From the results of model experiment convection current of molten glass the authors tried to analyse the flow mathematically and to introduce the experimetl formula on the convection current.
The temperature of glass in the vertical section at the hot spot t_s, the temperature of glass in the descending flow layer near the side wall t_w, the thickness of the layer δ, the depth of neutral point Z_n, the average velocity of of current at the layer upper than the neutral point v_u were formulated as the equation (3), (6), (18) and (14) respectively.

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The system Al₂O₃-SiO₂ is studied using co-precipitated gels with pH values in the range 5.0-8.0, obtained by the addition of urea to alcohol solution of mixed siliconethylester and aluminiumethylester.
Following the combustion of urea between 500°-600°C, the system appears to be amorphous up to about 1000°C. At this temperature an exothermic reaction occurs and a spinel-type phase is formed. It has various amounts of Si⁴⁺ substituted with Al³⁺ in the position of four co-ordination and its lattice parapeter increases as the Al₂O₃/SiO₂ ratio increases.
Mullite is formed at higher temperatures by the thermal decomposition of the spineltype phase. In the temperature range 1000°-1200°C mullite exhibits two-dimensional grain growth. The exothermic peak of the DTA-curve at about 1200°C is related to the velocity of formation of mullite and at temperatures higher than 1200°C mullite shows a remarkable grain growth. Chemical composition of mullite is not always constant and the possible existence of mullite containing more silica than 3:2 is suggested without saying the existence of the ratio Al₂O₃/SiO₂=3/2-2/1. According to the electron-microscopic studies mullite showed always a needlelike morphology irrespective of the mode of formation.
It is suggested from the measurement of the lattice spacing that the cristobalite formed in the silica-rich gel has some solid solution of alumina and is different from that produced from silica ge lalone.

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It was reported already that the forsterite porcelain containing excess silica has the narrow range of sintering temperatur and the low softening temperature under load, and the present paper concerns with the influence of excess magnesia on the properties.
A mixture of Mg(OH)₂ and SiO₂ (α-quartz) corresponding to the molar ratio of 2.2 MgO: 1.0 SiO₂ was prepared from the materials of high purity.
It was heated with the velocity of 200°C/hr., to various temperatures, kept constant for 15 min., and cooled rapidly.
X-ray analysis was used to identify the minerals in the fired specimens. It was confirmed that the formation of forsterite began at about 1000°C and was completed at 1460°C. The amount of quartz and periclase decreased with temperature; the former disappeared at about 1400°C, while the latter remained even at the highest temperature of the heat treatment. The inversion of quartz occured at 1240°C, but it disappeared at 1460°C.
The specimen clacined previously at 1460°C and mixed with 5% kaolin could be sintered between 1380°-1500°C.
By X-ray method only the existence of forsterite was detected from the sintered body. The dielectric loss of this specimen was very small.
Forsterite porcelains made from sea water magnesia and silicious stone were also studied; six batches having the compositions, MgO 57-70, SiO₂ 42-30% were calcined until the X-ray proved the termination of the forsterite forming reaction. They were ground and mixed with 5% of kaolin. The sintering temperatures were 1340°-1520°C, increasing with the increase of MgO.
The coefficient of thermal expansion, the thermal conductivity, and the thermal shock resistance have increased with the increase of MgO- content. The trend of the softening temperature was reflected the relations in the phase diagram. The bending strength became lower with increasing MgO, which presumably came from the effect of the aggregation of forsterite crystals.
Although X-ray failed to detect the periclase clystals in the specimens containing 58-62% MgO, it is highly probable that the mineral composition are the same with the forsterite porcelain which does not contain excess of MgO.
These two, however, showed some difference in the properties; especially, the softening temperature under load of the former was considerably higher than that of the latter. Microscopic observation showed that the excess MgO was disolved in the glass matrix, and made the refractive index higher. Moreover, such a texture may be considered as being responsible for the higher softening temperature under load.
The electrical properties of the specimens made from pure chemical as well as natural raw materials were excellent.
It was concluded that the forsterite porcelain having the base composition of 58-62% MgO are the best for using as the vacuum tube.

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