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Transformation of Quartz to Cristobalite 35 of obtaining an understanding of processes of this kind is by detailed microscopic studies. Observation of many microstructures leads one to the conclusion that a variety of different effects occur in different systems and that the definition of the terminal stages of sintering as a "coalescence" process is a gross oversimplification. V. Summary and ConclusionsExperimental observations of the sintering rate and resulting microstructures for a number of systems indicate that solution and reprecipitation does not take place for specimens of Al&silicate glass, in which the high viscosity of the liquid phase prevents appreciable rates of solution. Systems such as this are best described in terms of the Mackenzie-Shuttleworth theory .'Microstructures of CoO-B&, forsterite-talc, forsteritekaolin, MgO-kaolin, WC-Co, and Tic-Ni indicate that these materials densify by a solution-precipitation process in which the liquid phase must wet the solid and penetrate between grains, must have some solubility for the solid, and must be fluid enough for solution and precipitation to take place rapidly. In these cases rapid densification, spheroidal or prismatic grain development, and high densities with a moderate liquid content result.The time and particle-size dependence of the sintering rate for MgO-kaolin, MgO-eutectic liquid, WC-CO, and Tic-Ni is found to be in good agreement with theoretical expectations and previous experimental results for metal systems.Observation of microstructures indicates that the termination of sintering can result from many different causes in addition to the coalescence of solid grains. It is best to refer to this stage as a "terminal" stage of densification rather than to imply that coalescence is the only process that can occur. AcknowledgmentThe transformation of quartz to cristobalite was studied in the temperature range 1400' to 1650' C. The process which was believed to be a direct one was found to be a consecutive reaction involving an intermediate transition phase. Both Madagascar and Brazilian quartz were used to study the transformation reaction. The reaction rate was found to be much faster for the Madagascar quartz than for the Brazilian quartz, although no Merence in their impurity content was found in the spectrographic analysis. An approximate consecutive reaction kinetics was applied and the deviation of the ezperimentally observed values of the transition phase at a particular temperature from that of the mathematical prediction was noted and is discussed.

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The effects of chloropicrin fumigation of field soils before replanting apple are frequently predicted from bio-assays using pot-grown apple seedlings. The results of 506 such bio-assays carried out during 12 years are discussed with reference to modifications which were made to the test method. The interpretation of the most recent and valid bio-assay results is considered in relation to the different purposes for which such tests may be conducted.When all test soils were amended with phosphorus (to compensate for the eradication of vesicular-arbuscular mycorrhizal fungi by fumigation) the proportion of old orchard soils yielding economically significant growth responses increased from 39% to 68%. When, additionally, plant growth was assessed by shoot fresh weight (rather than shoot length) the proportion further increased to 82%.The traditional method of expressing growth responses to soil sterilisation, as proportionate growth increases, was found to be subject to skewing effects of non-biological fertility factors; this feature was avoided when results were given as actual growth increases. The latter value therefore is considered preferable when assessing the relative disease status of soils for etiological purposes (as opposed to predicting the cost-benefits of field soil treatment).The data indicated that disease severity was on average diminished in the most acidic soils (pH 4.8-6.1), but did not support the generalisation that disease severity increases progressively with soil pH.

Recent investigations into the mineralogical constitution of fireclays have revealed the presence of a micaceous mineral in all samples examined. Naturally-occurring clays may contain up to 30 % of this type of mineral, and exceptional concentrations have been found in the Yorkshire area. Tests on the purified mineral from this source have proved it to be a hydrous mica of an unusually large grain,size.