The variable viscosity which is shown by colloids generally as the rate at which they are sheared is varied leads to more striking anomalies in the behaviour of concentrated suspensions such as clay pastes when they are subjected to forces tending to make them flow. Bingham and Durham (l) first showed that when clay pastes were forced through capillary tubes, not only did the amount of clay squeezed out per second increase more slowly than the pressure applied, indicating a diminishing coefficient of viscosity, but also that there was evidence of a limiting shear, due to the consistency or cohesion of the particles, below which no flow would take place. It is the existence of this latter criterion that distinguishes clay from a weak suspension, e.g. of sand grains in water. This combined effect of cohesion and viscosity Bingham calls plasticity, and modifies Newton's law of viscous flow by the addition of a constant to allow for the cohesive force. Nevertheless, owing to the fact that the coefficient of viscosity is a function of the velocity gradient instead of being constant as in a homogeneous liquid, and that this velocity gradient is not known in the capillary tube viscometer, results as between one viscometer and another, or between two flow tubes of different size cannot be reduced to a common basis for comparison except by empirical trial. Keen and Scott Blair(2), Scott Blair and Crowther(3), and Schofield and Scott Blair(4) have repeated Bingham's experiments in an improved apparatus of the flow tube type, which they call a “plastometer.”