The property of viscosity is possessed, by any fluid, and. although the methods of investigation are widely different in each case, it may therefore be examined, in either liquids or gases, or in solutions or mixtures, as long as only a single phase is present.In this thesis, work is described on the viscosities of various types of mixtures; they are, briefly, (1) solution mixtures, where various degrees of reaction may be expected between the substances in solution; (2) mixtures of gases with vapours; (3) emulsions, which, though strictly speaking not single phase liquid mixtures, are yet as a rule sufficiently homogeneous to permit of the measurement of viscosity. Part I of this thesis contains the results obtai