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53 2                        METALLURGY   OF   CAST   IRON.
the iron comprising- the casting or test bar whose strength, etc., has been recorded. If for each test of all such castings or test bars we had a standard system, we could then by referring to the tests of any mixtures in our own practice which had recorded similar physical qualities in a test bar, be at once in a very favorable position to obtain or produce a similar casting, having like physical qualities. Some might suggest chemical analyses of the castings being recorded in order to give a base for making comparisons and duplication of like castings. This would work admirably in all cases, but of the two methods the physical test is often more economical and practical for adoption by some founders, for the reason, that there are some who can generally conduct physical tests, but who cannot maintain a laboratory with its chemist, or engage outsiders. Even where founders are equipped with laboratories, the physical tests are necessary as a "hand-maid," to tell what is being achieved, and still further argue for the advisability of a standard system of physical tests.
If there were no difference in the "grade" of an iron to make a difference in the hardness, strength, contraction, etc., of mixtures or castings, then we would not require any physical tests, but when we consider mixtures of iron can be made ranging all the way from 600 to 4,000 pounds, with one square inch area bars twelve inches between supports, it plainly illustrates the benefits to be derived by accompanying a casting with tests obtained from the same ladle or iron by means of suitable test bars, whether the strength is obtained by means of transverse or tensile tests to make comparisons.esented upon which to form mixtures, to duplicate fairly the 4t grade " ofed standards seen in the next chapter., if they have not drawn              '1