In previous reports a method of spray drying was developed which produces the fine spherical particles. From the statistical point of view the effects of slip concentration, atomizing velocity and the shape of rotating disc on the distribution of the particle size were studied. It was revealed that the spherical form of the particles thus obtained gives the uniform density distribution in the green body formed by dry pressing and improves considerably the mechanical properties such as hardness.
The present paper contains the results of the studies on the effects of grading of the particles on the hardness distribution in the bodies fired at 1200°-1300C. The relation between the hardness and the density of green body was found to be represented by empirical formulae;
ρ_A=1.35+0.0175R^-1
ρ_B=1.22+0.0193R^-1,
where ρ_A and ρ_B stand for, respectively, the densities of the particles of the irregular and the spherical shape, and R^-1 is the scratch hardness.
For the packing of the particles of uniform size whose mean radius is γ_p the following relations were established;
O₀=0.29-15.0×10^-4γ_p
O_c=0.24-17.1×10^-4γ_p
U₀=0.26-8.4×10^-4γ_p
U_c=0.26-18.2×10^-4γ_p
48<γ_p<88μ,
where, O₀ and O_c are, respectively, the hardness at the perifery and at the center of the end surface of the cylindrical specimen at which the pressure was applied, and U₀ and U_c are, respectively, the hardness at the corresponding points of another end surface.
In the case of mixed powders the condition of uniform packing does not always coincide with that of closest packing, namely, the maximum of the mean density of the green body was confirmed to occur at the size distribution of 20% 88, 20% 62 and 60% 48, while the best condition for the uniform packing would be with the mixture of 50% 88μ and 50% 66μ (Fig. 13, 20).
The hardness distribution of fired body seems to be governed predominately by the degree of sintering; when fired at 1200°C the original distribution in the green body disappeared, and at 1300°C the hardness decreased greatly owing to the evolution of gases in the body, the trend has been noted especially with the finer particles.
This decrease of hardness is considered to be due to the fact that the composition change in each particle has occured as the result of the centrifugal separation in the process of the atomizing of the slip. 20 Fig., 13 Refs.

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The stain and frost which are formed at the surface of some lenses or prisms give the difflculties in using the glass in optical system. They are the abnormal surface layer formed by the surface chemical change.
The authors have carried out the investigations on the nature of the surface layer formed by the attack of gases and liquids on the two kinds of heavy crown glasses, one having high, and the other low chemical durability. The Chemical agent used for attacking the glass surface were; i) water vapor, ii) wet CO₂ gas, iii) wet HCl gas, iv) wet SO₂ gas, v) 1N-HCl solution, vi) 1N-H₃PO₄ solution, vii) 1N-Na₂CO₃ solution, viii) 1N-NH₄Cl solution.
In order to unveil the nature of the surface layer and to know how it was formed the authors have used; a) the observation of the glass surface by phase microscope, b) the measurement of haze value, c) investigation by electron diffraction, d) observation of dew-drops on the surface, e) growth of mould, f) observation of the surface with an electron microscope.
The results of the investigations are summarized as follows: It was revealed that the frost is formed by the attack of gases on the glass surface forming, respectively, Ba(OH)₂. 8H₂O or BaCO₃ by water vapor, BaCO₃ by CO₂, BaCl₂⋅2H₂O by HCl, BaSO₄ by SO₂. In this case the haze per cent is comperatively large.
By the attack of liquid a thin film having the structure of α-quartz mixed partly with Ba-salt is formed. In this case the haze per cent does not become large although interferrence color is observed. It was concluded that the stain of glass surface is observed when a SiO₂-rich film is formed at the surface after the metallic ions were leached out.

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The characteristics of a newly installed vacuum high-frequency induction furnace used for high-temperature experiments are described. A 10, 000 cycle per second motorgenerator of 15kVA output was used as the power source. The heating system, including a water-cooled primary coil 20cm in diameter, a heat insulator and a graphite susceptor 12cm in outer diameter, 14cm high, and 2cm in wall thickness, was placed in a stainless steel vacuum vessel (70cm in inner diameter and 60cm in length) having a steel water jacket.
Graphite powder of 150-200 mesh/in. was used as the heat insulator. A graphite crucible 8cm in outer diameter and 12cm high, set in the graphite suscepter surrounded with a graphite powder insulating layer of 3cm, could be heated up to 3000°C in two hours using a power input of about 7kW. When the graphite crucible itself was used as the susceptor, the insulating layer was 5cm in thickness and the time of heating up to 3000°C was reduced to about one hour with the same power input. When the temperature of the crucible was raised to 2600°C or above, the vaporization of carbon became so noticeable that the fine particles of carbon blew out from the inside of the crucible and carbon vapor deposited in laminar form on the wall of the hole in the lid of the crucible.

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[in Japanese]

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