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£34                     METALLURGY OF CAST IRON.
all of them, the same size bar must be used. One size bar cannot be used for one per cent, silicon iron and then dropped and another taken up to test percentages above or below this. (See Chapter LXVIL, page 520.) Whatever size of a common sense bar the testers use, in making comparison through any range of work, they must stick to that one, and then, if they desire to make comparison with outside records that have been obtained with standard bars other than the one size they use, they would then be compelled to make tests with the same size of bars which was used to obtain the outside test. Of course, if a firm desired, they could cast the three sizes of bars together, mentioned on page 533, with the same ladle of iron, and thus always have at hand records by which they could make comparisons on a moment's notice, with any outside tests that had been obtained with either of the three standard sizes of bars mentioned herein.*
The following Tables, 108 to 113, pages 536 and 537, display tests of the author's proposed three sizes of standard bars, accompanied with a chemical analysis of the various mixtures shown to still increase their value. A study of these Tables (combined with those of Chapter LX., page 460), the author believes, will sustain him in his advocacy of the i^j-inch, ijHs-incli and iff-inch round test bars as well fitted for and to maintain a standard of comparative-physical tests.
The tests presented are obtained from the actual mixtures used for pouring castings in the various specialties mentioned, and, as seen, are arranged in the order of their strength. Double the amount of tests were made, but those shown illustrate the relation of the different areas in strength per square inch as
* For three other standards, see pages 573, 577 and 571;.n page 573, which show that test bars should not be smaller than \y* inches in diameter, and cast on end, us such will tfive truer results than the [J4™inch round bar in general practice, especially in making comparison <>i; the widest ranges in grades.temper '' or damp-              ,| |