The author and Mr. Kumahiko Hasegawa have described on the relation between the natural and artificial iron oxides which were reduced with H₂, CO, and coal gases, and their magnetic properties. It is also discussed that the magnetic properties of the reduced iron oxide heated in air and vacuum are variable.
The present paper follows the previous one, and the following items are described as the many experimental results.
1. Measurement of permeability by ballistic galvanometer.
2. The permeabilities of Fe₃O₄ and FeO.
3. The relation between the permeability of iron oxide and field intensity.
4. The relations between vacuum heating of the natural and reduced iron oxides, and their permeabilities.
5. The relations between the FeO contents of the natural and reduced iron oxides, and their permeabilities.
6. The state of combination of iron with oxygen and solid solution between iron oxides.
The maghetic permeabilities of Fe₃O₄ and FeO are determmed as 6.27 and 2.43 respectively.
After vacuum heating of the samples at above 600°C, though chemical compositions of the iron oxides do not alter, their permeabilities show the remarkable increase, and the relations between the permeabilities of the reduced iron oxides and the field intensities approach gradually to those of natural magnetic oxide.
Then by the vacuum heatings of reduced and natural iron oxides, there are interesting tendencies between the Feo contents and permeabilities. Any sample approaches gradually to the theoretical combined state of FeO and Fe₂O₃, namely Fe₃O₄, after the treatments.
Estimating the combined states of oxygen from the above relation, the natural and reduced iron oxides consist partly of the cubic form of Fe₃O₄, and partly of Fe₂O₃+Fe₃O₄, Fe₃O₄+Fe₂O₃, Fe₃O₄+FeO, FeO+Fe₃O₄, and FeO+Fe at the state of solid solution, and FeO in free state.
By vacuum heating of iron oxide, the cubic form of Fe₃O₄ are disintegrated from these solid solutions and their permeabilities are increased.

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Recently a new testing method for an opaque body called "Sump" was invented by Mr. J. Suzuki, the spelling of which came from the capitals of Suzuki's Universal Micro Printing. In this method we require no test pieces cut from body as in ordinary microscopic examination, but only use celluloi, giving some plasticity with a solvent like amil acetate, and directly stick it tightly on any surface. Soon after we tear it off from the body and bring it under a common simple microscope getting a precise micro-structure very clearly.
The writers tried to apply this method to iron and steel and designed a small portable box which contained in good order various et hing and cleaning reagents etc., and went out to many works with this box and with a small grinder to take various Sump printings.
In the first time we studied the technical skill of gaining clear micro-structures for steels of carbon 0.06 0.24 0.43 0.60 % to find that the polishing degree of test surface is almost the same with common microscope samples but requires some deeper etching to get clear structures of their ferrite and pearlite etc, and even for quenched steel we could see clearly in Sump printing those fine structures of martensite and troostite etc. in 500 magnification, and on the other hand various flaws of steel were also revealed distinctly by this printing. Next, for grey and white cast irons we could find their structure and small blow holes more easily, and especially in the former the difference of graphites and holes which has been very difficult to distinguish in ordinary metal microscope, appeared very clearly as graphites were white, holes were black, and is very interesting for cast iron investigators. Furthermore we succeeded to apply this method on the testing of defects, structures, or homogeneity of heavy materials e.g. chilled iron rolls etc.
Thus in this method we could prove the possibility of examining and studying the structure, cracks etc. very simply for iron and steel without any distinction of their size and form, with no fear of spoiling them.
Therefore the application of this new method will be very wide in all sides in future not only in iron and steel but also probably in all kinds of other metals.

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