up to 1800" C, and on pure silica up to 2050" C, using a rotating crucible viscometer.The viscosity isotherms exhibit no appreciable inflection and show an exponential increase in viscosity with increasing silica content. From the slope of the linear relation between log 7 and 1/T, the heat of activation for viscous flow is evaluated.A fundamental change in the silicate lattice occurs on the addition of about 10 mole % alkali metal oxide and about 20 % alkaline earth oxide. Thereafter, up to the orthosilicate composition, the results are consistent with the concept that discrete ions constitute the units of flow. Recently available partial molar volume results which indicate the magnitude of the Si-0-Si angle in liquid silicates are combined with the present measurements to ascertain the constitution of these ions. Si3096-rings are the fundamental building units, and the ions obey the general formula (Sin02n+3)6-. They are present in melts containing from about 10 to 60 mole % metal oxide. The model is shown to satisfy both molar volume changes and the variation of the activation energy for viscous flow with composition.* All compositions refer to the molar percentages.

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