As an example in point, may be cited the case of the natural minerals Analcite and Leucite, which Lemberg was able to reciprocally transform from their natural condition of soda- and potash-alumina silicates merely by alternate treatment with solutions of potassium and sodium chlorids respectively. (See chap. 3, p. 37). The same is true in the case of the zeolitic matter of the soil. There is nevertheless a distinct preference in the direction of the retention of potash as against soda; so that in the case of alkali soils, a large excess of potash is found to be present in the zeolitic form, notwithstanding the presence of sometimes very large amounts of the chlorid, sulfate and carbonate of soda. This preferable retention of potash is, of course, of material advantage in the case of the use of soluble potash-fertilizers, as well as in preventing the waste of the potash of the soil itself. ABSORPTION, OR CONDENSATION, OF GASES BY SOILS. Like all bodies in a state of fine division, soils are capable of absorbing a not inconsiderable amount of various gases. It may be said that in general, other things being equal, the amount thus condensed on the surface of the soil-grains is more or less directly proportional to the facility with which the gas is condensed by either pressure or cooling. Hence the very large amount of water-gas or vapor which may be absorbed by soils, as shown in a preceding chapter. But excepting perhaps the case of ammonia, moist soils are less absorbent of gases than dry ones. Oxygen and nitrogen, the main constituents of the atmosphere, being difficultly condensable by either pressure or cold, are absorbed by soils only to a relatively small, yet by no means unimportant extent. The condensation of oxygen within the soil-mass is doubtless of considerable importance in the processes of oxidation, as is shown by its partial replacement by carbonic gas in the free air of the soil (see chap. 2, p. 17). The intensifying of oxidizing action caused by surface condensation is well illustrated in the case of finely divided platinum, in which hydrogen is brought to rapid combustion when mixed with oxygen; as well as by the effect of bedding tainted meat in charcoal powder, when all odors of decay disappear, both by absorption and oxidation, ammonia and carbonic gas alone ultimately escaping through the powder.