For most practical applications polycrystalline barium titanate ceramics are more important than those of single crystal, and recently, some efforts have been made to explain the phenomena concerning the microstructure of polycrystalline materials, for the purpose of finding out the method of preparation which guarantees good reproducible properties.
Micro- and domain structures of the specimens, whose mole ratios BaO/TiO₂ varies from 0.990:1000 to 1.020 1.000, and fired at the temperatures ranging from 1350-1460°C, were polished and etched by the techniques suitable for the polycrystalline titanates, and were examined under microscope.
The results obtained are summarized as follows:
(1) Barium titanate ceramics containing excess BaO have fine grain structure with an increase in number of void. The specimens fired up to 1460°C and cooled slowly gave an extrmely coarse-grain structure.
(2) Excess TiO₂ was found to be more effective than BaO for the grain growth, probably, by the mineralizing action of the former.
(3) Domain structure of barium titanate ceramics assumed many forms, in which, square net and herrung bone pattern were most distinguished, and the former was more flequently observed in the specimens containing excess of TiO₂.

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Glass rods for lamp and tube of about 4mm in diameter were provided with two electrodes at the distance of 30-40mm, and the apparent resistance, which is equivalent to the combination of volume and surface resistance connected pararell, were measured by the method of filament electrometer or direct deflection.
In the first series of the experiment the specimens were washed by loundry soap and city water. In the season of high humidity (summer) the apparent resistance has changed after the preparation of the specimens showing a clear corelation with relative humidity. After half a year, and in the season of low humidity (winter) some glasses showed comparatively high surface resistance, keeping the apparent resistance at a constant level, being very near to the volume resestance. In the glasses of higher volume resistance, however, the change of surface resistance due to the change of humidity was clearly observed. After a year a lot of specimens gave the apparent resistance of as high as 10-100 times under the same atmospheric conditions.
In the second series of the experiments the apparent resistance of the fresh specimen was compared with those aged a year and those treated with dilute hydrofluoric acid. It was observed that the apparent resistances of above three kinds of the specimens showed the similar trend of the change with the atmospheric conditions, and the initially quite different of numerical values came up to one and the same value with the laps of time.
In the last series of experiments the apparent resistance of the specimens washed with, acid, alkali and salts was measured in the cell, to which, after evacuation, water vapour was introduced up to a certain saturation pressure. It was confirmed that the characteristic properties of apparent resistance vs. vapour pressure varied remarkably with the method of surface treatment.

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Mullite is studied using co-precipitated gels obtained from mixed silicon ethylate and aluminium ethylate. Mullite is formed at higher temperatures by the thermal decomposition of the spinel-type phase. In the temperature range 1000°-1200°C mullite exhibites twodimensional grain growth in accordance with the equation dα/dt=Kα(1-α), where t=time, α=amount of mullite. An activation energy of 11-12kcal/mol is obtained. At temperatures higher than 1200°C, mullite shows a remarkable grain growth.
The lattice parameters a and c of mullite decrease, and the parameter b increases, when the temperature of heat treatment is increased. Also a is increased and b is decreased as the Al₂O₃/SiO₂ ratio increases beyond 3:2. The c parameter shows little change with Al₂O₃/SiO₂ ratio in the range 3:2 to 2:1. The time variations of a and c are thought to be due mainly to the chemical composition, and the time variation of b due mainly to a process of ordering of the lattice. The diagram of c/a-a/b shows clearly the influence of chemical composition, and suggests the possible existence of mullite containing more silica than 3:2.
According to the electron-microscopic studies, mullite showed always a needle-like morphology irrespective of the mode of formation.

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