Coral limestone resourses are very abundant in the Amami Islands, though very little have ever been reported on them. The object of this paper is to describe the results of the writers' works on the fundamental properties of three kinds of the “Ryukyu Coral limestone”, namely those produced in Amami-Oshima, Okinoerabu-shima, and Yoron-shima.
Chemical analysis, determination of specific gravity, various thermal tests, X-ray analysis etc. were conducted on these coral limestones and compared with that produced in Shiraishi, Kumamoto prefecture. Moreover, differential thermal analysis, X-ray analysis, slaking test and sedimentation test were made on these coral limestones calcined at various temperatures, with the following results.
(1) The Ryukyu coral limestones are mainly composed of calcite with the following physical properties and chemical compositions: true density 2.68-2.74, porosity 6.31-23.41%, water absorption 2.51-11.81%, CaO 52.30-54.65%, MgO 0.37-1.62%, Al₂O₃+Fe₂O₃ 0.37-1.78%.
(2) From differential thermal analysis and powder X-ray diffraction, the decomposition of these coral limestone begins from about 700°C and comes to end at about 900°C.
(3) When burnt to 1000°C, the coefficient of maximum expansion of all three coral limestones was 0.7-2.3%, while that of Kumamoto limestone showed a lower value, 0.6%.
(4) The effect of calcing temperature on the slaking velocity has been observed both on the powder and on the block samples of the coral limestones. The results show that the calcination at 900°C is suitable for the powder specimens, while that at 1000°C is desirable for the block specimens.

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In his previous papers, the writer has shown the presence of diffusion layers around the quartz grains which were under the course of dissolution into the feldspathic fusion. In order to make clear the actual mater of the layers, microscopical measurements were made as reported in this paper. On the glass formed around the remaining quartz grain, as shown in Fig. 1, the refractive index, n, and distance from the quartz surface, t, were measured microscopically, as many as possible, and then these data were plotted.
From the diagrams (Figs. 2 and 3) thus obtained, the following results are concluded.
(1) Gradients of refractive index are found around the quartz grains. In the quartz-feldspar system, refractive indices of glasses formed are functions mostly of the amounts of quartz dissolved, i.e., the concentration of SiO₂, so that the presence of refractive index gradient correspond to the presence of concentration gradient of SiO₂, suggesting the formation of the diffusion layers around the quartz grains.
(2) In the fusion of potash feldspar, the diffusion layers are continuous and of linear gradient, while in the fusion of soda feldspar, there appeared temporarily two layers s howing a discontinuity; but in the course of prolonged soaking time this discontinuity is gradually diminished, and they become one layer.
(3) The higher the holding temperature is, the thiner the diffusion layer becomes.
(4) In the stationary state, the thickness of diffusion layers formed in the potash feldspar fusion is of the same order with the one formed in the soda feldspar fusion, only the former being somewhate thicker.
(5) The temporary formation of the double diffusion layers in the fusion of soda feldspar, as described in (2), may due to the singular fusion of the soda feldspar (plagioclase) used, by which the formation of melt is fast in earlier stage and then much slower down at a certain temperature range.
(6) Furthermore, it is inferred that the dissolution velocity of quartz grains into the feldspathic fusion is determined by the diffusion velocity of SiO₂ in the diffusion layers.

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The change of the viscosity of glass with time at constant temperature was analyzed by applying the concept of the network temperature (or the fictive temperature), and the annealing as well as several other problems on glass related to its viscosity were discussed.
The discussion was based on the following equation which represents the first approximation of the network temperature:
dτ/dt=K(T)(T-τ)…(1)
where K(T)=K₀e^-g/T, and τ is the network temperature which is essentially equal to the fictive temperature, T is the absolute temperature, t is the time, and K₀ and g are certain constants.
Integrating above equation, we get
logτ-T/τ₀-T=-K(T)(t-t₀)…(2)
which means that τ=τ₀ when t=t₀
The values of K(T) for various temperatures were calculated by applying equation (2) to the experimental data given by H. R. Lillie on the time dependence of the viscosity of glass, and it was found that K(T) changed linearly with 1/T
Therefore, it was concluded that the change of the viscosity of glass with time at constant temperature takes place, as the first approximation, according to the equation (2).
On the other hand, the following equation was derived:
K(T)(t_s-t₀)=C₀ (3)
where t_s is the time at which the value of viscosity reaches its equilibrium value, and C₀ is a constant chosen properly.
t_s-curve, which is obtained by connecting the points corresponding to the time t_s on a number of experimentally obtained viscosity vs. time curves, may be given by experimentally.
Following several interesting conclusions were derived by combining the concept of the fictive temperature with the t_s-curve:
(1) Careful annealing of glass is the heat treatment to make its fictive temperature, maintained before the heat treatment, approach as near as possible to the room temperature.
(2) The fact that different annealing schedules, particularly those for the cooling rate, are necessary for different kinds of glasses is caused by the different values of K(T) given by the following equation for these glasses:
-dT/dt=K(T)ε=C₀ε/t_s-t₀…(4)
where ε is a constant and t_s is the time on the t_s-curve at a certain temperature.
(3) To anneal a glass within the shortest time, it should be kept at the temperature T_m given by the following equation:
g(τ-T_m)-T_m²=0…(5)
(4) The experimental results given by H. R. Lillie on the change of the viscosity of glass with continuously raising temperature may also be explained reasonably by the concept of the fictive tempereature and the t_s-curve.

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