little more than a haze at 150 diameters of magnification. The particles seen are clearly, in most cases, aggregates of the finer clay particles. The larger particles show the rounded appearance due to attrition and weathering. It would have been more instructive to have had the particles of the different classes all photographed on the same scale, but this is manifestly impossible. The lowest power which shows any of the clay particles to advantage is at least 150 diameters, and with the larger particles such a magnification would have been impracticable.

=253. Measurement of Crystal Angles.=—The fine silt particles rarely retain sufficient crystalline shape to permit of the measurement of angles and the determination of crystalline form thereby. The rolling and attrition to which the silt particles have been subjected have, in most cases, given to the fragments rounded or irregular forms which render, even in the largest silts, the measurement of angles impossible. For the methods of mounting minute crystals and the measurement of microscopic angles, the analyst is referred to standard works on mineralogy and petrography.

=254. Determination of the Refractive Index.=—For a study of the theory of refraction, works on optics should be consulted. The general principles of this phenomenon which concern the determination of the refractive power of fine earth particles are as follows: if a transparent solid particle is observed in the microscope imbedded in a medium of approximately the same refractive power and color, its outlines will not be clearly defined, but the imbedded particle will show in all of its extent the highest possible translucency. If, therefore, the form or perimeter of the particle is to be studied with as much definiteness as possible, it should be held in a medium differing as widely from it as possible in refractive power. For minerals, water is usually the best immersion material. On the other hand, when the internal structure of the particles is the object of the examination, it should be imbedded in oil, resin (Canada balsam), etc., or in some of the liquids mentioned below.

If particles of different refractive powers and the same character of surface be studied in the same medium, they will not all appear equally smooth on the field of the microscope. Some of the surfaces will seem smooth and even, others will appear rough and wrinkled. Those particles whose refractive index is equal to or less than that of the liquid appear smooth, because all the emergent light therefrom can pass at once into the environing medium. On the other hand, the surfaces of those particles which have a higher refractive power than the medium will appear roughened, because, on account of the unavoidable irregularities on the surface, many of the emergent rays of light must strike at the critical angle and so suffer total reflection, and consequently those portions of the surface will be less illuminated, producing the phenomenon of apparent roughness above noted. In the case of any given particle, liquids of increasing refractive power can be successively applied until the change in the appearance of the surface of the particle is noticed. The refractive index of the liquid being known, that of the particle is in this way approximately to be determined.

The following liquids, having the indexes mentioned, are commonly employed:

Substance. Refractive index. Water 1.333 Alcohol 1.365 Glycerol 1.460 Olive oil 1.470 Canada balsam 1.540 Oil of cinnamon 1.580 Oil of bitter almonds 1.600 Oil of Cassia 1.606 Concentrated solution of potassium and mercuric iodid 1.733 Concentrated solution of barium and mercuric iodid 1.775

The solution of potassium and mercuric iodid may also be used for all refractive indexes from 1.733 to 1.334 by proper dilution with water.

The mineral particle may also be imbedded in Canada balsam and over it a drop of a liquid of known refractive power placed. By a few trials one of the liquids will be found having practically the refractive index of the particle under examination.

=255. Examination with Polarized Light.=—The internal structure of a mineral particle can often be determined by its deportment with polarized light. The theory of polarization is fully set forth in works on optics and will not be discussed here. The principle on which the utility of polarized light in the examination of soil particles rests is found in the information it may give in respect of crystalline structure. The structure of mineral particles which make up the bulk of an ordinary soil is, as a rule, so thoroughly disintegrated that all trace of its original form is lost. Some particles may exist, however, in which there is no determinable element of shape and which yet possess an internal crystalline structure which the microscope with polarized light may be able to reveal.

=256. Staining Silt Particles.=—The finer silts and clays before microscopic examination should be colored or stained. The methods used in staining bacteria may be employed for the clay particles.

Evaporation to dryness with a solution of magenta will often impart a color to the clay particles which is not removed by subsequent suspension in water. The harder and larger silt particles are not easily stained, especially if they be firm and undecomposed. On the other hand, if the particles be broken and seamed, and well decomposed, the stain will be taken up and held firmly in the capillary fissures. Valuable indications are thus obtained respecting the nature of the silt particles. Particles of mica, chlorite and talc are easily distinguished in this way from the firmer and less decomposed quartz grains.

The staining of the particles after ignition and treatment with acids gives better results than the direct treatment. Particles of carbonate which are stained with difficulty before ignition take the stain easily afterwards on account of the decomposition produced by the loss of carbon dioxid. This is the case also with particles containing water of composition or crystallization.

=257. Cleavage of Soil Particles.=—A microscopic examination of the cleavage of soil particles may be useful in determining their mineral origin. The course followed by cleavage lines and their mutual position is dependent on the direction in which the separation of the mineral fragment takes place. The character of the microscopic fragments produced by crushing a soil particle is determined primarily by the system of crystallization to which it belongs. Perhaps the most distinguishing cleavage marks in soil particles will be found in fragments of mica and orthoclase. These characteristic forms are shown in Figs. 39 and 40. The first (Fig. 39) shows the pinacoidal cleavage in a fragment of mica. Fig. 40 illustrates the appearance of the cleavage lines in a fragment of orthoclase. Figs. 41 and 42 show the characteristic cleavage lines in fragments of epidote and titanite.

=258. Microchemical Examination of Silt.=—The methods of quantitative chemical examination of silts will be given in another part of this manual. Certain qualitative and microchemical tests, however, are useful in identifying silt particles. For instance, any soluble iron mineral will be detected, even in minute quantity, by the blue coloration of the solution produced by the addition of potassium ferrocyanid. Manganese will be revealed by fusion with soda and saltpeter on platinum foil, in the oxidizing flame, producing the well-known green coloration due to the sodium manganate formed.

More valuable indications of the character of the fragments examined are obtained by microchemical processes. The best method of decomposing the silt particles for this purpose is by treatment with hydrofluosilicic acid. When the particles are composed of silicates, pure hydrofluoric acid is to be preferred.

The method of treatment is essentially that of Boricky.[165] The slide used is protected by a film of Canada balsam, and a few of the silt particles are placed thereon, and fixed in place by slightly warming the balsam. Each particle is then treated with a drop of hydrofluosilicic acid, care being taken not to let the drops flow together. The acid must be pure, leaving no residue on evaporation. The acid should be prepared by the analyst from a mixture of barium fluorid, sulfuric acid and quartz powder, or the commercial article should be purified by distillation before using. The acid should be kept in ceresin or gutta-percha bottles and must be applied with a ceresin or gutta-percha rod. Each particle should be as completely dissolved as possible by the acid, and the rate of solution may be hastened by gentle warming, provided the heat is not great enough to remove the balsam and allow the acid to attack the glass. The bases present in the silt particles crystallize on drying as fluosilicates. In case of a too rapid crystallization, the mass may be dissolved in a drop of water or of very dilute hydrofluosilicic acid, and allowed to evaporate more slowly. Some fragments need more than one treatment with acid to secure complete solution, and particles of mica may even resist repeated applications. In such a case the decomposition may be made in a platinum crucible with hydrofluoric acid, adding afterwards an excess of hydrofluosilicic acid and evaporating to dryness. The crystals may then be dissolved in a little water and a drop of the solution allowed to crystallize on the slide.

=259. Special Reactions.=—The number of microchemical reactions is very great, but there will be given here only some of the more important for silt identification.

_Sodium._—Sodium mineral fragments dissolved in hydrofluosilicic acid and dried give the combinations shown in Fig. 43. With sodium and aluminum the forms shown in Figs. 44 and 45 are obtained. With an increasing amount of lime in the mineral, the crystals tend to become longer. For microscopic work it is not advisable to try to produce the tetrahedral crystals of the double uranium sodium acetate because the commercial uranium acetate often contains sodium and even the pure article will often take up sodium from the bottles.

_Potassium._—Fragments containing potash give isotropic clear cubes, or octahedra of low refracting power, or combinations of these forms with each other and with rhombic dodecahedra. These crystals have the composition K₂SiF₆. Their forms are shown[166] in Figs. 46 and 47. In case much sodium be present, the first crystals obtained may be strongly double refractive rhombohedra, but on dissolving in water and allowing to recrystallize, the normal forms will be obtained. If the crystals be dissolved in hydrochloric or sulfuric acids, and treated with platinum chlorid, the characteristic yellow octahedral crystals of K₂PtCl₆ will be obtained. Ammonium and cesium compounds also give this reaction.

_Lithium._—When fragments containing lithium are treated with the solvent mentioned, monoclinic crystals are produced on drying. These crystals dissolved in sulfuric acid and freed from calcium sulfate by treatment with potassium carbonate give aggregates of lithium carbonate resembling a snowflake. At a high temperature lithium solutions treated with sodium phosphate give spindle-shaped crystals of lithium phosphate. The double lithium aluminum silicofluorid is shown in Fig. 48. The ease with which traces of lithium may be detected by the spectroscope renders unnecessary any further description of its microchemical reactions.

_Calcium._—Nearly all mineral particles, save quartz grains, contain calcium. When these particles are dissolved by treatment with hydrofluosilicic acid, they form on drying hydrated monoclinic crystals of calcium silicofluorid (CaSiF₆ + 2H₂O). These crystals assume many forms, some of which are shown in Figs. 49 and 50. These crystals are easily decomposed by sulfuric acid, the well-known long prismatic crystals of gypsum taking their place. On treatment of silt particles containing lime with hydrofluoric and sulfuric acids, only a part of the lime passes into solution if the content thereof be large. Where but little lime is present and the sulfuric acid is in large excess, all the lime passes into solution and the characteristic gypsum crystals appear as in Fig. 51.

_Magnesium._—Rhombohedral crystals of magnesium silicofluorid separate from the solution of particles containing magnesium in hydrofluosilicic acid. They have the composition MgSiF₆6H₂O and their common forms are shown in Fig. 52. Quite characteristic also are the crystals of struvite (NH₄MgPO₄ + 6H₂O), which are produced in a very dilute solution of the magnesium compound first obtained by carefully adding ammonium hydroxid and chlorid until a faint alkaline reaction is produced, and then placing a drop of dilute sodium phosphate at the edge of the solution. The crystals should be allowed to form slowly in the cold. Their form is shown in Fig. 54.

No. Diameter in mm. Name. Magnification. Diameters. 1 1.0–0.5 coarse sand ×10 2 0.5–0.25 medium sand ×10 3 0.25–0.1 fine sand ×10 4 0.1–0.05 very fine sand ×30 5 0.05–0.01 silt ×30 6 0.01–0.005 fine silt ×150 7 0.005–0.0001 clay ×150

_Barium._—From solution of barium bearing minerals in hydrofluosilicic acid fragments, no characteristic crystals, are obtained. Treated with hydrofluoric and sulfuric acids the barium is left as sulfate. If this salt be dissolved in boiling oil of vitriol and a drop of the solution placed on the slide, a mixture of rectangular tablets and St. Andrew’s cross-shaped growths will be separated before any crystals of gypsum which may be present appear. When strontium is present, the barium sulfate residue obtained by treatment with hydrofluoric and sulfuric acids should be fused with sodium and potassium carbonate, washed with water until the sulfuric acid is removed, the residue dissolved in hydrochloric or nitric acids, and the solution treated with potassium chromate. Pale yellow crystals of barium chromate are thus obtained, which resemble in form those secured by dissolving the barium sulfate in oil of vitriol. Strontium is not precipitated by this treatment. If potassium ferrocyanid be used instead of barium chromate with the hydrochloric acid solution, crystals of barium potassium ferrocyanid are formed of a bright yellow color and rhombohedric shape.

_Strontium._—From a hydrofluosilicic acid solution, strontium crystallizes in columns or tablets of the monoclinic system as strontium silicofluorid, SrSiF₆. On treating these with sulfuric acid, rhombic plates of strontium sulfate are formed, which serve to distinguish this element from calcium. On treatment of the particles of the original mineral with hydrofluoric and sulfuric acids, the strontium remains in the insoluble residue. When this residue is treated with boiling oil of vitriol, rhombic plates of celestine are separated. If the residues above mentioned be dissolved by fusion with the alkaline carbonates, washed with water, dissolved in hydrochloric acid and treated with oxalic acid, octahedral crystals of strontium oxalate are formed.

_Iron._—Mineral particles containing iron give crystals, when treated as is first described above, which are fully isomorphous with those obtained from magnesium. By moistening the crystalline mass with potassium ferrocyanid, the presence of iron is at once revealed by the blue coloration produced.

_Aluminum._—No crystals containing aluminum are formed from the mineral particles containing this substance when dissolved in the solvent already mentioned. If, however, the gelatinous mass be dissolved in a little sulfuric acid and a fragment of a cesium salt added, beautiful crystals of cesium alum are obtained, illustrated in Fig. 53.

_Phosphorus._—When a mineral fragment containing phosphorus is treated according to the usual analytical methods for securing the ammonium magnesium phosphate, crystals are obtained of the form shown in Figs. 54 and 55. A phosphatic fragment of silt may be identified when soluble by treatment with nitric acid and ammonium molybdate. On slowly drying, rhombohedral crystals are produced, yellow by reflected, and green by transmitted light. Their form is shown in Fig. 56.

=260. Petrographic Examination of Silt Particles.=—The larger silt particles and the minute fragments of minerals in the soil can best be studied in thin sections. For this purpose the following plan, proposed by Thoulet, may be used. Mix the soil minerals in considerable proportion—Thoulet recommends ten per cent, but a greater percentage is often better—with zinc oxid and make into a paste with sodium silicate. The paste should be worked to the consistence of putty and then rolled into little tablets about one-eighth of an inch thick and an inch in diameter. After drying a day or two without heating, the tablets become hard enough to mount and grind like rock sections. These tablets are mounted in Canada balsam on glass slides and ground as thin as possible with fine emery on the turn-table or glass plate, as rock sections are treated. As these tablets are not as strong as rock sections usually are, they require care in this treatment. Some of the grains also are apt to be torn out in the process of grinding and to compensate for this loss a number of slides should be prepared with each lot of soil minerals. When this operation has been successful, the optical properties of the various minerals can be studied as in rock sections.

As the iron oxid contained in the soils obscures the transparency of the minerals, it is well to treat a portion of the material under examination with hot hydrochloric acid for a short time to remove this oxid and then prepare slides with the cleansed material and compare results with the untreated. As the acid will dissolve phosphates and carbonates, and will partly or wholly decompose some other minerals, the operator must be guided by his judgment in its use.

=261. Machine for Making Mineral Sections.=—A convenient apparatus for this purpose has been described by Williams[167] and is represented in Fig. 57. It is supported on a substantial table provided underneath with electric batteries and a motor for driving the cutting disks seen on the top. The table is three feet six inches square and two feet nine inches high.

The grinding apparatus consists of two circular disks of solid copper, nine inches in diameter, and three-eighths inch thick, which may be used alternately as different grades of emery are required. They are attached either by a screw or square socket to a vertical iron spindle which revolves smoothly in a conical bearing. The grinding disk is surrounded when in use by a large cylindrical pan of tin, which is not shown in the cut, which has an opening in its center to allow of the passage of the spindle.

The sawing apparatus consists of a horizontal countershaft placed on a different part of the table and connected with the motor by a separate belt. It carries at one end a vertical wheel of solid emery, and at the other an attachment, level-table and guide for the diamond-saw. A small water-can with spout, not shown in the cut, is suspended over the edge of the table to keep the saw wet when it is in use.

The machine is very conveniently driven by a storage battery when street circuits cannot be drawn on.

For the details of making mineral sections, the works on petrography may be consulted.

=262. Separation of Silt Particles by Specific Gravity Solutions.=—In silt separates the specific gravity of the different mineral particles present may vary from graphite (1.9–2.3) to hematite (5.2–5–3).

The following list gives the specific gravities of some of the more common minerals which may be met with in soils:

Gypsum 2.31 Albite 2.56–2.63 Quartz 2.65 Talc 2.74 Chlorite 2.78 Muscovite 2.85 Calcite 2.5–2.78 Dolomite 2.90 Tourmaline 2.94–3.3 Biotite 3.01 Apatite 3.16 Pyroxenes 3.22–3.5 Epidote 3.39 Titanium Minerals 3.48–4.75 Iron oxids 5.2–5.3

The finest particles of silt are separated by gravity with great difficulty, inasmuch as they tend to remain suspended in the solutions for an indefinite period. With the coarser silts, however, useful data are often obtained by this method. The separation is preceded by extraction of the particles with hydrochloric acid to remove encrusted soluble matter, and by ignition to destroy any traces of organic matter. Those mineral matters which are soluble in acid or are changed by ignition must, of course, be sought for in separate portions of the silt,

=263. Thoulet’s Solution.=[168]—The standard solution is of such a density that particles of 2.65 specific gravity-will just float thereon, using for this purpose a solution of about 2.7 specific gravity. The solution from which the above standard is prepared is made as follows:

One part of potassium iodid is weighed and placed in a beaker and one and one-quarter part of mercuric iodid is placed on top of it. Then water is added in the proportion of ten cubic centimeters to 100 grams of the mixture, and after some time (twelve to twenty-four hours), with occasional stirring, the salts will nearly completely dissolve. Filter from the undissolved residue and evaporate in a porcelain dish until crystals form on the surface of the liquid. Allow to cool, pour off the liquid from the crystals and evaporate the liquid for another crop. The first solution, after cooling, has a specific gravity between 3.10 and 3.20, the second a specific gravity of 3.28, practically the limit of density of the solution. The solution of 2.7 specific gravity and other densities are made by cautiously adding a few drops of water at a time and ascertaining the specific gravity by the Westphal balance or other convenient method.

The strong solution, according to Goldschmidt,[169] may be prepared directly by using potassium iodid and mercuric iodid in the ratio of 1 : 1.24. Twenty-five cubic centimeters of water, 210 grams of potassium iodid, and 280 grams of mercuric iodid afford a solution of 3.196 specific gravity at 15°, on which fluorspar fragments will float.

=264. Klein’s Separating Liquid.=—A solution of cadmium borotungstate, of the composition 2H₂O,2CdO,B₂O₃,9WO₃ + 16H₂O, has been proposed by Klein[170] for separating silt particles. This salt is obtained by dissolving pure sodium tungstate in five times its weight of water, adding one and a half parts of boric acid and boiling until, complete solution takes place. On cooling; the borax is separated in crystalline form. The mother-liquor after the removal of the crystals is carefully concentrated by boiling. By stirring the cold solution, there is a further separation of sodium borate and polyborate. This operation is continued until glass will float on the mother-liquor. The salt in solution then has the following composition: 4Na₂O,12WO₃,B₂O₃. To this boiling concentrated solution, is added a boiling saturated solution of barium chlorid, in the proportion of one part of the chlorid to three parts of the original double tungstate. An abundant pulverulent precipitate is formed, making the whole mass mushy. The mass is filtered under pressure and well-washed with hot water. The residue is then suspended in hot water containing one part in ten of hydrochloric acid of 1.18 specific gravity. It is then evaporated to dryness in the presence of an excess of hydrochloric acid and decomposed, by which process hydrated tungstic acid is separated. The boiling mass is taken up with water and the boiling continued for two hours with occasional addition of water to take the place of that evaporated, and the tungstic acid separated by filtration.

From the solution, beautiful quadratic crystals separate having the composition 9WO₃,B₂O₃,2BaO₂H₂ + 18H₂O. These are purified by several recrystallizations and freed from any scales of boric acid by washing with alcohol. Any reducing action, revealed by a violet coloration of the crystals, can be avoided by adding a few drops of nitric acid. From a boiling solution of these crystals, the cadmium salt desired is obtained by treatment with the proper amount of cadmium sulfate solution to precipitate the barium. The barium sulfate is separated by filtration. The cadmium borotungstate is soluble in less than ten parts by weight of water. From this solution it is obtained in pure form by evaporation under a vacuum, or by carefully concentrating on a water-bath and cooling. A saturated solution of these crystals at 15° has a bright yellow color and a specific gravity of 3.28.

If a dilute solution of the above salt be carefully evaporated on a water-bath, any violet color which may be present disappears when the specific gravity reaches 2.7. If the evaporation be continued until a crystal of augite will float on the hot liquid, crystals may be obtained on cooling which, dissolved in as little water as possible, make a solution which will almost support olivine. If the two solutions be united, the specific gravity of the mixture is 3.30–3.36. The highest attainable specific gravity; _viz._, 3.6, is produced by continuing the evaporation on a water-bath until the liquid will support olivine, and then allowing to stand in a closed place for twenty-four hours. The crystals of cadmium borotungstate thus obtained are freed as much as possible from the mother-liquor by drainage and then melted at about 75° in their own water of crystallization. A liquid is thus obtained on which spinel will float. The same concentration may also be obtained by careful heating on a water-bath. At its highest specific gravity this solution has an oily consistence and this renders its practical use in the separation of fine particles somewhat restricted. By filtering the liquor when a crystalline crust begins to form during evaporation, a cold solution of 3.360–3.365 specific gravity is obtained which is found practically useful. It has a higher specific gravity than Thoulet’s mixture, is not injurious to any of the mineral particles, not even of iron with which it is brought into contact, but the trouble of preparing it is far greater than that of the mixture of mercuric and potassium iodids.

=265. Rohrbach’s Solution.=—The solution of barium mercuric iodid recommended by Rohrbach[171] for this purpose was originally prepared by Suchsin. The solution must be rapidly prepared on account of the tendency of the barium salt to decomposition. The solution is prepared by weighing rapidly 100 grams of barium and 130 grams of mercuric iodid, mixing the two salts well in a dry flask and adding twenty cubic centimeters of water. The mixture is raised to a temperature of 150°–200° on an oil-bath. The formation and solution of the double salt are promoted by constant stirring.

After solution, the liquor is boiled for a few minutes and then evaporated on a water-bath until it will bear a crystal of epidote. On cooling, a small quantity of a yellow double salt is separated by crystallization and the resulting mother-liquor is dense enough to carry a fragment of topaz. Inasmuch as the liquor is filtered with difficulty, the clear mother-liquor should be separated by decantation after standing for several days. This solution has the disadvantage of not being dilutable with water, the addition of which causes a separation of red mercuric iodid. Were this solution not so easily decomposed, it would prove of high value in silt separation.

=266. Braun’s Separating Liquid.=—In many respects the separatory solution proposed by Braun[172] is superior to those already mentioned. It is the commercial methylene iodid, CH₂I₂, which has at 16° a specific gravity of 3.32, at 5° of 3.35, and at 25° of 3.31. It is a strongly refractive liquid having a refractive index of 1.7466 for the yellow ray.

As a separating medium the liquid is open to two objections; _viz._, first, it cannot be diluted with water and, second, it turns brown on heating or on long exposure to the sunlight.

When dilution is necessary, it should be accomplished with benzene or xylene. To bring the diluted liquor again to its maximum density, the benzene must be removed by evaporation, which causes a considerable loss in the liquid. When this substance becomes opaque, the transparency may be restored by removing the separated iodin by shaking with potash lye, washing with pure water, drying by the addition of pieces of calcium chlorid and filtering. The same result may also be reached by freezing and separating the liquid portion. The frozen portion on melting will have the density of the original liquid.

=267. Method of Bréon.=—Instead of a solution of a salt, Bréon[173] has proposed to use salts in a fused state for separating mineral particles. Lead and zinc chlorids may be used for this purpose in a melted state, having the specific gravities of 5.0 and 2.4, respectively. By mixing the molten salts in different proportions, any desired specific gravity between the extremes mentioned may be secured. The fusion is accomplished at 400° in a test-tube. The silt is added gradually with constant stirring until a sharp separation is secured between the sinking and floating particles. After cooling, the tube is broken, the two parts separated, and the silt recovered by dissolving the mixed salts in hot water containing a little nitric acid. Only the coarser silts can be separated by this method. Fused silver nitrate, melting point 198°, specific gravity 4.1, has also been used for separation.

=268. The Separation.=—Forty cubic centimeters of the solution in the Thoulet process are placed in the separatory tube A, Fig. 58, together with from one to two grams of the silt and the stopper F inserted. The tube G is connected with a vacuum apparatus by means of which any air particles adhering to the mineral fragments are removed. The silt which sinks in the solution is removed after G has been disconnected by opening the cock C and sucking through B at I. The cock C is closed and the separated particles washed into a beaker at H after opening D. Water is next added to the materials left in A in quantities previously determined to secure a given specific gravity and thus a second, a third, etc., separation secured. An intimate mixture of the solutions in A can be effected by closing D, opening C, and blowing through B in such a way that no liquid is allowed to pass through C.

The quantity of water to be added in each case to secure a given specific gravity is determined by the formula _v_₁ = (_v_(D − _d_))/(_d_ − 1), in which _v_ is the volume of the solution, D its specific gravity, and _d_ and _v_₁ the specific gravity desired and volume of the water to be added.

_Example._—Let the specific gravity of the original solution be 3.2, its