Studies of the gas-phase reaction of water vapor and parts-per-million concentrations of nitrogen dioxide were made at atmospheric pressure and near ambient temperatures. Of special interest in the work was careful chemical analysis-water to values less than 2 ppm and nitric oxide and nitrogen dioxide to values less than 5 ppm-and absence of mist. The initial rate of disappearance of nitrogen dioxide was first order with respect to water and second order with respect to nitrogen dioxide. Nitric oxide and nitric acid were the principal products. A third-order rate constant of 5.50 (±0.29) X 104 I.2 mol-2 sec"1 was found at 25°C. Results of the work are applicable to air pollution studies and to the understanding of the mechanism for the absorption of nitrogen dioxide into aqueous solution.

The Influence of moisture on the thermal degradation of wood waste was Investigated by pyrolyzlng ground samples in a batch fluid-bed reactor at between 320 and 470 °C in helium at 101-104 kPa. The wood samples were heated at about 300 °C/ min, so that drying and pyrolysis were simultaneous. The presence of moisture increased the yield of char by as much as 5 yield %, relative to dry samples. Moisture suppressed tar formation from demineralized samples at 470 °C but enhanced the formation of tar above and below 390 °C from samples containing ash. Ash and added calcium catalyzed the conversion of tar into aqueous product, while moisture interacted with tar formation or trapping reactions to Increase the yield of char. Moisture also modulated the activity of the ash components for the conversion of tar into aqueous product. The native mineral components in the wood waste gave effects equivalent to ion-exchanged calcium.

Houston, Texas 77004. Dr. Attar obtained his Ph.D. in Chemical and Environmental Engineering from the California Institute of Technology, and his B.Sc. and M.Sc. from the Technion, Haifa, Israel. Dr. Attar is an active researcher and consultant on coal desulfurization, liquefaction, and gasification.

Thrombus formation and tissue overgrowth were observed in nine Björk-Shiley aortic prostheses recovered six months or longer after implantation. These pathologic findings may be attributed to the flow characteristics of the prosthesis. The open disc of the valve separates the flow into two unequal regions. Varying degrees of thrombus formation were observed in the minor outflow region, including the depression in the aortic face of the disc and the metal strut bridging this area. Tissue overgrowth was noted along the perimeter of the prosthesis adjacent to the minor outflow region. That overgrowth further reduced the available cross section for flow in this already constrained area. In vitro velocity measurements with a laser-Doppler anemometer identified a zone of stagnation about 20 mm wide near the aortic face of the disc. The average velocities in the major and minor outflow regions were around 100 and 25 cm/sec, respectively, and the corresponding peak-shear stresses were approximately 700 and 150 dynes/cm2. There is reason, then, to attribute the thrombus formation and tissue overgrowth to the stagnation zone and the low shear in the minor outflow region.

The presence of the S02-radical ion in aqueous solutions of sodium dithionite has been previously reported. 1 Electron spin resonance (e.s.r.) measurements showed that the S02-concentration varied as the square root of the dithionite concentration, suggesting a monomerization reaction of the form fe» s2o4-2 2S02-(i) Ad This reaction has received significant polarographic study by Cermák2 and chronopotentiometric studies by Fischer8 and co-workers. Based on experimental observation in a 0.1 M solution of sodium hydroxide, Cermák reported a value of 6 X 10-8 mole72 l.-7' sec.-1 at 40°for the quantity km/ where km is