Many works of previous investigators on various types of inorganic glasses containing both phosphorus and silicon were collected. On these results a reasonable interpretation and a consideration from a view-point of structural chemistry were given. Then the following conclusions were obtained.
1) Silico-phosphate glasses are classified into two types, that is low oxygen type (I) and high oxygen type (II). In the intermediate composition, there is a region in which the crystallization of the melt occurs easily.
2) Examples of (I) are sodium silica-phosphate glass and calcium metaphosphate glass etc., and a sole example of (II) is fused calcium magnesium silico-phosphate.
3) The mechanism of the action of SiO₂ on vitrescence of silico-phosphates is different between (I) and (II) which means the structural difference between them.
4) (I) is formed when SiO₂ content is of low value, while a considerable amount of SiO₂ is necessary to obtain (II).
5) Various physical and chemical properties of (I) change in different ways with the increase of SiO₂ content. All these results are possible to be explained using the concepts of complexing of ions, breaking of P-O-P bond and formation of P-O-Si structure.
6) In (I), oxygen has a tendency to be given from the Si-O part to the P-O part. Thus P-O-P bond is easily broken and Si-O ion is easily condensed and devitrified as the crystal of silica. This fact is a reason that a considerable amount f o SiO₂ entered into (I) with difficulty. From this principle, many results can be interpreted.
7) In (II), neither P-O-P structure nor P-O-Si structure exists and the P-O ion exists as a isolated PO₄ tetrahedron. This glass consists of the P-O part with high structural regularity and the Si-O part with low, and the P-O part is dissolved in the Si-O glassy part.
8) Although these glasses are named “silico-phosphate glasses”, it is considered that (I) is a real phosphate glass and (II) belongs to a silicate glass essentially. The structure of (II) is more approached to crystalline state as compared with (I).

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This is the second paper in a series of articles concerning “Thermodynamic Approach to the Structure of Silicate Glasses”. By applying the method presented in the first paper of the series [This Journal, 68, 204-210 (1960)], calculations of free-energy changes and equilibrium constants have been made for reactions of forming metasilicate building units of sodium, calcium and magnesium from the combination of other types of silicate building units including SiO₂ at temperatures of 298°-1, 500°K.
The values of equilibrium constant, K, for the above types of reactions of forming sodium metasilicate building unit were decreased with the increase of temperature and were also decreased, at a given temperature, with the decrease in difference of the silicon-oxygen ratio, N_Si, between each reactant and the resultant; for example, the values of log₁₀ K for the reaction, n[(4Na⁺)(SiO₄^4-)](N_Si=0.250)+n[SiO₂] (N_Si=0.500)→←2[(2nNa⁺)(Si_nO_3n^2n-)](N_Si=0.333), at 298° and 1, 100°K were respectively 3.37 and 0.74, the values of log₁₀ K for the reaction, n[(6Na⁺)(Si₂O₇^6-)](N_Si=0.286)+[(2nNa⁺)(Si_2nO_5n^2n-)](N_Si=0.400)→←4[(2nNa⁺)(Si_nO_3n^2n-)](N_Si=0.333), at 298° and 1, 100°K respectively 0.59±0.30 and 0.04±0.26, and the values of log₁₀ K for the reaction, n[(14Na⁺)(Si₆O₁₉^14-)](N_Si=0.316)+[(2nNa⁺)(Si_2nO_5n^2n-)](N_Si=0.400)→←8[(2nNa⁺)(Si_nO_3n^2n-)](N_Si=0.333), at 298° and 1, 100°K respectively 0.29±0.37 and -0.02±0.32, where n denoted an integer and (Si₂O₇^6-) and (Si₆O₁₉^14-) were linear silicate anions. The effect of replacing sodium oxide component by calcium and/or magnesium oxides on the value of K for the reactions of forming metasilicate building unit was almost within the range of suggested errors. It can be said, therefore, that the value of K for the reactions does not depend predominantly on the kind of cations Na⁺, Ca²⁺ and Mg²⁺, but depends mainly on both the temperature at which the reactions occur and the N_Si ratios of the reactants.
The micro-inhomogeneity expected for the ionic structure of metasilicate glasses including one or more oxides of sodium, calcium and magnesium in the internal equilibrium state at and above room temperature, can be concluded from the above results as follows: The glasses have the “micro-inhomogeneous structure” containing different types of SiO₄ groups, among which one or more types of SiO₄ groups of linear silicate building units may be included; the structural micro-inhomogeneity of the glasses is increased with temperature; at an given temperature the difference in the structural micro-inhomogeneity may be approximately negligible among the metasilicate glasses of 2-4 component systems including one or more oxides of sodium, calcium and magnesium;

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