VISCOSITY 227 



yet not until there has been found an elastic colloidal system 

 which exhibits true viscosity can elastic structure be disregarded 

 as a possible factor in anomalous flow. 



In an attempt to straighten out these difficulties, Bingham 

 introduced a new interpretation of the word "plastic"— new 

 because he apphed it to liquids as well as solids. To be plastic 

 in the Bingham sense, the ratio of flow to force in a liquid must 

 be variable. Consequently, materials that give curved flow- 

 pressure graphs are said to be plastic. 



One of the characteristics of plastic, or non-Newtonian, 

 liquids is the possession of a yield value. They require an initial 

 force to start flow. Yield value is the minimum shearing stress 

 in dynes per square centimeter required to produce continuous 

 deformation. Sheppard, while accepting in the main the plastic- 

 ity idea, holds to the opinion that non-Newtonian behavior is a 

 criterion of molecular aggregation, or of structural features, 

 such as account for elastic properties. This suggests that we 

 may be dealing with two types of viscosity — true (inner friction) 

 and structural. 



These questions of theory, which are rather out of the province 

 of the biologist and must be left to the rheologist to solve, need not 

 seriously bother the worker on protoplasm, because the most that 

 he can hope for is a rough approximation of the consistency (a safe 

 term under all conditions) of protoplasm. However, the biolo- 

 gist must realize that these questions exist and that the material 

 with which he works is not a pure liquid or true solution but 

 exhibits anomalous flow and is therefore not obedient to the laws 

 of Newton, Poiseuille, and Stokes. 



Some workers will dissent from this last statement, which to 

 others is as true as any that can be made of protoplasm. We 

 read that protoplasm is a pure solution. The declaration is 

 made by those who view it as a system of no great complexity. 

 If this well-nigh inconceivable concept is for the moment 

 accepted, we must at least give to protoplasm the same degree 

 of anomalous behavior as that possessed by a protein or a soap 

 solution, for the dry weight of protoplasm is half protein, with 

 a fraction of soap, and neither exhibits true viscous flow. 



Studies of flow under varying stresses show that colloidal solu- 

 tions of the lyophilic type are mostly anomalous in character. 

 The viscosity is to be regarded as "apparent" and should be 



