TABLE 2. Quenchings which determine the limits of the field of forsterite (2MgO.SiO2); see also Table 9. Phases present, glass + 2MgO.SiO2. Quenchings which determine the limits of the field of clinoenstatite (MgO.SiO2); see also Table 10. Phases present, glass + MgO.SiO2. Quenchings which determine the limits of the field of SiO2; see also Table 11. Phases present, SiO2 + glass. Quenchings which determine the limits of the field of sillimanite (Al,O,.SiO2); see also Table 12. Phases present, glass + Al ̧O.SiO2. TABLE 6. Quenchings which determine limits of field of cordierite Quenchings which determine the limits of the field of Al.O. While it is not our present purpose to discuss each of these fields at length, it may be well to state here certain special facts concerning the fields and their respective crystalline phases. Fields of Stable Compounds. The components MgO, Al2O3, SiO2 and the compounds 2MgO.SiO2, MgO.Al.O, and probably Al2O3.SiO, do not dissociate when melting, therefore the composition of each necessarily lies within its respective field. The accurate determinations of the limits of these fields were in most cases made without difficulty. The limits of the field of MgO and certain portions of the fields of Al2O,, ALO.SiO2, MgO.ALO, and 2MgO.SiO2, however, could not be determined with extreme accuracy for the reason that the temperatures required are beyond the range of the platinum quenching furnace, 1600°. Due to the pecu 3 liar circumstance, however, that the excess glass from compositions containing between 65% Al0, and 85% AlO did not crystallize readily even when slowly cooled from the high temperature (2100) obtained in the iridium furnace, it was possible to obtain Al2O3 (corundum) and spinel" each alone imbedded in glass in certain compositions. This enabled us to determine more accurately than would otherwise have been possible the limits of certain portions of the fields of Al2O, and spinel. Aside from these data the limits of the portions of the fields mentioned which are at temperatures too high for direct study were determined indirectly from certain theoretical reasonings based on experimental evidence. This evidence and theory will be taken up later when we come to consider the boundary curves and quintuple points. Fields of Unstable Compounds. Clinoenstatite, MgO.SiO2, and cordierite have no definite melting points, each being a crystalline substance of such a nature that when pure it dissociates when melting begins, therefore their compositions lie outside of their respective fields. The determinations of the limits of these two fields involved no especial difficulty since each lies wholly within the range of temperatures possible in the platinum quenching furnace. The determination of the composition of such unstable crystalline substances may be, however, a matter of extreme difficulty. Of the two such substances which we have to consider in this ternary system one, MgO.SiO2, is a definite compound of constant composition; the other, cordierite, is affected by solid solution, and is so finegrained when crystallized from a glass at temperature below that at which it begins to melt, that its homegeneity cannot be accurately tested. (See p. 316.) MELTING TEMPERATURES WITHIN THE FIELDS. Of the eight fields of the diagram two have melting temperatures all of which may be determined in the platinum quenching furnace. These are the fields of MgO.SiO2 and the ternary compound (cordierite). 6 Microscopial examination of various iridium furnace melts containing only spinel and a small percentage of glass would seem to indicate that spinel in ternary mixtures takes up a small amount of SiO, in solid solution. This together with the extent of the solid solution between Al2O, and MgO.ALO, in the binary system is indicated in fig. 1. AM. JOUR. SCI.-FOURTH SERIES, VOL. XLV, No. 268.-APRIL, 1918. Part of the melting temperatures of all the other fields, except the field of MgO, may be thus determined. Tables 8-13 show these ascertained melting temperatures for various compositions within each field. From these temperatures and others referred to later the isotherms of fig. 6 were drawn. TABLE 8. Quenchings which determine melting temperatures of various compositions within field A-6-4-3-7-B; Primary phase is (MgO.Al,O,) spinel. Quenchings which determine melting temperatures of various compositions within the field G-6-4-5-F; Primary phase is 2MgO.SiO2. Composition Weight Per cent Temp. Time in Phases present Glass Quenchings which determine melting temperatures of various compositions within field is F-5-1-E; Primary phase is MgO.SiO2. TABLE 11. Quenchings which determine melting temperatures of various compositions within field E-1-2-D; Primary phase is SiO2. Quenchings which determine melting temperatures of various compositions within field D-2-3-7-C; Primary phase is Al2O3.SiO2. Quenchings which determine melting temperatures of various compositions within field 1-2-3-4-5; Primary phase is the ternary |