The methods employed to determine the deformation under stress of soft solids are largely empirical in character. To evaluate the working properties of materials that are molded or extruded, and the “creep,” “cold flow,” or “plasticity” of materials that yield slowly under load, recourse must be had to comparative tests simulating as closely as possible the conditions to which the material will be subject in service. As service conditions can seldom be exactly duplicated in any practical test, particularly with respect to the time element, these methods always involve some uncertainty as to the validity of the parallel between test and service behavior, and the results are in any case limited in their application. There is, therefore, a need for the development of methods of determining, from simple general tests, the character and amount of deformation under given conditions. While much of the theoretical background for this purpose is available, little progress has been made in applying it to the test methods adapted to materials of the type in question.