V = content of the absorption tube obtained by filling with mercury and
weighing.
P′ = weight of the empty tube filled with air at 100°.
pl = weight of the air in the tube (pl = V × specific gravity of the air
at 100°).
pt′ = weight of the tube (pt′ = P′ − pl).
P² (second weighing) = weight of the tube filled with the substance with
the included air at 100°.
v^s = volume of the substance calculated according to the formula
v^s = (P² − P′)/(s^s − specific gravity of air).
s^s = specific gravity of the substance.
vl = volume of the air in the flask filled with the substance (vl = V −
v^s).
pl′ (weight of this included air) = vl × specific gravity.
p^s = weight of the substance (pl = p² − pt′ − pl)
P³ = weight of the apparatus at the end of the experiment.
sg = specific gravity of the gas employed for saturation.
pg (weight of the gas remaining over the substance) = vl × sg.
pa (weight of the absorbed gas) = P³ − pt′ − p^s − pg.
p^s gram of substance absorbs pa gram of the gas and 100 grams of
substance would absorb (100 × pa)/(p^s) grams.
The specific gravities of the gases employed are calculated from the
tables given by Landolt and Börnstein in “Physical and Chemical Tables,”
page 5.
The specific gravity of the quartz sand employed was 2.639; of the
quartz powder, 2.622; of the kaolin, 2.503; of the humus, 1.462; of the
iron hydroxid, 3.728; and of the calcium carbonate, 2.678.
One liter of ammonia, at a pressure of 760 millimeters of mercury and a
temperature of 0°, weighs 0.7616 gram; one liter of carbon dioxid,
1.9781 grams; one liter of aqueous vapor, 0.8064 gram; and one liter of
dried air, 1.2931 grams.
At a pressure of 720 millimeters, and at 20° temperature, a liter of air
saturated with aqueous vapor at 0° weighs 1.1383 grams; saturated at
8.6°, 1.1362 grams; saturated at 10°, 1.1358 grams; saturated at 14°,
1.1340 grams; saturated at 18.2°, 1.1330 grams; saturated at 20°, 1.1321
grams; saturated at 30°, 1.1313 grams.
The general results of the experiments are as follows:
ABSORPTION AT 0°.
Aqueous vapor Ammonia. Carbon dioxid.
from saturated
air.
Grams. Cubic Grams. Cubic Grams. Cubic
cm.[H] cm.[H] cm.[H]
100 grams quartz 0.159 197 0.107 145 0.023 12
„ „ kaolin 2.558 3,172 0.721 947 0.329 166
„ „ humus 15.904 19,722 18.452 24,228 2.501 1,263
„ „ Fe₂(OH)₆ 15.512 19,236 4.004 5,275 6.975 3,526
„ „ CaCO₃ 0.224 278 0.256 320 0.028 14
Footnote H:
Reduced to 0° and 760 millimeters pressure mercury.
The foregoing methods will suffice to show the procedures to be followed
in estimating the maximum amount of any common gas or vapor a given
quantity of soil may be made to absorb. We pass next to consider the
quantities of gases or vapor soils _in situ_ may hold.
=284. Method of Boussingault and Lewey.=[184]—This method is the oldest
and most simple procedure for estimating the nature of the gases held in
a soil _in situ_.
For the purpose of collecting the sample of gas from the soil a hole,
thirty to forty centimeters in depth, is dug, and a tube placed in it in
a vertical position, having on its lower extremity a bulb perforated
with fine holes. The hole is filled and the earth closely packed around
the tube which is left for twenty-four hours. At the end of that time
the tube is slowly aspirated until a volume of gas approaching from five
to ten liters is obtained.
_Estimation of Carbon Dioxid._—The carbon dioxid in the sample of gas is
estimated by allowing it to bubble through a solution of barium
hydroxid.
_Estimation of the Oxygen._—The oxygen is estimated in a separate sample
of the gas by means of potassium pyrogallate.
The chief objection to this simple process is the uncertainty of being
able to obtain an average sample of the occluded gas. In digging the
hole and refilling, there must evidently be a considerable disturbance
of the original distribution of the gas or vapor.
The methods of Pettenkofer[185] and Aubry[186] are essentially like that
just described. Pettenkofer found the largest quantities of carbon
dioxid in the earth gases in July, August, and September, and the
smallest quantities in the winter months.
No greater detail concerning these methods of the direct aspiration of
the air is considered necessary inasmuch as the methods about to be
described, while more elaborate, are superior in accuracy to the older
methods mentioned. In general, in these experiments, it is deemed
sufficient to determine the carbon dioxid only.
[Illustration:
FIGURE 62.
SCHLOESING’S SOIL-TUBE FOR COLLECTING GASES.
]
=285. Method of Schloesing.=—The apparatus used by Schloesing[187] in
the collection of the soil gases consists of a steel tube (Fig. 62) a
little over one meter in length, ten millimeters in external diameter,
and one and one-half to two millimeters in internal diameter. The end
which penetrates the soil is made slightly conical for a distance of
twenty-five to thirty centimeters. By reason of the shape of the tube,
when it is driven into the soil all connection between the orifice in
the point of the tube and the external air is prevented. The obstruction
of the internal canal of the tube is prevented by introducing a thread
of steel which penetrates the whole length of the tube. This thread,
represented by A, B, C, D, is flush with the interior extremity of the
tube at D. It extends for about three centimeters above the upper end of
the tube in order to be easily handled when it is to be removed.
For the purpose of driving the tube into the soil its upper part is
covered with a cylindrical piece of steel, EF, in the interior of which
are freely engaged H and A. This head piece rests upon a ring of steel,
K. This ring is fastened solidly into the tube. On striking the piece EF
the tube and the steel wire in the center are driven together into the
soil. The tube is flattened at L and L′ in order to be embraced by the
key MM, the employment of which is necessary in order to revolve the
tube around its axis when it is being driven into the soil. When the
tube has been driven to the depth desired, the steel wire is withdrawn
and it is immediately connected at H with the rubber tube N (Fig. 63)
belonging to the system PQT, and furnished with a pinch-cock X. The
system PQT comprises the following elements: PQT made of a capillary
glass tube in the form of a T. The lower end of the tube P is closed by
the larger glass tube O, sealing the end of P with a little mercury. O
is held to P by the cork S, which is attached firmly enough to prevent O
from dropping off, but is furnished with a canal in order to allow the
air to flow in or out freely. This system is connected with the system
UV by the rubber connection T. U is a glass vessel having the
constrictions as indicated in its stem above and below the bulb. V is a
glass vessel of convenient size connected with U by the rubber tubing as
indicated. The capacity of the cylindrical portion of U should be from
fifteen to eighteen cubic centimeters.
[Illustration:
FIGURE 63.
SCHLOESING’S APPARATUS FOR COLLECTING GASES FROM SOIL.
]
To take a sample of soil gas, V is lifted above U. The air is driven
from U and escapes through O, which acts as a true valve. When the
mercury has completely filled U the pinch-cock X is opened and V
depressed gradually. The gas coming from the soil is thus collected in
U. A few cubic centimeters of the soil gas are collected in this way,
the pinch-cock X is again closed and V is raised in order to drive the
whole of the contents of U again through O. In this way the whole of the
air which the capillary vessel originally contained is removed and all
parts of it remain filled with soil gas. Two or three operations, using
from five to ten centimeters of soil gas in all, will be sufficient to
completely free the apparatus from its original content of air. U is
then entirely filled by depressing V, and it is then hermetically sealed
at the two constricted points by means of an alcohol lamp. The sealed
tube can then be transported to the laboratory and its contents
subjected to eudiometric analysis.
Without displacing the tube from the soil, several samples of gas can be
taken from the same spot. A sufficient number of the bulbs V should be
at hand to hold the required number of samples. Instead of submitting
the sample to eudiometric analysis it is usually sufficient to determine
the quantity of carbon dioxid which it contains, inasmuch as numerous
experiments have shown that in 100 parts of soil gas the oxygen and
carbon dioxid together constitute twenty-one parts. No appreciable trace
of marsh gas, or other combustible gas, has yet been detected in
ordinary arable soils. These gases have only been found in special soils
from marshes, in the neighborhood of gas wells, etc., and not in arable
soils.
[Illustration:
FIGURE 64.
SCHLOESING’S APPARATUS FOR DETERMINATION OF CARBON DIOXID.
]
=286. Apparatus for Estimating the Carbon Dioxid.=—The apparatus used
for determining the carbon dioxid in Schloesing’s work consists of the
apparatus shown in Fig. 64. A represents a glass vessel surrounded by a
jacket of glass, full of water, and sealed on its lower part to the tube
BC of about six millimeters internal diameter. On its upper part it is
sealed to the capillary tube D. The tube BC is graduated from C in
hundredths of the volume of DAC, which volume is about twelve cubic
centimeters. On its lower part it is connected by a rubber tube with a
reservoir F which is capable of being raised or lowered. GHK are
capillary tubes connected together by the rubber tubes L and M, which
are furnished with pinch-cocks. The tube G is connected to a vacuum by
the rubber tube N. The rubber tube should be of very small internal
diameter and from forty to fifty centimeters in length. To the tube H
are sealed, at right angles, the branch D and another branch O. This
last dips into a little mercury which the tube P contains. It serves as
a valve, permitting the exit of the gases but not their entrance. The
tube K carries some lines engraved on its inferior part and is sealed to
the system of the two bulbs Q and R. The bulb Q contains a concentrated
solution of potash. It carries a number of pieces of glass tubing for
the purpose of increasing the surface of the potash solution.
All the parts of the apparatus are fixed upon a rectangular board,
nineteen centimeters broad by twenty centimeters long. This forms one of
the faces of a wooden box to which it is hinged and which serves for the
transportation of the apparatus in a vertical position. The graduation
of the tube BC is recorded behind this tube upon a card fixed upon the
board. By means of these two graduations, the height of the mercury in
the tube BC is most easily read, even when the tube is not perfectly
vertical. Each one of the pinch-cocks L and M, on its upper part is
fixed in a sort of guard which prevents it from being displaced
laterally during the processes of the manipulation, thus avoiding all
danger of breakage.
After the operation is finished a little air is sent into Q in such a
manner as to sensibly lower the level of the solution of potash, and the
upper extremity of R is closed with a rubber stopper. Afterward, the
apparatus can be transported without any danger of the potash becoming
engaged in the tube K and reaching the measuring tank A.
To proceed to the analysis, a stake is driven into the soil to which all
of the apparatus can be fixed. At the side of the stake the apparatus
for taking the sample, already described, is driven into the soil and
this apparatus is connected by the tube N with the apparatus for
determining the carbon dioxid. The pinch-cocks L and M being closed, F
is lifted until the mercury which runs from it fills A and approaches D.
During this time the air which the apparatus contains has been driven
out through O. The tube NGD is freed from air by opening the pinch-cock
L, lowering F and drawing into A the gas coming from the soil; afterward
closing L and driving out the gas through O. After two or three rinsings
of this kind, which employ altogether only ten to twelve cubic
centimeters, the gas which is to be analyzed is sucked into A. For this
purpose F is lowered until the mercury in the tube BC is very near C.
The pinch-cock L is closed and M opened. The reservoir F is displaced
little by little by pressing lightly against the rectangular board in
order to give it greater firmness in such a way as to fix the level of
the mercury exactly at C, and the line is noticed where the solution of
potash in K stands. The gas contained in the apparatus is under a
pressure, the difference of which from the external pressure is
represented by the column of the potash solution between the mark just
noticed and the level of the same solution in the bulb R. In order to
absorb the carbon dioxid, F is lifted until the mercury stands between D
and E. The gas thus passes from A into Q. It gives up immediately its
carbon dioxid to the potash solution. It is then made to come again into
A, and afterward a second time into Q in order to free it from the last
trace of dioxid. Finally it is made to return to A and F is kept at such
a height that the potash solution maintains in the tube K the same level
as at the commencement of the operation. The gas is then at the same
pressure to which it was subjected before absorption. The level of the
mercury is then read on BC. At the time the apparatus is used, the
measuring tube A should be slightly moist. If it is not so, a small
quantity of water should be introduced which is afterward rejected, but
which leaves a sufficient quantity of moisture upon the internal walls
of A. In this way the gas will always, before or after absorption of
carbon dioxid, be saturated with vapor of water, and the figure read in
the last place upon the tube BC represents the percentage of carbon
dioxid in 100 parts of the gas extracted from the soil supposed to be
saturated with vapor at the temperature of the experiment.
During the course of the analysis, the temperature of the measuring
flask, which is almost entirely surrounded with water, does not vary
sensibly, but in a series of experiments which are executed at different
times, the temperature of the measuring apparatus, which is that of the
ambient air, may change much. It may oscillate between 10° to 25°, and
exceptionally between 0° and 30°, whence there are notable variations in
the tension of the vapor of the gas measured. If it should be desired to
calculate to 100 parts of dry gas the observations made at 30° upon 100
parts of saturated gas, it would be necessary to increase the percentage
of carbon dioxid by about ¹⁄₂₅ of its value. It is noticed that with the
apparatus described above, the gas upon which the estimation is really
conducted comprises not only that which the measuring apparatus contains
from E to C before the absorption of the carbon dioxid, but also the
small quantity which remains in the capillary tube KME at the moment
when closing the pinch-cock M, after the second rinsing, the gas from
the soil is aspired into EAC. On the other hand, there is left in the
same tube KME, when the final reading is made, some gas which belongs to
that which has been measured at the end. These two small gaseous
portions which we consider in the tube KME to be sensibly equal, do not
contain any carbon dioxid and may be left out of consideration. That is
why the volume of the measuring apparatus is limited to E and the
graduation of the tube BC is in hundredths of the volume comprised from
E to C. In reality the two portions are not absolutely equal because the
two successive levels of the potash solution, which limit them in the
tube K, are not absolutely identical. These two levels can differ in
such a manner as to correspond to a volume of about ¹⁄₁₀₀₀ of the
measuring apparatus. Thus the estimation is really made upon a volume of
gas which may be greater or less by ¹⁄₁₀₀₀ than the volume of EAC;
whence there might result an error of ¹⁄₁₀₀₀ in the estimation of the
carbon dioxid, an error which is wholly negligible.
As a result of numerous analyses it is concluded, first, that the oxygen
exists normally in the atmosphere of soils in large proportion; second,
very probably the gaseous atmosphere of arable soils, to a depth of
sixty centimeters, contains scarcely one per cent of carbon dioxid and
about twenty per cent of oxygen; third, the highest percentages of
carbon dioxid correspond to epochs of highest temperature and periods of
greatest calm; fourth, the proportion of carbon dioxid increases
ordinarily with the depth at which the samples are taken. This
disposition of the carbon dioxid would appear almost necessary, since
near the surface the internal atmosphere is almost constantly diluted by
external air by virtue of diffusion. Fifth, from one epoch to another
the composition of the atmosphere of the soil can undergo considerable
variation.
=287. Determination of Diffusion of Carbon Dioxid in Soil.=—The method
proposed by Hannén[188] is a convenient one to use in studying the rate
of diffusion of carbon dioxid in soils. A large Woulff’s bottle with
three necks serves for the reception of the gas. The two smaller outer
necks of the bottle carry two glass tubes bent outwards and provided
with stop-cocks. One of these passes to near the bottom of the bottle
and the other just through the stopper. The middle tubule of the bottle
is of a size to give in section an area of about twenty-two square
centimeters. It is made with a heavy rim two centimeters wide and plane
ground. This rim carries a plane-ground glass plate with a circular
perforation in one-half of it, of the size of the opening in the central
tubule of the bottle. A glass cylinder, carrying a fine wire-gauze
diaphragm near the lower end, fits with a ground-glass edge air-tight,
over this aperture, being held in position by a brass clamp. The
ground-glass plate moves air-tight between the cylinder and the bottle,
so that the cylinder can be brought into connection with the bottle or
cut off therefrom without in any way opening the bottle to the air. The
plate and all ground movable surfaces should be well lubricated with
vaseline.
The experiment is carried on as follows: The glass cylinder is filled
with the soil to be tested, closed above with a rubber stopper carrying
a gas tube, and then by moving the perforated-glass plate brought into
connection with the bottle. The side tube, with short arm inside the
bottle, is then closed, and carbon dioxid introduced through the other
lateral tube until the gas passing from the tube at the top of the
cylinder is pure carbon dioxid.
The lateral tube is then closed and the bottle is placed in a water-bath
and kept at a constant temperature of 20°. When the temperature within
and without the apparatus is the same the reading of the barometer is
made, the stopper removed from the top of the cylinder, and the process
of diffusion allowed to begin. After from six to ten hours the glass
plate is moved so as to break the connection between the cylinder and
bottle. The carbon dioxid remaining in the bottle is driven out by a
stream of dry, pure air. The air is allowed to pass through the
apparatus for about ten hours. The carbon dioxid driven out is collected
in an absorption apparatus and weighed. The absorption apparatus should
consist of a series of Geissler potash absorption bulbs and finally a
=ᥩ= form soda-lime tube. In front of the absorption apparatus is placed
a drying bulb containing sulfuric acid. Inasmuch as the temperature and
pressure can be readily determined, the weight of carbon dioxid obtained
is easily calculated to volume.
The weight of 1,000 cubic centimeters of carbon dioxid at 0° and 760
millimeters pressure is 1.96503 grams. Therefore one milligram is
equivalent to 0.5089 cubic centimeter of the gas. The volume of the
bottle should be carefully determined by calibration with water. The
results should be calculated to cubic centimeters per square centimeter
of exposed surface in ten hours. The depth of the soil layer is
conveniently taken at twenty centimeters.
=288. Statement of Results.=—
THE SOIL PACKED LOOSELY IN THE DIFFUSION TUBE.
DIFFUSION TIME, TEN HOURS.
Diameter of Weight of Pure carbon Carbon dioxid Cubic
soil soil taken, dioxid at at end of centimeters
particles, grams. beginning of experiment, of carbon
millimeters. experiment, cubic cm. dioxid
cubic cm. diffused for
each square
cm.
0.01–0.071 520 2549.4 1230.3 59.9
0.071–0.114 550 2545.9 1269.2 58.0
0.114–0.171 590 2556.4 1354.2 54.6
0.171–0.250 620 2538.9 1336.1 54.6
0.250–0.500 660 2532.0 1374.5 52.6
0.500–1.000 680 2528.2 1440.2 49.5
1.000–2.000 690 2496.6 1396.9 50.0
Mixture of 720 2514.3 1572.5 42.8
the above
In greater detail the calculation and statement of the results may be
illustrated by the following data:
In the first experiment given in the above table the diameter of the
soil particles varied from 0.010 to 0.071 millimeter. The weight of soil
in the diffusion tube was 520 grams. The volume of gas, at 0° and 760
millimeters, before the diffusion began was 2549.4 cubic centimeters.
The volume of carbon dioxid under standard conditions remaining after
ten hours of diffusion was 1230.3 cubic centimeters. This volume is
calculated from the weight of carbon dioxid obtained in the potash
bulbs, each milligram being equal to 0.5089 cubic centimeter of carbon
dioxid. The volume of carbon dioxid diffused is therefore 2549.4 −
1230.3 = 1319.1 cubic centimeters. The per cent of carbon dioxid
diffused is 1319.1 ÷ 2549.4 = 51.74. The volume of carbon dioxid
diffused for each square centimeter of cross section of the diffusion
tube is 1319.1 ÷ 22 = 59.9 cubic centimeters.
The carbon dioxid should be passed long enough to secure complete
expulsion of the air before the determination is commenced.
=289. General Conclusions.=—The general results of the experiments with
the diffusion apparatus to determine the effect of the physical
condition of the soil upon the rate of diffusion are as follows:
1. The diffusion of carbon dioxid through the soil is, at a constant
temperature, chiefly dependent upon the pores in the cross section of
the column of soil. Therefore, the absolute quantity of the diffused gas
is greater the larger the total volume of the pores and _vice versa_.
2. Every diminution of the volume of the pores, whether secured by
pressure of the soil or by an increase in the moisture thereof, is
followed by a decrease in the volume of diffused gas. The giving up of
the carbon dioxid present in the soil atmosphere to the upper atmosphere
by the method of diffusion is therefore the less the finer the soil is,
the more compressed the soil particles are, and the larger the water
capacity of the sample and _vice versa_.
3. The quantity of diffused carbon dioxid is diminished according to the
measure of compression to which the soil is subjected but is not
strictly proportional to the height of the soil layer.
4. In soils in which rain water percolates slowly the diffusion of the
carbon dioxid on account of this property is depressed to a greater or
less extent.
AUTHORITIES CITED IN PART FIFTH.
Footnote 179:
Proceedings of the American Association for the Advancement of
Science, 1872, p. 328.
Footnote 180:
Die Landwirtschaftlichen Versuchs-Stationen, 1889, S. 197.
Footnote 181:
König, Untersuchung Landwirtschaftlich und Gewerblich Wichtiger
Stoffe, Ss. 64–66.
Footnote 182:
König, op cit. supra.
Footnote 183:
Forschungen auf dem Gebiete der Agricultur-Physik, Band 15, S. 190.
Footnote 184:
Annales de Chimie et de Physique, Tome 37, 1853; Encyclopedie
Chimique, Tome 4, p. 154.
Footnote 185:
Zeitschrift für Biologie, Band 7, S. 395 and Band 9, S. 250.
Footnote 186:
Jahresbericht für Agriculturchemie, Band 1, S. 160.
Footnote 187:
Annales de Chimie et de Physique, 1891, Sixième Série, Tome 23, pp.
362, et seq.
Footnote 188:
Op. cit. 5, 1892, Ss. 8, et seq.
PART SIXTH.