Varying Oxygen Pressure Research Articles

Sintering of Zinc Oxide

The fundamental processes involved in the sintering of ionic solids are highly complex. Of particular significance are the initial stages of sintering during which surface effects are of prime importance. The sintering of zinc oxide has been studied by the method of changes in surface area at temperatures from 300° to 1000°C. and with varying oxygen pressures (from 10‐5 to 760 mm. of Hg). It has been demonstrated that significant sintering occurs at temperatures far below the Tammann temperature. It also has been shown that under specific conditions a very high defect concentration may exist in the surface of the material and that this is closely related to the phenomenon of sintering. Correlation has been made with an electron‐microscope investigation. Detailed X‐ray studies on single‐crystal specimens of zinc oxide have shown that interstitial zinc constitutes the defects. Activation energies have been deduced for the sintering process which suggest that the rate‐controlling step is diffusion in the surface zone in the low‐temperature region which changes to lattice or complex diffusion and viscous‐flow mechanisms about the Tammann temperature.

Streptothrix Interproximalis. N. Sp. an Obligate Microaerophile from the Human Mouth

Isolation.-Materia alba from the gingival and interproximal portions of the central lower incisors was emulsified in sterile water, by very diligent shaking. Proportionate amounts of this emulsion were inoculated into tubes of equal parts of melted Martin's agar and pleuritic fluid, and the tubes carefully manipulated to make roll tubes. These tubes were then connected by airtight rubber tubing, to cultures of freshly inoculated B. subtilis, and grown at 35 C. (average mouth temperature) for 5 days when they were opened for examination. A small, round, white, elevated colony was fished and transplanted to slants of the same medium, and grown aerobically, anaerobically and at partial tension. Very slowly, during 5 days, the partial tension culture grew, to form white, slightly raised, rather dry but tenacious colonies. The aerobic and anaerobic tubes gave absolutely no growth. Subcultures were made on the same medium, and grown under the three varying oxygen pressures, with the uniform result that the partial tension tube gave a growth, while the aerobic and anaerobic tubes were consistently without growth. The colonies were practically invisible until the 4th day, and attained their maximum growth by the 5th or 6th day. Six consecutive subcultures were made, at the end of 5 days each, from the partial tension tubes, when finally the aerobic tube showed a faint growth. This tube, however, was tightly capped with a rubber hood to prevent drying of mediums, and in this manner a somewhat reduced oxygen pressure may have been produced. This aerobic strain was carried through 2 subcultures, when it died out.