The tension phenomenon described herein occurs in a wide variety of porous materials, including sands, clays, agricultural soils and porous rocks. When some of the water is removed from a water-saturated porous system, the residual water evidently remains physically interconnected, judging from the fact that water can be transmitted through the system at reduced water content by suction.

The removal of water may result in contraction of the system, as in the case of clay, or in the entry of air, as in the case of sand. The liquid phase and the solid phase in contact with it comprise a closely linked force system. Equilibrium can be established between the system at reduced water content and a separate mass of water at reduced pressure through a porous membrane in contact with both.

The equilibrium tension required in the external water phase is considered an attribute of the moist, porous, system itself. From this point of view, the tension originates through the combined action of the internal forces of the system in a virtual displacement of water. It follows from this and from the principle of virtual work that the tension is numerically equal to the differential work done by the internal forces per unit volume of water absorbed.

The movement of water, under tension, through porous systems represents a special class of flow phenomena in which tensiometers or equivalent devices are required for measuring the hydraulic potential. Flow patterns can be determined in much the same way as in systems characterized by positive hydrostatic pressures, but special attention must be paid to the Darcy coefficient (the capillary conductivity) which varies with the tension.

The theoretical conditions for the equilibrium of water in the soil and for emergence from the soil have been developed in terms of the tension and certain applications have been indicated.

The phenomenon referred to in the soil science literature as moisture tension has been recognized for almost forty years and has been used as a means of explaining the absorption and movement of water in the soil. It is closely related to osmotic pressure but its mechanism cannot in general be identified with the traditional mechanisms of osmotic pressure. Moisture tension has been observed in wet clay soils and in other finely divided porous systems containing interstitial water, but the phenomenon is not confined to colloidal systems, since moist sand and moist porous rock, such as pumice, show similar effects.