This paper discusses the recent development of rotary percussive drilling specially with reference to the relations between thrust, revolution per minute, torque and penetration rate. Sizing tests of cuttings and theoretical considerations were made to analyze the drilling mechanism of rotary-percussive drilling.
The followings are brief statements of conclusions.
1. Penetration rate is increased in accordance with the increasing thrust. The above relation is linear in the case of rotary percussive drilling but it shows parabolic curve in the case of rotary drilling. When thrust reaches a certain point (in this experiment, 170 to 190 kg. in thrust), both system of drilling has same penetration rate. If the thrust is increased over this point, rotary drilling will show higher penetration rate, on the contrary, rotary percussive drilling shows better results under lower thrust.
2. Penetration rate is increased in accordance with the increasing rotation specially in rotary drilling. This increasing rate is, however, pretty small in the case of rotary percussive drilling when number of percussion is remained constant. Then, optimum rotation of rotary percussive drilling would be 200 r. p. m. in this-experiments. From above mentioned standpoint there would be optimum r. p. m. in relation with each number of percussion in the case of rotary percussive drilling.
3. Large cuttings are seen when thrust is increased. Increasing rate is, however, big in rotary drilling. Also better petter penetration rate is obtained when revolution per minute is increased but size of cuttings is getting smaller and smaller.
4. Essencial power for drilling is generally big in rotary percussive drilling needing the power for impact. Rotary percussive drill will need 1.5 to 2 times big power for rotation compared with conventional rotary drill.

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In previous paper, the author culculated stress distribution around the two tunnels, which excavated horizontally parallel and the other case vertically doubled.
In this paper, the author dealt with again the same problem and tried to find the stress distribution in the following case:
(1) two circular tunnels have the same radius, but not excavated horizontally parallel or vertically doubled.
(2) two circular tunnels have not the same radius, one is large, the other small. In this paper, we can see the great compressive stress at the near side of those two tunnels.

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The principle of the method of the quantitative X-ray diffraction analysis, and the errors which may happen in the process of analysis were mentioned.
The relations between the particle-size of quartz powder and the reproducibility of X-ray diffracted intensities were examined both in enough amount and in submicro amount of samples. And we made the. calibration curve of quartz (about 2μ) in submicro amount of samples (total weight including the internal standard material: about 2mg.), and estimated the errors in these cases.
This X-ray diffraction analysis method is “Internal Standard Method” using fluorite powder as the internal' standard material and calcite powder as the diluent for quartz (in our case, quartz+calcite=about lmg.). The measurements of the X-ray diffracted intensity-ratio of quartz to fluorite were performed by the Fixed Counts Method (FCM).
From the result by the FCM (total counts: 30, 000) we could recognize the extent of the error may be about ±5%.
But the X-ray diffracted intensity of quartz differs from the crystallite- or particle-size, and on determining of quartz percent in an unknown sample, therfore, if we do not consider of the particle-size and not compensate the intensity, the result obtained just using our calibration curve of quartz (2μ) may give a larger error. The studies on this point may be performed later.

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Separation of rock crystal from silicate sands which is a mixture of feldspar, augite, hornblende and andesite by using a inductive singleroll type electrostatic separator has not been made public, because of many difficulties with respect to their electric properties, e. g. the lack of their surface electric conductivity and the similarity in dielectric constant.
The author has developed the idea of preconditioning the surface of sand particles with a solution of electrolytes, and the following results were obtained.
1) The separation of rock crystal from other silicate sands can be accomplished by preconditioning tnem with dilute water solution (2-4%) of electrolyte such as NaCl, KCl, MgCl₂, or an artificial sea-water. In preconditioning sands are immersed in electrolyte solution for a while, filtered, washed by pure water softly and then dried. As a result of this procedure an electro-conductive tiim is rerainea seiectiveiy on the surface of gangue particles, . while such a film is hardly held on the surface of rock crystal particles owing to their smoothness.
2) Quantity of electrolyte abstracted by unit weight of sands increases with the concentration or eiectrulyte solution, being constant in mol with the same kind of sand constituents.
3) As an example, in treating the sands of 1.2-1.6 mm in size, preconditioned by 2 percent NaCl solution, the 50-70 percent of rock crystal was recovered with the purity of 99.69 percent SiO₂, by keeping negative pole at 16, 800 volts and 22.5mm apart from grounded charging roll.

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The authors have investigated on the physical and chemical properties of galena produced from typical lead-zinc mines in Japan. Differential thermal analyses, thermal balance measurements, x-ray study and measurement of electrical properties were carried out. It was found that these properties are influenced by minor impurities. The main results obtained are as follows:
(1) Differential thermal analysis curves showed two exothermic reaction at 770°C and 840°C, and a slight endothermic reaction at 850°-855°C.
(2) According to the thermal balance measurement, weight began to increase at 310°C and changed to decrease at 750°C, a little inflection was observed on the decreasing curves at 850°-900°C.
(3) Besides lead sulfide, lead sulfate, lead oxide and metallic lead were determined in the heated samples by x-ray spectrometer.
(4) Because of minor impurities, exothermic reaction caused by oxidation of disordered samples began at lower temperature than that of ideal sample.
(5) Semiconductivity of galena depends on its minor impurities, especially silver and bismuth.

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