=172. Lysimetry.=—The process of measuring the capacity of a soil to permit the passage of water and of collecting and determining the amount of flow and determining soluble matters therein is known as lysimetry. In general, the rate at which water will pass through a soil depends on the fineness and approximation of its particles. Water will pass through coarse sand almost as rapidly as through a tube, while a fine clay may be almost impervious. The study of the phenomena of filtration through soil, and the methods of quantitatively estimating them, are therefore closely related to porosity. Two cases are to be considered, _viz._: First, percolation through samples of soil prepared for analysis, and second, the passage of the water through soil _in situ_, whether it be virgin or cultivated. The determination of the rate of flow through a soil in laboratory samples, gives valuable information in respect of its physical properties, while the same determination made on the soil _in situ_, has practical relations to the supply of moisture, to growing plants, and the waste of valuable plant food in the drainage waters. The determination of the rate of flow of water through a small sample, disturbed as little as possible in its natural condition, is classed with the first divisions of the work, inasmuch as the removal of a sample of soil from a field, and its transfer to the laboratory, subjects it to artificial conditions, even if its texture be but little disturbed by the removal. =173. Calculation of the Relative Rate of Flow of Water Through Soils.=—There will evidently be one space, or opening, into the soil for every surface grain, as pointed out by Whitney,[118] and the approximate number of grains, or of openings, on a unit area of surface may be found by the following formula: N = (√((M × W)/(V))^⅔ where N is the number of grains, or openings, on one square centimeter of surface, M is the approximate number of grains in one gram of soil, W is the weight of soil, V is the total volume of the soil grains and the empty space. If the grains are assumed to be symmetrically arranged and the spaces between them cylindrical in form, the radii of the spaces can be found by the following formula: _r_ = √(V₁)/(πNL) where _r_ is the radius of a single space, V is the total volume of the empty space, N is the number of grains or spaces on one square centimeter of surface, and L is the depth of the soil. If the space within the soil is completely filled with water the relative rate of flow of water through the soil will be according to the fourth power of the radius of a single space multiplied by the number of spaces on the unit area of surface, as shown by the following formula: T₁ = (N(_r_)⁴T)/(N₁(_r_₁)⁴)