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The Niobium-Niobium Carbide System 1471 identically to the test samples showed that free molybdenum and tungsten were indeed present after the first visible melting.The periodic variation of the minimum solidus temperature may be seen in Fig. 1, where the ratio Ts/Tm from Table V is plotted as a function of the group of the periodic table. Group VII exhibits a minimum.A chemical analysis of the NbC melting point cylinder gave a empirical formula of NbCo-ses, with free carbon not detectable.Acknowledgments.-The authors thank Dr. E. VanKooten and Mr. G. C. Heasley for the chemical analyses, Messrs. O. R Simi and J. A. Mariner for the spectrographic analyses, and Mr. C. G. Hoffman for the metallographic examinations.