When two liquids are mixed in a certain ratio, it would not appear to be a very difficult problem to calculate the fluidity of the mixture, provided that the properties of the pure components are known and they are inert. In fact the problems at first appeared to be so simple that the assumptions involved were not even considered. After many years of groping for the solution, we have arrived at the conclusion that the problem is still unsolved. An important investigation in this field concludes: “The results of this paper also are consequently indecisive. All previous formulas are certainly useless.” A textbook of Rheology puts it succinctly, “The law of ideal mixtures is unknown.” It is not altogether unsatisfactory that the old erroneous assumptions have been admitted to be unworthy of confidence. As Hatschek says, “The ideal mixture curve for viscosity is almost certainlynota straight line.” This at least opens the way for new work, however.