A theory is developed to calculate the surface shear viscosity of a mixed film if the flow parameters of each single component are given. This theory starts from the model of Eyring, assuming that the free energy of viscous flow for the mixture is proportional to the mole fractions of the component in the surface and to the free energy of viscous flow of the single components. To test this equation the surface viscosity of mixed adsorbed films of two saponins, digitonin and senegin is considered. Large deviation from this behaviour, as in the case of dilute cholesterol monolayers penetrated by digitonin, points to an interaction.

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The enzymatic activity of cellulase has been estimated by the decrease of viscosity of a hydroxyethylcellulose solution as a function of incubation time. This decrease is a function of the applied shear stress. It is proposed that the viscosity be measured in an ordinary Ostwald viscometer under conditions where the viscosity is Newtonian, that is, the decrease of viscosity by enzymatic activity is independent on the shear stress.

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