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
[Illustration:
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.