=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.