of measurement of the hydrometer. Sand particles may take less than a second. Stoke's law provides the theoretical basis to calculate the relationship between sedimentation velocity and particle size. ASTM provides the detailed procedures for performing the Hydrometer test. Clay particles can be sufficiently small that they never settle because they are kept in suspension by Brownian motion, in which case they may be classified as colloids. Mass-volume relations There are a variety of parameters used to describe the relative proportions of air, water and solid in a soil. This section defines these parameters and some of their interrelationships. The basic notation is as follows: , , and represent the volumes of air, water and solids in a soil mixture; , , and represent the weights of air, water and solids in a soil mixture; , , and represent the masses of air, water and solids in a soil mixture; , , and represent the densities of the constituents (air, water and solids) in a soil mixture; Note that the weights, W, can be obtained by multiplying the mass, M, by the acceleration due to gravity, g; e.g., Specific Gravity is the ratio of the density of one material compared to the density of pure water (). Specific gravity of solids, Note that specific weight, conventionally denoted by the symbol may be obtained by multiplying the density ( ) of a material by the acceleration due to gravity, . Density, Bulk Density, or Wet Density, , are different names for the density of the mixture, i.e., the total mass of air, water, solids divided by the total volume of air water and solids (the mass of air is assumed to be zero for practical purposes): Dry Density, , is the mass of solids divided by the total volume of air water and solids: Buoyant Density, , defined as the density of the mixture minus the density of water is useful if the soil is submerged under water: where is the density of water Water Content, is the ratio of mass of water to mass of solid. It is easily measured by weighing a sample of the soil, drying it out in an oven and re-weighing. Standard procedures are described by ASTM. Void ratio, , is the ratio of the volume of voids to the volume of solids: Porosity, , is the ratio of volume of voids to the total volume,