It is postulated that (A) the material is isotropic, (B) t~e vo.lun:e change an? hysteresis are negligible, and (C) the shear is proportional to the tractlOn* In sl~ple s~ear In a plane previously deformed, if at all, only by uniform dilatation or contractIOn. It IS deduced that the general strain-energy function, W, has the form

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The hydrodynamical theory of a Newtonian fluid flowing through a capillary viscometer is quite simple and yields a formula for the fluidity in terms of the measured efflux. In the case of non-Newtonian fluids, on the other hand, the theory is not so simple. The usual practice among experimental rheologists is to assume some particular form of fluidity function, after which the corresponding efflux formula is obtained by an integration. If the efflux formula agrees with the experimental efflux data, the assumed fluidity function is thereby established as the correct one. However, it is sometimes difficult to find a fluidity function leading to an efflux formula which agrees satisfactorily with the data. In such cases the experimenter needs a more powerful method of analysis.

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The molecular mechanism of stress relaxation in a linear polymeric material is conceived of as a process of diffusion in phase space of molecular configuration. The analysis is an extension of the Rouse theory of linear visco‐elasticity to finite, three‐dimensional deformations. A molecule, either open end or crosslinked, is treated as a chain of submolecules, each just long enough that its net length follows approximately the Gaussian distribution law. Variables are separated by transformation to normal coordinates, and diffusion equations are obtained of the form where ρn is the density in configuration space of the n‐mode of molecular configuration, and Dn is the diffusion constant characteristic of the mode. An exact solution exists in the form Here βn − 1 is the measure of the recoverable strain in the un direction of the n‐mode, the instantaneous value of the corresponding partial stress being proportional to βn2 − 1. Any partial stress obeys the Boltzmann superposition law, but the total stress does not. The transform equations connecting such pairs of functions as creep and relaxation in the linear theory are here not valid; but, as in the linear theory, in a given case of a single imposed strain the stresses are expressed in terms of a distribution function of relaxation times. Expressions are obtained for the tangential and normal stressed in continuous simple shear at a finite rate. To explain non‐Newtonian viscosity it is necessary to postulate either that the intertangling of the molecules is reduced or that rheological flow units develop in the material which are larger than the individual molecules.