According to Newton’s law of fluid friction, the tangential or shearing stress in a liquid in motion is proportional to the rate of change, with time, of the angle of shear. Real liquids may be divided into two categories: “Newtonian” or “viscous” liquids, for which the factor of proportionality, the “viscosity,” is independent of the “rate of shear”; and “non-Newtonian” liquids, for which the factor depends upon this rate. The “Newtonian” behavior of many ordinary liquids, and of some glasses, has been verified for considerable ranges of shear rate. “Non-Newtonian” behavior has been observed for such materials as certain colloidal solutions, asphalt, and other bituminous products; no single value of “viscosity” is sufficient to describe the flow of such materials. (See also Section 9.) If the stress is measured in dynes × cm.⁻², and the rate of shear in radians × sec.⁻¹, the viscosity will be given in dyne × sec. × cm.⁻² or the equivalent unit, gram × sec.⁻¹ × cm. ⁻¹ This unit is called the poise and is used in the tables in this section.

The viscosity of a given material depends upon the pressure and temperature. Other factors of great importance for geological applications remain to be investigated, such as the effect of dissolved gases . . .