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According to Newton's law of fluid friction, viscosity is defined as the ratio of the shearing stress to the rate of shear. If the stress is measured in dynes/cm2 and the rate of shear in sec−1, the viscosity is given in dynes sec/cm2, or equivalently in gm/cm sec. This unit is called the poise and is used in the tables in this section. The unit of kinematic viscosity is the stokes; it is the viscosity in poises divided by the density in gm/cm3.

Real liquids may be categorized as "Newtonian" or "viscous," and "non-Newtonian". The former show viscosities independent of rate of shear over wide ranges of shear rate. Silicates are believed to fall in this category, although verification of true viscous behavior in glasses becomes difficult as the viscosity becomes high. Non-Newtonian behavior is observed in colloidal suspensions, solutions of high polymers, greases, asphalt, etc. No single value of viscosity is sufficient to describe the flow of such materials. For many liquids, viscous flow is a simple activated process, and the viscosity, η, is well represented as a function of temperature by the expression η = η0 exp (E η /RT). Such a law is fairly well obeyed by SiO2 and by some binary silicate liquids. It is not obeyed by water, B2O3, many complex silicates, or binary barium silicates. The viscosities of some binary calcium silicates and some more complex silicate liquids can be represented by a sum of terms, each of the form given in the above expression,

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