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The series of mixtures of thoria and ceria, in which is included the \Velsbach man,tle of commerce, is in many respe,cts the most interesting of the mixtures studied. In it are exhibited in striking manner the characteristics of an optimunl proportion of the constituents, a fundamental difference in the optical properties of the hot and cold mantle, and a greatly different behavior under different kinds of heating. A full discussion of this series therefore covers many of the characteristics found in others, which can, in their turn, be described briefly in terms of their likeness or unlikeness to the thoria-ceria mixtures. Gross Ph%,sical Characteristics. Pure ihorium oxide,"ThO2, is white, of high reflecting power (8 5 per cent.), and does not show any color change on heating in a quartz tube in the Bunsen flame. Pure cerium oxide (CeO~) is either white or brownis,h white, depending on the method of preparation. Both varieties turn deep yellow or brown, in heating to a few hundred ~legrees, a characteristic which persis,ts in the mixtures of ceria with thoria to very low percen, tages of the former. It is exhibited clearly in the regular commercial mantle (approximately I per cent. ceria) when this is turned low. * Based on a lecture delivered by Doctor Ives at a meeting of the Section

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