ABSTRACTMethods of determining the constitution of glasses are discussed rather than the presentation of data on glass constitution. The equilibrium diagram method involves the destruction of the glassy condition but indicates the compounds which may be formed. The meaning of the change in state at a temperature some 40° below its softening point is still obscure. Light‐absorption studies indicate solvation. Data on five different physical properties are interpreted in the light of equilibrium diagram data as follows: (1) specific volume, (2) thermal expansion, (3) refractive index, (4) electrical resistance, and (5) vapor tension and volatilization, the first three of which are not decisive. Electrical resistance data indicate presence of Na2O.2SiO2. Volatilization data indicate presence of Na2O.2SiO2and K2O.4Si