_Extent of Shrinkage._—The extent of this shrinkage in drying, and subsequent expansion in wetting, have been measured by the writer by the use of the sieve cylinder described below (chapt. 11, p. 209), as serving for the determination of the water capacity of soils. When a soil of the kind above referred to is placed in the sieve cylinder in the tilled (flocculated) condition, then allowed to absorb its maximum of water and then dried at 100 degrees C., the contraction in drying can be very strikingly seen, and its amount measured by filling up the empty space with mercury; then measuring the latter after expelling the surplus by means of a ground glass plate laid on top. The contraction of several heavy clay soils, thus measured, has been found by the writer to range from 28 to as much as 40 per cent of the original bulk.[32] The soil thus contracted, when again wetted, does not return altogether to its original bulk, but remains in a more or less compacted condition, like that of a soil which has been rained upon. [32] Wollny (Forsch. Vol. 20, p. 13 ff., 1897) records similarly high shrinkages in his experiments. The expansion and contraction of a heavy clay soil on wetting and drying are well illustrated in the figure below, in which the soils are shown in the shallow cylinder which serves for the determination of water-holding power (see chapt. 11, p. 209). The middle figure shows in profile the expansion of a dry, pulverized “black adobe,” struck level, when allowed to absorb its maximum of water; it rises above the rim of the sieve-box to nearly the half height of the latter. The outside figure to the right shows the same soil after drying; that to the left, a red clay soil similarly treated. It is easily seen that these variations in volume may bring about very marked results in the fields; the surface of which, apart from the cracks usually formed, may be several inches lower in the dry season than during wet weather. [Illustration: RED CLAY SOIL. BLACK “ADOBE” CLAY SOIL. FIG. 13.—Expansion on Wetting and Contraction on Drying of heavy clay soils.] _Contraction on Wetting._—In the case of alkali soils containing much carbonate of soda, a very notable _contraction_ occurs in _wetting_ the loose, dry soil. The cause is here obviously the collapse of the crumbs, formed in dry tillage or crushing, into single grains, closely packed. The same result is observed in the naturally depressed “alkali spots” (see chapt. 22).