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344                      METALLURGY   OF   CAST   IRON.
able conditions because of its being possible to keep the carbon lower and the better to add other metals, as Spiegel and ferro-manganese, which alloy with the fluid metal without having their original properties destroyed to any great degree. Tensile strengths ranging from 45,000 to 50,000 pounds per square inch have been obtained by air furnace meltings with mixtures of iron, steel, etc., but to obtain castings equal to those of steel proper .we must have them cast by regular steel founders. Whenever we desire to improve the strength of cast iron by mixture with steel, the lower carbon or soft steels will be found to give the best results, and air furnace meltings excel those of a cupola, especially if charcoal irons are used. In mixtures with'the latter, from 15 to 30 percent, of soft steel scrap may often be advantageously used. For further information on the steel question, see pages 265, 267, 271, 272 and 276, and the " Moulder's Text-Book."
The following is an extract of a valuable paper which was presented by Dr. Richard Moldenke before the Pittsburg Foundrymen's Association, Oct. 24, 1898. This extract gives a description of the pyrometer which the doctor used for testing the temperature of molten metal, etc., and of its value in other lines, also of tests he made as found in Tables 77 to 81. In looking about for a pyrometer, the doctor's attention was naturally directed to the latest and admittedly the best form of a pyrometer for very high temperatures  the Le Chatelier. In referring to this instrument and to his tests, the doctor says: " This pyrometer consists essen-verberatory or air furnace, in mixture with cast iron, we have more favor-the metal was held in the cupola but a short time, compared to that generally occupied in ordinary shop practice. The longer heated or semi-molten iron remains in contact with incandescent fuel or is exposed to gases, the more sulphur will be absorbed  up to the limit of the iron'sding   its mate..	2111 3Os	i m.	mi 3os	3OS.	3111.	im ^os	Jill   l^S	i in.