220 FLUIDITY AND PLASTICITY The following table, taken from the work of Bingham and Green on paints, proves the validity of the general law of plastic flow expressed in Eq. (73). The friction, when expressed in terms of shear—and not in terms of pressure—is nearly constant and not a function of the dimensions of the capillary. It is a fact, however, that the rate of flow is not directly proportional to the shear, when the shear is too small, but when the shear is suffi- ciently high the relation becomes linear, as is proved by plotting 200 25 JO 75 VOLUME ^PERCENTAGE CLAY FIG. 79.—The relation of fluidity and friction to volume concentration of solid in clay suspensions. the values in the table, Fig. 78. The table also indicates that the mobility is a constant independent of the rate of flow or of the dimensions of the capillary. The reason for the rate of flow- shear curve not being linear as the rate of flow is decreased will be considered when we come to discuss the theory of plastic flow. By measuring the fluidity of suspensions containing increasing amounts of solid in suspension, Bingham and Durham found it possible to obtain a concentration which would possess zero fluidity when the shear was very small. Conversely, by measur- ing the friction of suspensions containing decreasing amounts of solid, it is possible to find a concentration which would have