Chapter VI: Introduction (5)

The peculiar odor is intensified proportionately as the volume of the liquid is decreased. To this one cubic centimeter, is added some pure crystallized potassium carbonate. The liquor is immediately troubled and some hours are required for it to become clear again. Meanwhile there is formed upon its surface a resinous ring almost invisible, amounting at most to from ten to twenty milligrams of a matter which has not been identified with any known principle. The reactions described above, however, permit of its general character being known. This resinous matter contains the odoriferous principle, the composition of which is not yet definitely known.

=96. Specific Gravity.=—The density of a soil depends on its composition, the fineness of its particles and upon the packing which it has received. It has in other words an apparent and a real specific gravity. It is easy to see that a soil in good tilth would weigh less per cubic foot than one which had been pressed closely together, as in a road or well-pastured field. Ordinary soils in good tilth have an apparent specific gravity of about 1.2, and when entirely free from air, a real specific gravity of about 2.5. If the apparent specific gravity of a soil sample were 1.2 and the air were removed, leaving a vacuum in the interstices of the soil, the apparent specific gravity would not be sensibly increased. The figure 1.2 is the apparent specific gravity of a mixture of soil material which is about 2½ times heavier than water, and of an extremely small proportion by weight of air which is about 1000 times lighter than water. The figure 2.5 is about the true specific gravity of the real soil material, and shows that this material is about 2½ times heavier than an equal volume of water.

The weights of a cubic foot of different kinds of soil as given by Schübler[71] are as follows;

Pounds.
Sand 110
Sand and clay 96
Common arable soil 80 to 90
Heavy clay 75
Vegetable mold 78
Peat 30 to 50

In general the specific gravity of soil decreases inversely as its content of humus.

=97. Determination of Specific Gravity.=—The ordinary method of proceeding to determine the true specific gravity is by means of a pyknometer. The pyknometer should have a capacity of from twenty-five to fifty cubic centimeters.

From ten to fifteen grams of earth dried to constant weight at 100° are taken, boiled for a time with a few cubic centimeters of water to remove air and poured into the pyknometer. All soil particles are washed out of the vessel in which the boiling took place into the pyknometer with freshly boiled distilled water and after cooling to the temperature at which the calibration took place, the pyknometer is filled with distilled water at the given temperature and weighed. If the soil contain materials soluble in water, alcohol of definitely known specific gravity may be employed and the number thus obtained calculated to a water basis.

The calculations when water is used are made as follows:

Grams.
Weight of pyknometer 13.4789
„ „ pyknometer full of distilled water at 20° 62.8934
„ „ water in pyknometer 49.4145
„ „ dry soil taken 10.0000
„ „ pyknometer + dry soil + filled with water at 20° 67.9834
„ „ soil and water 54.5045
„ „ water 44.5045
„ „ water displaced by ten grams soil 4.9100

Then specific gravity = 10.000 ÷ 4.9100 = 2.04.

=98. Specific Gravity of Undried Soils.=—It is often desirable to determine the specific gravity of an undried portion of the soil. For this purpose a portion of the sample is dried at 100° to determine its percentage of moisture. The specific gravity is then determined on a ten gram sample of the undried soil as just given. The actual weight of soil taken is calculated from the percentage of moisture obtained in the first instance. In the case given if the percentage of moisture at 100° be ten then the actual weight of dry soil taken is nine grams. This number is therefore used in making the calculations. In all statements of specific gravity taken in the manner described the temperature at which the pyknometer is calibrated should be stated and all weighings where water is involved made at that degree.

=99. Volume of Soil.=—If it be desired to calculate the volume occupied by a soil it is easily done by dividing the weight of water displaced by the weight of one cubic centimeter of water of the temperature at which the determination took place.

In the case given one cubic centimeter of water at 20° weighs 0.998259. Then 4.9100 ÷ 0.998259 = 4.9186 cubic centimeters = volume occupied by ten grams of dry soil excluding interstitial spaces between particles.

=100. Volumetric Methods.=—The water displaced by a given weight of soil may also be measured volumetrically by the method of Knop.[72]

Place 200 grams of the soil in a flask of from three to five hundred cubic centimeters capacity. Add a measured quantity of water, and shake thoroughly to eliminate air, and fill up to the mark from a burette. The quantity of water required to complete the volume subtracted from the number expressing the volume of the flask will give the volume of water displaced by the earth.

Another method consists in thoroughly shaking about thirty grams of the soil in a graduated cylinder with fifty cubic centimeters of water containing a little ammonium chlorid and after twenty-four hours recording the volume occupied by the whole. The increase in volume over fifty cubic centimeters shows the quantity of water displaced. This method may also be used to determine the volume occupied by a soil when saturated with water. The above methods are only to be used when approximately correct results are all that are desired.

=101. Apparent Specific Gravity.=—The apparent specific gravity of a soil is obtained by dividing its volume, interstitial spaces included, by the weight of an equal volume of water.

The real and apparent specific gravities of six samples of soil are given below.[73]

Real specific gravity 2.5445, 2.6315, 2.6508, 2.6400, 2.7325, 2.6603
Apparent specific
gravity of air-dried
soil 1.0940, 1.1710, 1.3570, 1.2810, 1.4060, 1.2730
Apparent specific
gravity of soil dried
at 125° 1.0990, 1.1770, 1.3750, 1.2910, 1.4640, 1.2850

It is to be noted that in computing the apparent specific gravity of a soil dried at 125° the volume occupied by the water is assumed to occupy the same space as if it existed in a free state. The volume of this water is therefore to be subtracted from the contents of the flask before proceeding with the computations.

=102. Determination of Apparent Specific Gravity.=—Place in small quantity portions of the air-dried sample properly prepared, into an open glass cylinder, holding one liter, and about 170 millimeters high (if the height is exactly the mentioned one, the diameter of the cylinder will be 86.6 millimeters); pack the sample by striking the bottom of the cylinder hard against the palm of the hand after each new filling; close the cylinder thus filled by a glass plate and weigh on a balance sensitive to 0.1 gram; deduct the weight of the cylinder and glass plate, and the weight of one liter of soil in approximately similar conditions as it is found on the dry land prepared for cultivation, is thus ascertained. The weight of one liter of the soil in grams multiplied by 2000 will give in kilograms the weight of the surface soil from a hectare (2.47 acres) of the field from which the sample is taken when the depth of this is calculated at twenty centimeters.[74]

RELATION OF THE SOIL TO HEAT.

=103. Sources of Soil Heat.=—The heat of the soil comes from three sources, _viz._: solar heat, as the sun’s rays, heat of chemical and vital action within the soil, and the original heat of the earth’s interior. The latter is sensibly a constant quantity, and of great value to plants. The heat of chemical and vital action is not great in amount except in a few special cases but is often, as in germination, of the greatest importance to plant growth. The sun, therefore, remains the greatest source of heat of practical importance in relation to the production of crops. Dark-colored soils, absorbing most and radiating the fewest rays, must attain the highest temperature. Schübler’s classical researches on soil temperatures, show that there is at times a difference of over 7° in temperature between white and black soils, all other conditions being alike. Schübler’s researches, being made on dry soils in the laboratory, do not, however, apply wholly to conditions in the field.

=104. Influence of Specific Heat.=—The heat which a soil receives and retains is largely due to the specific heat of the soil. The specific heat of a body is expressed by a number which shows the amount of heat necessary to raise a given weight of the body 1° of temperature, as compared with the amount necessary to raise the same weight of water 1°. The specific heat of the soil is usually between 0.20 and 0.25, while that of water taken as the standard is unity.

=105. Influence of Moisture.=—The moisture of the soil possesses great influence on the soil temperature, so much so that a dry, light-colored soil may attain a greater degree of warmth than a moist, dark-colored one. The action of water in reducing soil temperature is easily explained. In our latitude, we see the water in all its forms, solid, liquid, and gaseous, and we know that these forms are the direct result of temperature. The changing of water from the solid to the liquid or gaseous form is performed at the expense of heat; the more water evaporated from the soil the more heat must be used for the evaporation. Therefore, the more water contained in the soil at any given time the lower must be its temperature during subsequent exposure to sun heat because of the greater evaporation. The experiments of Liebenberg, Pattner, Schübler and Dickenson have practically settled all the questions of soil temperatures. The radiation of heat from the soil, and the consequent cooling propensity of the latter, are directly proportional to the absorptive power of the soil. Two soils of like absorptive power towards heat possess, as a rule, equal radiating power.

In a general way, it can be said the greater the heating capacity and conductivity of a soil the more readily and rapidly does it give off its heat and become cooled.

=106. Absorption of Solar Heat.=—The quantity of heat absorbed from the sun by the earth is an important factor in the growth of vegetation. As has been established in the physics of heat, a black surface, other things being equal, will absorb a larger amount of heat than one of any other color; so, other things being equal in the physical and chemical composition of a soil, variations in the amount of organic matter producing greater or less black coloration will affect the heat absorption. Thus, black soils, in the conditions above mentioned, will absorb more heat than lighter colored soils. As a result, the vegetation in such soils gets an earlier start in the Spring and matures more rapidly. As an illustration of this it may be noted that the black prairie soils of Iowa produce uniformly crops of maize which are matured before the early frosts, while crops grown on lighter soils much farther South often suffer injury from that source.

DETERMINATION OF SPECIFIC HEAT.

=107. General Principles.=—The quantity of heat stored in any given weight of soil is capable of being measured and compared with the quantity stored in an equal weight of water at the same temperature. The ease, however, with which disturbing influences operate during the determination makes the manipulation somewhat difficult. The specific heat of the containing vessels must be carefully determined. Fortunately this has been done for most materials and the data thus obtained are recorded in standard works on physics. The material operated on must be protected from thermal influences from sources not controlled by the experiment and even the heat of the operator’s body may often disturb the conduct of the work. The general conditions which should control the experiment as well as the details thereof are given in the following method which, however, the ingenious analyst may profitably simplify.

=108. Method of Pfaundler.=—The process of estimating the specific heat of soils by the method of mixture, is essentially that of Regnault and is described as follows by Pfaundler[75].

The apparatus used is illustrated in Fig. 13.

A and A′ show the heating apparatus. It consists of a vessel of sheet iron in which a test tube E is fixed by means of a cork. The test tube holds the soil whose specific heat is to be determined. The apparatus contains water, which is brought to the boiling point by means of a lamp, and the excess of steam is conducted away, as indicated in the figure, through one of the axes of the apparatus; the opposite axis is, of course, closed. It requires about thirty-five minutes boiling to bring the contents of the test tube to the temperature of the aqueous vapor. The exact temperature at which the water boils is determined by observing the barometer at the time and consulting a table of the boiling temperature of water at different barometric pressures.

The calorimeter is shown in the figures B and B′. It consists of a wooden box closed on one side by a glass plate G and on the other to the heighth F by a small board on which a calorimeter of ordinary construction is placed. The cylinder of the calorimeter is seventy millimeters high and forty-seven millimeters in diameter.

FIGURE 13.

REGNAULT’S APPARATUS FOR DETERMINING THE SPECIFIC HEAT OF SOILS.
]

This part of the apparatus is supported by triangular pieces of cork. A delicate thermometer is fastened to the top of the box of the calorimeter and the value of the degrees is so arranged that about twelve of them correspond to about one degree C. The scale of the instrument can be arbitrarily fixed and the temperature of any part of it determined by comparison with a delicately graduated thermometer.

Near the thermometer in the calorimeter is a stirrer made of a very thin copper disk with a bent rim. This stirrer is operated by means of a silk cord moved by appropriate machinery.

The reading of the thermometer is made through a glass plate and this should be protected from the heat of the body of the observer by a paper screen.

The test tube E is first filled with the substance, whose specific heat is to be determined, and weighed. It is then placed in the water bath until constant weight is reached. After constant weight has been obtained the apparatus is again dried and the exact weight of the moisture lost thus determined. The test tube is then placed in the apparatus A closed with a well-fitted cork, the top covered with cotton and heated in the aqueous vapor for about one hour. The heating apparatus should be far removed from the calorimeter so that the temperature of the latter cannot be influenced thereby. Meanwhile the calorimeter is filled with water which has stood in the room for a long time until it has acquired, as nearly as possible, the room temperature.

The quantity of water is such that the water value of the whole of the calorimeter together with the immersed portions of the thermometer and stirrer shall amount to exactly 100 grams. A few minutes before bringing the substance into the calorimeter, the stirring apparatus is put in motion and the temperature observations are commenced. These should be at intervals of twenty seconds and should be continued until ten observations have been made. Meanwhile the height of the barometer is also read. A few seconds before the tenth interval the apparatus A is brought quickly to the calorimeter and its contents emptied into it at the moment of the tenth interval. The apparatus A should be removed as quickly as possible after its contents are emptied.

After the introduction of the substance and its thorough incorporation with the water of the calorimeter by the stirring apparatus, the thermometer is again read, at intervals of twenty seconds, until its maximum has been reached and as much longer thereafter as may be necessary to show that an appreciable fall of temperature has taken place. The test tube, in which the substance was heated is weighed and the exact quantity of the added substance thus determined.

In order that the sample of soil may be easily removed from the test tube in which it is heated, it is best to have it molded into appropriate forms before being placed in the heating tube. This is easily accomplished by pressing it into molds of convenient shape and of a size so that six or eight pieces (best of cylindrical shape) will be necessary to give the quantity sufficient for the experiment. Since some soils will not retain their shape after molding, the molds may be made of zinc foil whose water values in the calorimeter are previously determined and they can be placed with their contents in the calorimeter thus securing the total immersion of all the particles of soil in the water. With very dusty materials, it is necessary that these little cylinders should be closed with pieces of foil at the ends in order to prevent the particles of dust from escaping and rising to the surface of the water.

Another source of error consists in the solution of the soluble salts which the soil may contain. This is avoided by the use of turpentine instead of water. If the cylinder containing the soil be made water-tight, this danger from the solubility of the salts in water is avoided. Another method of correcting these errors is in making a blank experiment in which a quantity of the earth taken is kept at the temperature of the water in the calorimeter until both are of the same temperature. The earth is then mixed with the water and the change of temperature produced noted. In this way the corrections made necessary by the solution of the salts in water and other causes are determined.

=109. Method of Calculating Results.=—Let t represent the mean temperature of the beginning period of the experiment, and v equal the loss in heat per interval. Let t′ and v′ represent the same values for the end period. Let θ₁, θ₂, θ₃, etc., represent the temperature at the end of the first, second and third intervals of the middle period and θ₀ the temperature at the beginning of the middle period and θₙ the end temperature of of the middle period. Let τ₁, τ₂, τ₃, ... τₙ, represent the mean temperature of the single intervals; then τ₁ = (Θ₀ + Θ₁)/(2); τ₂ = (Θ₁ + Θ₂)/(2), and τₙ = (Θ_{n–1} + Θₙ)/(2). The constant C represents the correction which must be applied in order to determine the true increase of temperature in the calorimetric system. The expression θₙ − θ₀ + C represents the true temperature increase of the calorimetric system which we may represent by Δθ and θₙ + C represents the true maximum, that is, the end temperature, which by exclusion of external influences is reached. The correction C, as already indicated, is to be added to θₙ − θ₀ when it is positive and is to be subtracted therefrom when it is negative. The numerical value of C is usually very small, and, in the experiments indicated, varied between zero and one division of the thermometer employed, that is it seldom exceeded one degree.

=110. Illustration.=—The method of determining value of specific heat is best illustrated by an example:

In one determination the water value of the calorimetric system, including stirrer and thermometer was 2.50 grams, the weight of water added was 97.50 grams and the total water value of the system 100 grams. The substance was dried at 100° and weighed in five envelopes:

Total weight 31.423 grams.
The envelopes alone weighed 10.654 „
Weight of the soil taken 20.769 „

The envelopes holding the soil were made of brass with zinc ends, the specific heat of which is 0.0939 and the water value of the whole of the envelopes was 1.0004 grams. Since, however, the ends were soldered on with zinc the true water value was somewhat smaller being equal to 0.8692 gram. The data of the observations were as follows:

Corrected height of barometer 699.6 millimeters.
Intervals between the observations 20 seconds.

No. of Temperature on the
Observations. arbitrary scale of the
thermometer.
First Period { 0 162°.6
„ {10 162°.9 = θ₀ (Moment of
immersion.)

Second Period {11 185°.0
„ {12 200°.0
„ {13 206°.1
„ {14 209°.5
„ {15 210°.7
„ {16 211°.3
„ {17 211°.5 Differences.
„ {18 211°.5 0
„ {19 211°.5 0
„ {20 211°.5 0
„ {21 211°.5 0
„ {22 211°.4 –0°.1
= θₙ
–0°.1
–0°.1
Third Period {23 211°.3
„ {24 211°.2 –0°.1
„ {25 211°.1 –0°.1
„ {26 211°.0 –0°.1
„ {27 210°.9 –0°.1
„ {28 210°.8 –0°.1
„ {29 210°.6 –0°.2
„ {30 210°.5 –0°.1

From the twenty-second interval, the regular fall of temperature begins and 211°.4 is therefore taken as θₙ. The mean temperature of the beginning period is therefore (162°.6 + 162°.9)/(2) = 162°.75 = t. The value of v is (162°.6 − 162°.9)/(10) = –0°.03. For the end period the value of t′ is (211°.4 + 210°.5)/(2) = 210°.95 and the value of v′ is (211.4 − 210.5)/(8) = + 0.11. Then the sum of the observations from eleven to twenty-one inclusive = Σ′_{n–1}θ = 2280.1

(θ₀ + θₙ)/(2) = 187.15
The sum = 2467.25
nt = 1953.00
Difference 514.25
This difference multiplied by v − v′ = 0.14
gives a product equal to 71.995
This product divided by t′ − t = 48.20
gives a quotient equal to 1.49
nv = –0.36
The sum = 1.13 = C

Then Δθ = θₙ − θ₀ + C = 211°.4 − 162°.9 + 1°.13 = 49°.63. The true end temperature = θₙ + C = 212°.53. The zero point of the thermometer = 24°.70, and the actual rise of temperature = 187°.83. The rise of temperature due to the proximity of the warming apparatus at the beginning was found by experiment to be equal to 0°.1 of the division of the scale. On comparing the thermometer used with a standard centigrade scale it was found that one division of the calorimetric thermometer was equal to 0°.0858. Converting these numbers into expressions of the centigrade scale we have the following summary:

The true rise of temperature, Δθ = 4°.25 The true end temperature, θₙ + C = 16°.10 The temperature of the steam, as determined by the height of the barometer, was equal to 97°.70

From these data the specific heat is calculated according to the following formula:

Σ = 1/20.769 × ((100 × 4.25)/(97.70 − 16.10) − 0.8692) = 0.2089.

From this formula the following rule for calculating specific heat is deduced:

Multiply the water value of the calorimetric system by the true rise in temperature in degrees Celsius and divide the product by the difference between the temperature of boiling water under the conditions of the experiment and the true end temperature. From the quotient subtract the water value of the envelopes holding the soil sample. Divide the remainder by the weight of soil taken.

=111. Variations in Specific Heat.=—Different soils deport themselves very differently in respect of specific heat. In a large number of soils examined by Pfaundler, the specific heats were found to vary from 0.19 to 0.51. The highest specific heat was observed in the case of a peaty soil. Next to peaty soils came those soils which were highest in humus, and in general it was found that the specific heat varied directly with the humus content.

SOIL THERMOMETRY.

=112. General Principles.=—The measurement of the temperature of the soil at stated depths is often of use in analytical processes connected with agricultural chemistry and physics. The general principles on which the process rests, depend on bringing the bulb of the thermometer into as intimate contact as possible with the particles of soil at the depth required, disturbing as little as possible the normal state of the soil particles.

In the thermometer chiefly used for this purpose in this country, the stem is strong and carries the degrees figured on the glass. The whole is inclosed in a wooden case which is cut away to expose the face of the scale. The scale is about eleven inches long. The part which enters the soil is of varying lengths, according to the depth at which the temperature is desired.

=113. Method of Procedure.=—An excellent method of determining soil temperatures and of recording results is well illustrated by Frear.[76]

The thermometers are set in niches cut in a trench, the earth being afterwards carefully tamped about the bulbs to secure a good contact, the trench being filled at the same time. The surface of the soil is freed from vegetation and kept in good tilth.

The depths at which observations are made are at the surface and one, three, six, twelve, and twenty-four inches. The soil tested was moderately dark and loamy to a depth of seven inches and below that a stiff clay. Solid rock existed at from five to seven feet below the surface. Readings were made three times a day.

=114. Method of Stating Results.=—The individual readings of the thermometers should be entered at the time they are made. At the end of each month the mean of the readings should be determined, together with the maxima and minima, and a comparison made between the mean readings of the temperature of the air and maxima and minima. As a sample of the method of stating these mean results the data are given for the month of May, 1891, for the atmosphere, surface, and for the depths mentioned above:

MAY.
T° Fahrenheit.
ATMOSPHERE.
Monthly mean 57.1
Maximum 85.0
Minimum 31.0
Mean daily range 22.5
Greatest daily range 32.0
Least daily range 8.0
SURFACE.
Monthly mean 56.7
_Extremes._
Maximum (10th of month) 77.0
Minimum (5th) 36.0
Mean maximum 65.2
Mean minimum 49.9
_Range._
Monthly 41.0
Mean daily 14.9
Greatest daily (19th) 25.0
Least daily (21st) 4.0
ONE INCH.
Monthly mean 56.8
_Extremes._
Maximum (10th) 74.5
Minimum (5th) 36.5
Mean maximum 62.9
Mean minimum 49.5
_Range._
Monthly 38.0
Mean daily 11.9
Greatest daily (10 and 19) 20.0
Least daily (23rd) 1.0
THREE INCHES.
Monthly mean 56.7
_Extremes._
Maximum (31st) 71.0
Minimum (6th) 40.0
Mean maximum 60.9
Mean minimum 49.7
_Range._
Monthly 31.0
Mean daily 9.3
Greatest daily (19th) 15.5
Least daily (23rd) 1.5
SIX INCHES.
Monthly mean 56.3
_Extremes._
Maximum (31st) 66.0
Minimum (6th and 7th) 43.0
Mean maximum 56.7
Mean minimum 53.2
_Range._
Monthly 23.0
Mean daily 4.65
Greatest daily (8 and 19) 8.5
Least Daily (5th) 1.0
TWELVE INCHES.
Monthly mean 55.6
_Extremes._
Maximum (31st) 64.0
Minimum (6th and 7th) 46.0
Mean maximum 56.6
Mean minimum 54.4
_Range._
Monthly 18.0
Mean daily 2.18
Greatest daily (8th) 4.5
Least daily (3rd and 20th) 0.0
TWENTY-FOUR INCHES.
Monthly mean 53.1
_Extremes._
Maximum (31st) 58.0
Minimum (6th and 8th) 48.0
Mean maximum 53.4
Mean minimum 52.8
_Range._
Monthly 10.0
Mean daily 0.48
Greatest daily (23rd) 2.0
Least daily (on 12 days) 0.0

FIG. 14. SOIL THERMOMETER—Whitney and Marvin.
]

=115. Method of Whitney and Marvin.=[77]—The thermometer devised by Whitney and Marvin is shown in Fig. 14. The principle on which this modification depends is as follows:

A mercurial thermometer of the ordinary construction is liable to give wrong indications of the temperature because it is difficult to determine the temperature of the column of mercury from the bulb to the surface of the ground. To avoid this source of error the thermometer figured was constructed.

The bulb of the thermometer is made quite small and a slender portion of the stem extends into its spherical portion. The top portion of the thermometer stem does not differ in any essential respect from the stem of an ordinary thermometer.

The bulb is almost wholly filled with alcohol, which acts as the principal thermometric fluid and has the advantages of a high coefficient of expansion. The thermometer bulb and the stem of the thermometer up to a point convenient for graduation, are filled with mercury. In the drawing the mercury is represented by the heavy black marking in and just above the small bulb. The peculiar construction at this point is for the purpose of retaining the mercury about the point of the slender capillary stem inside the bulb and preventing the entrance of alcohol into the stem when the thermometer is horizontal.

In order to register the maximum and minimum temperatures a short column of alcohol is placed in the upper portion of the stem, above the mercury, and within this are arranged two small steel indexes, so constructed that they will not slide in the tube of their own weight, but are easily pushed upward by the mercury column or pulled downward by the top meniscus of the alcohol column. The indexes are set by means of a small magnet, the one being drawn down upon the top of the mercurial column and the other raised up against the meniscus of the alcohol column.

The rise of the mercury carries its index upward, leaving it to register the highest point reached, while the alcohol meniscus withdraws the other index and leaves it at a point representing the minimum temperature. It remains only to mention that the graduations are fixed in the usual way, having reference only to the positions of the mercurial column. Beyond the highest point supposed to be reached by the mercury, say about 120°, the graduations are extended in an arbitrary manner. The scale numbers represent temperatures by the mercurial column and are continued in regular sequence beyond the 120°. On this plan the readings for minimum temperatures are on a purely arbitrary scale and are converted into true degrees of temperature by use of a table prepared for each thermometer, which table embodies as well all the corrections for instrumental error.

The arrangement of the alcohol columns above the mercurial column and the indexes are shown enlarged at one side of the illustration. The readings of the maximum temperature are made from the bottom end of the index next to the mercurial column. The minimum temperature is the reading of the top of the uppermost index. Thus in the figure the maximum temperature indicated is 76.5°, and the minimum 125.7°, which, by reference to the table of correction for this thermometer, No. 10, is found to be 53.3°.

The use of mercury in the stem of the thermometer not only admits of the use of the index for registering the maximum temperature, but possesses the additional advantage of reducing the error due to uncertain temperature of the stem to about one-sixth what it would be if alcohol were used. Moreover, if necessary, as in the case with thermometers for greater depths than that figured, the ungraduated portion of the stem can be made of very much finer bore than the graduated portion, the effect of which is to diminish the objectionable error to a comparatively unimportant quantity.

The chief objection to thermometers of this construction is the liability of alcohol getting from the bulb into the stem during the processes of construction, graduation and subsequent handling, and the difficulty of safely shipping them.

When once set up, however, there seems to be little or no possibility of derangement and the error common to mercurial thermometers due to rise of the freezing point with age does not apply owing to the high coefficient of expansion of the alcohol used in the bulb.

APPLICATIONS OF SOIL THERMOMETRY.

=116. Estimation of the Absorption of Heat by Soils.=—A cubical zinc box, six centimeters square, is filled with the sifted air dried soil. The box, one side of which is left open, is encased snugly in a wooden cover, exposing only the open end, and placed for a few hours in the direct rays of the sun. The temperature is then taken at a given depth. The box may be provided with thermometers at different depths, the bulbs thereof extending to the center. In this case the box should be covered with thick felt instead of wood. The temperature of the layers of soils of different depths can thus be read off directly. The air temperature directly above the box should be accurately noted while the experiment continues.

Any other kind of box well protected against all heat save the direct sunlight on the open surface of the soil will answer as well as the one described.

To determine the action of moist earth in similar conditions the soil may be previously moistened; the per cent of moisture being determined in a separate portion of the soil or the amount of water added to the air-dried soil being noted.

=117. Estimation of the Conductivity of Soils for Heat.=—The bulb of a thermometer is placed in the middle of a mass of fine earth which is then exposed, best in a metallic box painted with lamp black, in a warm place. The time required for the thermometer to reach a certain degree is noted. By reversing the experiment and placing the mass of earth heated to a given degree in a cool place the conductivity can be determined by the time required for the mercury in the thermometer to fall to any given point.

The experiment may also be made by packing the soil by gently jolting it in a glass tube six to eight centimeters in diameter. One end of the tube is closed with a piece of metal or fine wire gauze painted with lamp black and is exposed to the source of heat. The bulb of a thermometer is placed at a given distance from the end of the tube and the time for the mercury to be affected observed.

COHESION AND ADHESION OF SOILS.

=118. Behavior of Soil After Wetting.=—The deportment of a soil when thoroughly wet in respect of its physical state on drying out is a matter of great practical concern to the agronomist. Some soils on becoming dry fall into a pulverulent state and are easily brought into proper tilth; others become hard and tenacious, breaking into clods and resisting ordinary methods of pulverization. The physical laws which determine these conditions depend largely on the principles of flocculation soon to be described. The present task is to describe briefly some of the methods of estimating the force of cohesion and adhesion.

=119. General Method.=—The fine earth, air-dried, is mixed with enough water to make a paste and molded into forms suitable for trial in a machine for testing strength of cement, etc. The forms most used are cakes three to five centimeters in length and one to two centimeters thick. These are used for determining crushing power. For longitudinal adhesion the paste may be molded in prismatic or cylindrical shape.[78] The prisms should show one to two centimeters in cross section or the cylinder be one to two centimeters in diameter. Before use they are to be exposed for several days until thoroughly air-dried. The force required to separate or crush these prepared pieces will measure the adhesive or cohesive property of the sample. A great number of trials should be made and the mean taken.

=120. Method of Heinrich.=[79]—This process consists in mixing the air-dried earth with water until its aqueous content is fifty per cent of the highest water capacity determined by experiment. The sample is next placed between two pieces of sheet iron of ten centimeters square, each of which in its middle point is provided with a hook. The thickness of the layer between the two pieces of iron should be about five to ten centimeters. The exuding particles of soil are cut off with a knife. The upper piece of sheet iron is next suspended by a cord in such a way that the iron piece occupies a horizontal position. A small basket is attached to the lower surface and sand added thereto, little by little, until the column of earth is separated. The sand basket and iron plate are weighed, and the total weight gives the power necessary to separate a column of soil ten centimeters square in cross section. The iron plates may be roughened so that the adhesion thereto of the soil is greater than its cohesive force.

=121. Adhesion of Soil to Wood, Iron, Etc.=—The adhesive power of moist soil for wood, iron, etc., is measured by Heinrich[80] in the following way: The soil is mixed with water, as above, until it contains just fifty per cent of its total water-holding content. It is then placed in a large vessel and the upper surface made as smooth as possible. A plate of wood, iron, etc., of ten centimeters square is then pressed on the surface until a complete contact is secured. This plate, by means of a hook and cord passing over a pulley, is then subjected to stress by weighting the cord which carries a basket for that purpose. The basket should be of the same weight as the plate in contact with the soil. The weight added to the basket necessary to separate the plate from the soil is taken to represent the cohesive force. The author of the method appears to take no account of the pressure of the air on the plate caused by the exclusion of the air from its under surface.

THE ABSORPTIVE POWER OF SOILS FOR SALTS IN SOLUTION.

=122. General Principles.=[81]—It is a fact of every-day observation that soils have a particular property of absorbing certain materials with which they come in contact. If it were not for this property all our wells would soon become unwholesome from the reception of decayed animal and vegetable matter carried to them in the drainage water from the surface. It is also a well-known fact that burying dead bodies prevents the gaseous products of decomposition from reaching and vitiating the atmosphere.

Besides this well-known power of soils to absorb the decomposition products of animal and vegetable matter, they also possess a property which is of far greater importance in plant economy; that is, the power of withdrawing and retaining certain mineral constituents from their solutions.

As far back as the sixteenth century mention is made by Lord Bacon of a process for obtaining pure water on the seashore by simply digging a hole in the sand and allowing it to fill with filtered sea water, which by this means is deprived of its salt. Although certain facts were observed by some of the earlier writers in regard to soil absorption, no systematic researches were conducted with a view of demonstrating the extent and cause of this power until within a comparatively few years.

In 1850 Prof. Way published in the _Journal of the Royal Agricultural Society of England_, the results of a thorough and most excellent investigation of the subject. Since then many distinguished chemists, such as Henneberg, Stohmann, Peters, Heiden, Knop, Ullik, Pillitz, Biedermann, Tuxen, and others have given their attention to this matter.

=123. Summary of Data.=—If a solution of a soluble sulfate, chloride or nitrate of an alkali or an alkaline-earth metal be placed in contact with a soil, the result is that the soil takes up a part of the base but none of the acid. This absorption of base is attended with the liberation of some other base from the soil which combines with the acid of the solution. Any alkali or alkaline earth base has the power of replacing any other such base. However, if soluble phosphates and silicates of these bases be placed in contact with the soil both the base and the acid are removed from the solution.

Peters[82] has shown that the amount of absorption depends upon the concentration of the solution, the relation between the quantity of solution and the soil and the kind of salt used. He treated 100 grams of earth with 250 cubic centimeters of solutions of different potash salts with the following results:

Strength of solution. ⅒ Normal. ¹⁄₂₀ Normal.
Grams Grams
Salt Used K₂O absorbed. K₂O absorbed.
KCl 0.3124 0.1990
K₂SO₄ 0.3362 0.2098
K₂CO₃ 0.5747 0.3154

Biedermann[83] proves that, for phosphoric acid at least, the absorption increases with the temperature.

It has also been found that the amount of absorption depends upon the time of contact between the soil and solution. Way found that the absorption of ammonia was complete in half an hour, while Henneberg and Stohmann[84] noticed that the phosphoric acid continued to be fixed after the expiration of twenty-four hours.

It is a very important fact that the absorption of a base is never complete; no matter how dilute the solution it will still carry a small portion of the base with it. Peters states that it requires about 28,000 parts of water to remove one part of absorbed potash and Stohmann found that it required about 10,000 parts of water to remove one part of absorbed ammonia. With phosphoric acid, the resulting compound seems to be much more insoluble.

According to Tuxen[85] the presence of salts of soda and potash in solution decreases the power of a soil to absorb ammonia compounds and the presence of sodium salts decreases the power of a soil to absorb potash. On the other hand the presence of potassium compounds considerably increases the absorption of phosphoric acid. He further affirms that the compounds of potash, phosphoric acid, etc., formed in the soil, are decidedly more soluble in sodium salts than in pure water.

=124. Cause of Absorption.=—The withdrawing and fixing of phosphoric acid from solutions by the soil is not very difficult to understand as this acid forms insoluble compounds of iron, lime, and magnesium, some or all of which are present in all soils. As to the absorption of the alkalies, the explanation is far more difficult as nearly all of their ordinary compounds are readily soluble in water.

As lime is usually found combined with the acid part of an alkali salt, from which the base has been absorbed by the soil, it might naturally be supposed that the absorptive power of the soil would depend upon the amount of lime present. Way found, however, that the addition of chalk in no way influenced the absorption of ammonia by a soil which contained but a small amount of lime. This fact was also confirmed by Knop[86] who found that chalk exerted no influence on the absorption of ammonia salts. These facts would seem to point to the conclusion that lime was present in sufficient quantity in these experiments, or that it is not essential to the phenomena of absorption. However, as any alkali or alkaline-earth base can replace any other such base, the presence of lime in the filtrate is probably more of an accidental occurrence, owing to the comparatively large amount of that substance in most soils, than a necessary condition, as any other base would doubtless answer in the absence of lime.

=125. Warington=[87] has shown that hydrated oxides of iron and aluminum, and especially the former, are capable of absorbing potash and ammonia, and as more or less of these hydrates exist in nearly all soils, a part, at least, of absorptive phenomena is to be ascribed to them.

=126. Way= tried to determine which of the constituents of a soil exercised chiefly the absorptive power. He passed a solution of ammonia through tubes containing pure sand and found that it came through apparently unaltered from the first, while a soil treated in the same way removed the ammonia for a considerable time. He concluded from this that the absorptive power does not exist in the sand. He next oxidized the organic matter in a soil with nitric acid and then treated it with ammonia in the same way. The first portions of the filtrate showed no ammonia in any form, hence he concluded that organic matter is not essential to the act of absorption. He further showed that clay alone is capable of causing absorption phenomena, by treating powdered clay tobacco pipes with ammonia.

Having shown that clay was the main constituent in a soil which caused the absorption of alkalies, he tried next to trace out the particular compound which caused the absorption. Having tried various natural silicates he at last succeeded in producing a hydrated silicate of aluminum and soda which exhibited displacement and absorptive properties very similar to those shown by the soil.

As Way had succeeded in producing an artificial hydrated silicate possessing absorptive properties, Eichorn[88] thought of trying natural hydrated silicates or zeolites and found that they exhibited the same power as Way’s artificial preparation. It has also been shown by Biedermann,[89] Rautlenberg,[90] and Heiden[91] that the absorptive power bears a close relation to the amount of soluble silicates present.

In view of these facts it is now generally accepted that the absorption of salts of the alkalies, accompanied by the change of base, is due chiefly to the presence of decomposed zeolite minerals in the soil.

Besides the purely chemical absorption of salts by the soil, we have a physical absorption of various substances similar to the action of charcoal when used as a filter.

=127. Conclusions of Armsby.=—The data connected with the absorption of bases by a soil have also been reviewed by Armsby.[92] He shows that the absorption is accompanied by a chemical reaction between the salt whose base is absorbed and some constituent of the soil, and this change seems to be due particularly to certain zeolitic silicates, although Liebig and others were disposed to credit this absorption largely to physical causes.

Knop advances the idea that the soil has the power of disintegrating salts in the presence of some substances like calcium carbonate which can unite with the acid. In experiments made with hydrous silicates it was shown that the absorption resembled in all cases like phenomena in the soil; hence the supposition already advanced in regard to the influence of such silicates is doubtless true.

In respect of absorption in general, the following conclusions were reached:

1. The absorption of combined bases by the soil consists in an exchange of bases between the salt and the hydrous silicates of the soil.

2. This exchange, which is primarily chemical, is only partial, its extent varying

(a) with the concentration of the solution, and

(b) with the ratio between the volume of the solution and the quality of soil used.

3. The cause of these variations is probably the action of mass or the tendency of resulting compounds to re-form the original bodies, the absorption actually found in any case marking the point where the two forces are in equilibrium.

=128. Selective Absorption of Potash.=—As a rule more potash is absorbed from the sulfate than from the chlorid. This fact would seem to point to the advisability of using sulfate as a fertilizer in preference to chlorid. However, as with the exception of nitrates, the absorptive power of a soil, for the salts used as fertilizers, is many times greater than it is ever called upon to exert in fixing applied fertilizers, we need not trouble ourselves in regard to the absorption of phosphoric acid, potash or ammonia, in so far as the practical side of the matter is concerned. For example, an acre of soil to the depth of nine inches weighs about 900 tons. Now it has been found by Huston,[93] that 100 parts of a soil experimented upon absorbed over 0.25 part of P₂O₅, hence 900 parts would absorb over 2.25 parts of P₂O₅; or an acre of this soil to the depth of nine inches would absorb over two and one-fourth tons of phosphoric acid. 500 pounds per acre is a large dressing of a phosphatic fertilizer for field crops and 500 pounds of a high grade fertilizer would contain about 100 pounds of P₂O₅; hence the power of such a soil to absorb phosphoric acid is more than forty-five times as great as it is ever likely to be called upon to exert in fixing the phosphoric acid added to it as a fertilizer.

Huston has further shown that an acre of soil nine inches deep will absorb more than 2.7 tons of potash (K₂O) from potassium chlorid from which salt less potash is absorbed than from the sulfate. Now one-tenth ton of potassium chlorid per acre would be a large dressing of potash, hence this soil possesses the power of absorbing more than twenty-seven times as much potash as is ever likely to be applied as a fertilizer.

In like manner it may be shown that the power of an acre of soil nine inches deep to absorb ammonia from ammonium sulfate is more than thirty-two times as great as it would be called upon to exert in fixing the ammonia from a dressing of one-quarter ton of ammonium sulfate per acre.

With sodium nitrate, however, there is no absorption; hence great care is necessary in the application of nitrogen as a nitrate, for, if it be put on in large quantities, at a season when the plant is not prepared to assimilate it, or during a period of heavy rains, there must unavoidably result loss from drainage. The best time to apply a nitrate is evidently during the active growing season.

=129. Whitney=[94] places great emphasis on the surface area of soil particles in respect to their power to absorb solutions of salts. The approximate surface area of a cubic foot of each of the different typical soils of Maryland is as follows:

Pine barrens 23,940 square feet.
Truck lands 74,130 „ „
Tobacco lands 84,850 „ „
Wheat lands 94,540 „ „
River terrace 106,260 „ „
Limestone subsoil 202,600 „ „

It will be seen that there are about 24,000 square feet of surface area in a cubic foot of the subsoil of the pine barrens, no less then 100,000 square feet or two and three-tenths acres of surface area in a cubic foot of the subsoil of the river terrace, and 200,000 square feet of surface area in a cubic foot of the limestone subsoil.

These figures seem vast, but they are probably below rather than above the true values, on account of the wide range of the diameters of the clay group. This great extent of surface and of surface attraction, which has been described as potential, gives the soil great power to absorb moisture from the air, and to absorb and hold back mineral matters from solution. A smooth surface of glass will attract and hold, by this surface attraction, an appreciable amount of moisture from the surrounding air. A cubic foot of soil, having 100,000 square feet of surface, should be able to attract and hold a considerably larger amount of moisture.

It might have been added that if the potential of the surface, separating the solution from the soil, be greater than the potential in the interior of the liquid mass, there will be a tendency to concentrate the liquid on this surface of separation. It has been shown that certain fluids have greater density on a surface separating the fluid from a solid. On the other hand, if the potential were low there might be no tendency for this concentration, and even the reverse conditions would prevail and the soluble substance could be readily washed out of the soil.

=130. Removal of Organic Matters.=—It is probably largely due to this straining power that organic matters are removed from solutions in percolating through the soil. Whitney[95] has observed that the organic matter may be coagulated and precipitated from solution by the soil constituents, and held in the soil in loose flocculent masses, while the liquid passes through nearly free of organic matter.

=131. Importance of Soil Absorption.=—The importance of the absorptive power of the soil can hardly be overestimated. By means of this power those mineral ingredients of plant food, of which most soils contain but little, are held too closely to allow of rapid loss by drainage, and still sufficiently available to answer the needs of vegetation, provided the store is large enough. The only important plant food liable to be deficient in the soil which does not come under the influence of absorption is nitrogen in the form of salts of nitric acid, and nature has made a wide provision for this element by binding it in the form of organic bodies which nitrify but slowly, and by supplying each year a small quantity from the atmosphere.

By means of the absorptive power of soils the farmer, if he puts on an excess of potash or phosphoric acid as a fertilizer, does not lose it but is able to reap some benefits from it in the next and even in succeeding crops. If it were not for this power the best method for applying fertilizers would be a much more complicated problem than it is at present; and it would be necessary to apply them at just the proper season and in nicely regulated amounts to insure against loss.

=132. Method of Determining Absorption of Chemical Salts.=—The soil which is to be used for this experiment should be treated as has been indicated and passed through a sieve the meshes of which do not exceed half a millimeter in size. From twenty-five to fifty grams of the fine earth may be used for each experiment.