276. Relation of Soil Composition to Gases.—The power of a soil for occluding gases rests primarily on its composition as determined by silt analysis. The discussion of this part of the subject is so nearly related to that of the physical properties of the soil that it might properly have been included in that part of the work. Since, however, we deal in this part more with the determination of the gas constituents of the soil, it was deemed preferable to place it after the silt analysis and as introductory to the general estimation by more strictly analytical processes of the chemical constituents of the soil.

277. Occurrence of Carbon Dioxid.—The amount of organic matter in the soil, according to Wollny,^[179] is no indication of the quantity of carbon dioxid when the organic matter is in excess. The percentage of carbon dioxid is only proportional to the amount of organic matter when this is in small quantities. Large quantities of organic matter do increase the amount of carbon dioxid, but the increase is not a proportional one, since a larger quantity of this gas in the air of a soil reduces the activity of the organisms which produce oxidation. Water and temperature have a greater influence on the oxidation, and act in an opposite direction to that of the organic matter. The amount of free gas in the soil affords no indication either of the intensity of the action of oxidation or of the amount of organic matter.

The addition of liquid manure to the soil results in a reduction of the decomposition of the organic matter when the quantity of the salts therein contained is greater than that already present in the soil. But if the liquid manure is dilute, and the absorptive power of the soil for salts is great, then the decomposition is promoted.

278. Absorption of Aqueous Vapor.—The power of a soil to resist drought depends largely upon its coefficient of absorption for aqueous vapor. Hilgard has shown^[180] that at temperatures between 7° and 21°, the amount of aqueous vapor absorbed by a thin layer of a clay or soil not unusually rich in humus, in a saturated atmosphere, is sensibly constant. In general, clay soils are more absorbent than sandy ones, yet there is no direct connection between the amount of clay present and the absorbent power of the soil. Evidently the hygroscopic coefficient is largely controlled by the presence with the clay of the powdery ingredients which determine its looseness of texture, and it is found that the finer silts themselves possess a considerable absorbing power. According to Whitney this is largely dependent upon the extent of the surface area of the soil grains and upon the size and arrangement of these grains. Again, the presence of hydrated ferric oxid materially influences this power, so that the amount of iron present must always be taken into consideration.

279. Methods of Study.—The study of the deportment of a soil with vapors or gases may be divided into two general classes. The first depends on the subjection of a sample of soil to the saturating influence of a given vapor or gas and measuring the amount thereof absorbed, either directly by increase of weight, or by the diminution in the amount of gas originally supplied. The maximum absorbent capacity of a soil under given conditions for a gas or vapor is in this way determined.

In the second class the determination consists in accurately estimating the amount of gas which is absorbed by a soil in natural conditions or in situ, thus giving the natural percentages of the gaseous constituents of the soil.

In the first case in general, the principle of the method depends upon the exposure of the soil for a given time under given conditions, to an atmosphere of the gas to be absorbed. The principle of the second class of determinations depends upon the extraction, usually by means of suction, from a given mass of soil of the gaseous matters therein contained. The general details of the methods of procedure for the first class are found in the following directions for manipulation:

280. Determination of the Maximum Hygroscopic Coefficient.—The fine earth, in Hilgard’s method, is exposed to an atmosphere saturated with moisture for about twelve hours at the ordinary temperature (60° F.) of the cellar in which the box should be kept. The soil is sifted in a layer of about one millimeter thickness upon glazed paper, on a wooden table, and placed in a small water-tight covered box, twelve by nine by eight inches, in which there is about an inch of water; the interior sides and cover of the box should be lined with blotting paper, kept saturated with water, to insure the saturation of the air.

Air-dried soil yields results varying from day to day to the extent of as much as thirty to fifty per cent, nor have we any corrective formula that would reduce such observations to absolute measure. Knop’s law, that the absorption varies directly as the temperature, while applicable to low percentages of saturation, is wide of the truth when saturation is approached. The ordinary temperature of cellars will serve well in these determinations without material correction.

After eight to twelve hours the earth is transferred as quickly as possible, in the cellar, to a weighed drying tube and weighed. The tube is then placed in a paraffin bath; the temperature gradually raised to 200° C. and kept there twenty to thirty minutes, a current of dry air passing continually through the tube. It is then weighed again and the loss in weight gives the hygroscopic moisture in saturated air.

The reason for adopting 200° C. as the temperature for drying instead of 100° is that water will continue to come off from most soils at the latter temperature for an indefinite time, a week or more, before an approach to constancy of weight is attained; and that up to 200° only an arbitrary limit can be assigned for the expulsion of hygroscopic moisture. Moreover, the great majority of soils, especially those poor in humus, will reabsorb moisture from a saturated atmosphere to the full extent of that driven off at 200° C.

281. Estimation of the Absorption Power of Soils for Aqueous Vapors.^[181]—Method A.—The fine earth, ten to twenty grams, is spread out on a surface of about twenty-five square centimeters, and left for several days with the observation of the temperature of the air and the loss of weight determined from time to time. This evaporation is continued until the weight remains practically constant. Afterwards by drying the sample at 100° the amount of hygroscopic moisture is determined. A similar result can be reached if the sample is first dried at 100°, or over sulfuric acid at ordinary temperatures, and then the increase in weight observed which the sample acquires on being exposed for several days to the atmosphere under ordinary conditions. Soils with about the same content of humus show variations in the power to absorb aqueous vapors which are almost proportional to the amount of clay which they contain. With the increase of humus substance, the power of the soil for absorbing moisture is increased, so that a sandy soil which is rich in humus often will retain as much moisture in an air-dried state as a clay soil which is poor in humus. If the experiment is carried on by drying over sulfuric acid instead of at 100°, the sample should be left from four to seven days in order that a constant weight may be reached. Even after this time the loss in weight is 0.2 to 1.5 per cent less than when the sample is dried at 100°.

Method B.—In order to determine the amount of aqueous vapor which a soil will absorb in an atmosphere saturated with the vapor the following method is used:

The sample of air-dried soil in a flat dish of given surface; viz., about twenty grams of soil to twenty-five square centimeters surface is placed in a vessel over water without contact with the water, and the whole of the apparatus is covered with a glass bell-jar. The sample is weighed at intervals of six or eight hours until no appreciable increase of weight is observed. An empty vessel of the same size and character as that containing the soil is kept under the bell-jar, also in the same conditions, so that any increase in weight by the deposition of moisture on this vessel may be determined. This increase in weight is to be deducted from the total increase in weight of the vessel and the soil. Sandy and loamy soils become saturated in this manner in the course of the first twenty-four hours and remain after that unchanged in weight. Very clayey soils, and also those which are very rich in humus, require a much longer time, three or four days even. In this case it is better to take a smaller sample of the soil; viz., ten grams. The temperature of the air within the glass vessel, of course, must be taken into consideration.

Method C.—The same flat dish and the same quantity of soil as in the other methods are taken in this one. The sample is left out over night where it can be fully saturated with dew. The amount of dew which appears on the bushes should be noted and also the temperature of the air and the percentage of clouds in the sky. An experiment should also be made on spots of earth which are entirely free from vegetation in order that the difference in the amount of water absorbed in places practically devoid of dew and in places where the dew is abundant may be observed.

Method D.—Deeper flat dishes should be used for this determination so that the depth of soil contained in them shall be from one to three, or even six centimeters. The sample of soil should be completely air-dried and in a state of fine subdivision. The vessels containing the soil should be placed in a locality saturated with aqueous vapor or in the open air during the night where they are subjected to the influence of the cooling of the atmosphere and the deposition of dew. Note should be made of the different amounts of moisture absorbed by the layers of earth of different thicknesses in a given time. Observation should also be made of the depth to which the moisture sinks in the sample of soil under consideration.

282. Estimation of the Absorption Power of the Soil for Oxygen and Atmospheric Air.^[182]—From fifty to one hundred grams of air-dried soil are placed in a glass vessel of about 500 cubic centimeters capacity, and the flask closed with a stopper after the addition of enough water to make the percentage of moisture in the soil about twenty. After from eight to fourteen days the air contained in the vessel is analyzed for oxygen, nitrogen, and carbon dioxid, with special reference to the determination of how much oxygen has disappeared and how much the carbon dioxid has been increased. As an alternative method, twenty-five grams of the soil may be moistened with tolerably concentrated potash lye in a small glass vessel, which is itself joined with air-tight connections to an azotometer in which a known volume of air is confined by quicksilver. The glass vessel is frequently shaken during the progress of the experiment. The diminution of the volume of air in the apparatus after from one to four days gives approximately the quantity of oxygen absorbed.

283. General Method of Determining Absorption.—This method, due to Freiherrn von Dobeneck,^[183] is as follows: The soil, in a state of fine powder, is dried at 100° to 105° to a constant weight. It is then placed in an absorption tube of the following construction:

The absorption tube consists of a ᥩ shaped wide glass tube, both ends of which are supplied with small glass tubes sealed upon the end of the ᥩ tube, and those are furnished with tightly-ground glass stop-cocks. Above these stop-cocks these small tubes are bent in opposite directions at right angles. On the bend of the ᥩ is sealed another tube which is furnished with a ground glass stopper. Through this opening the ᥩ tube can be filled with the sample of soil. When the tube is filled, the glass stopper inserted, and the two stop-cocks on the small tubes closed, the contents of the tube are completely excluded from the external atmosphere. Many of these tubes can be used at once so as to hasten the progress of the work.

The tubes after being filled are placed in a drying oven with the stop-cocks open. The stop-cocks are then closed before the tubes are removed, when they are placed in a desiccator for cooling preparatory to weighing. The weighed tubes are held in a tin box which can be placed in a water-bath which is kept at a given temperature by means of a thermostat. The top of the tin box should be hinged and made of a thick non-conducting material so as to prevent any rapid change of temperature within. On the inner side of the box a small thin-walled glass tube is carried around four times. One end of this tube passes through an opening in the side of the box by means of which it can be connected with the gas apparatus outside. The other end of it is connected directly with the absorption tubes.

The absorption tubes are so connected among themselves that when ammonia or carbon dioxid is employed the gas passes through one of the tubes before it can reach the next, and so on. For experiments with water-gas, however, that is, air charged with aqueous vapor, the arrangement must be different. While in the case of ammonia and carbon dioxid the composition of the gas is not changed by passing through the samples of soil, the case is quite different when air charged with aqueous vapor passes through. In the latter case the amount of aqueous vapor in the air would be notably lessened in passing from sample to sample on account of the retention of a part of the aqueous vapor by the soil. In this case, therefore, the saturated air, after it has passed through the glass tube around the inside of the box in order to reach the proper temperature, is conducted into a receptacle of glass which has a number of connections equal to the number of absorption tubes so that the saturated air can pass directly into each one of them.

The gases which are to be used for the experiments are prepared in proper apparatus and are forced through the samples of soil, either by pressure as in the case of ammonia or carbon dioxid, or by means of aspirators as in the case of air saturated with aqueous vapor.

The carbon dioxid employed is purified by passing over sodium carbonate and calcium chlorid.

The ammonia is prepared by the action of finely powdered lime on ammonium chlorid, and is dried by passing over lime and sticks of potassium hydroxid.

The air which is to be saturated with aqueous vapor, in order to purify it from dust, carbon dioxid, and ammonia, is passed through two flasks in which are contained respectively, diluted sulfuric acid and potash lye. It is afterwards thoroughly saturated with aqueous vapor at the temperature desired.

Various kinds of soil material may be employed as follows:

(1) Pure quartz sand.—Freed from all fine particles by subjection to silt analysis, afterwards boiled with hydrochloric acid and washed with water to free it from all clayey materials. The sand prepared in this way should be passed through different sieves in order to prepare it in different states of fineness.

(2) Quartz powder.—Prepared from pure quartz crystals by grinding in an iron mortar.

(3) Kaolin.—Material such as is used in the manufacture of the finest porcelain which, after being freed of all foreign matter, is rubbed to a fine powder in a porcelain mortar.

(4) Humus.—Washed with ether and alcohol, boiled with hydrochloric acid, washed, dried and reduced to a state of fine powder.

(5) Iron oxid.

(6) Calcium carbonate.—Precipitated, washed, and dried.

(7) Soil mixtures.—Prepared artificially by mixing the kaolin, quartz, and humus, above mentioned.

The quantity of gas absorbed by each of these materials is determined by filling the tubes, as above mentioned, with the dried material. The content of each tube is previously determined by filling with mercury and weighing. Having determined the weight of the substance to the exclusion of the air contained within its pores, it is treated with the gas in the apparatus described above and weighed from time to time until no further increase of weight takes place.

The method of calculating the results is shown in the following scheme:

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

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:

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.

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.

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.

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.

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.