THE term viscosity has frequently been used in a qualitative sense in describing the properties of solids and particularly of metals, but its use in the quantitative sense which has been developed through the study of liquids is a comparatively recent development. Viscosity may be defined quite generally as the ratio of the shearing stress in any plane to the rate of shear or velocity gradient perpendicular to the plane. Thus defined it appears always to be a variable quantity for solids, its value depending on a number of factors but particularly on the magnitude of the shearing stress. No emphasis appears, however, to have been laid on the importance of the degree of variability of the viscosity in relation to the rupture of plastic solids. RESP-1028