If one alkali oxide is gradually replaced by another in a series of glasses, some physical properties vary in an extremely non-linear manner. In particular, the electrical resistivity and, to a lesser extent, the expansion coefficient show pronounced maxima while the viscosity shows a deep minimum; the variation of molar volume however is nearly linear. No comparable effects have been reported in solid solution crystals but some slight effects have been reported in fused salts. Mixed alkali glasses also show a unique mechanical relaxation phenomenon at temperatures of 100-200°C. Various theories have been proposed, mostly with a view to explaining the marked effect on resistivity, but none can be said to account adequately for all the data.
