Viscosity of Simple Soda‐Silicate Glasses, 500° to 1400°CComparison of the results given by English with those of Washburn, Shelton and Libman, indicates a discrepancy in the absolute values of log10viscosity amounting to 0.6, those of Washburnet al., being relatively too high. If correction for this is made, the isothermal curves of log10viscosity as a function of soda content are smooth up to 50% Na2O, showing no inflection. The observations as a function of temperatureTare all represented within accidental error by an equation of the type log10η=−A+B× 103/ (T−T0) where all three constants vary regularly with the composition.Change of Viscosity of Glass (6SiO2, 2Na2O) due to Molecular Substitution of CaO, MgO and Al2O3for Na2OThe effect is clearly brought out by plotting (from the results of English) the change of log10ndue to the substitution as a function of temperature. The curves each show a sharp bend at a temperature between 840° and 1050°C, which is designated the aggregation temperatureTa. If we divide these curves by the corresponding percentage substituted, we get curves for each oxide which are straight and parallel below the aggregation temperatures, the slopes (increase of change of log10nper 100°C) being −0.056 (CaO), −0.055 (MgO), −0.018 (Al2O3) per per cent oxide substituted. For substitution of 1/2 molecule the slopes are −0.325 (CaO), −0.23 (MgO) and −0.18 (Al2O3) per 100°. At the aggregation temperature the change of log10nper per cent is a minimum, 0.03 to 0.06 for CaO, 0.12 for MgO, 0.07 for Al2O3.Evidence of Aggregation in Glasses, from Viscosity Measurements. The sharp bends in the plots of change of log10ndue to substitution of an oxide for Na2O, suggest the beginning of molecular aggregation at these temperatures. These aggregation temperatures are close to the devitrification temperatures, but the effect on the viscosity curves cannot be due to actual devitrification since it does not change with time. Taking the aggregation temperatures as equal to devitrification temperatures, additional isotherms are roughly sketched into theequilibrium triangle of the system Na2O─CaO─SiO2.Change of Viscosity of Glass (4SiO2, 2Na2O) due to Substitution of B2O3for SiO2The change of log10n(from the results of English) is plotted as a function of temperature, and also the change of log10nper per cent B2O3. The curves are more complex than