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equal. PYROMETRY. Undoubtedly the most convenient pyrometer for kilns is the Seger cone. Heintz suggested in 1886 that glass mixtures of varying fusibility would make better pyro- meters than the alloys made by Prinsep in 1828. Seger took the matter up and made a series of cones which, revised, 70 CERAMIC CHEMISTRY. serve all practical purposes. Seger instituted cones 1 to 35, of which 4 to 35 consisted of clay, quartz and spar or whiting, while in 1 to 3 iron oxide was used as flux. Then Cramer prepared the more fusible series 01 to 010 by adding boric acid, and Hecht extended them down to 022 with the aid of lead oxide. These cones, as left by Seger, were subject to two serious inaccuracies : cones of the same number did not always go down together, and the grada- tions in temperature between consecutive numbers was not uniform. However, Simonis calibrated them in 1908, and the official Berlin cones may now be regarded as fairly reliable. But those containing iron oxide are dubious in action if heated quickly or in a reducing atmosphere. Cone 1 melts at 1,150 C., 2 at 1,170, and so on by intervals of 20 up to 36 (1,850 C.). Then the downward series goes from 0.01 to 010 (1,130 to 950), and thence by grada- tions of 30 to 022 (590). Cones should be placed in various parts of the kiln, protected from direct contact with the flame and from