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388                       METALLURGY   OF   CAST   IRON.
per cent. For the purposes of these experiments, however, the actual expansion or contraction was not required.
The significance of these diagrams is qualitative and comparative; and for this use of them the reading of the pencil-travel in inches is accurate, the apparatus and operation being the same in all the tests recorded. With this explanation I return to Fig. 74, In each of the four casts shown, two test bars, i x i^ inches in section and 4 feet long, were cast "open-sand" side by side in the same mould. Tests Nos. i, 3, 5 and 7 were poured from the respective ladles which brought about 100 pounds of the iron direct from the cupola. These tests comprised the softest iron of each cast and had the least expansion and contraction, as is shown by the diagram. For tests Nos. 2, 4, 6 and 8, the grade of the iron was changed, by means of pouring about half of the hundred pounds contained in the ladle coming direct from the cupola into an empty ladle, the bottom of which was covered with about three-quarters of a pound of brimstone. The metal in the ladle having the sulphur was then agitated with a half-inch wrought iron rod until fuming ceased, after which all dross was skimmed from the surface, when each ladle was poured into its respective test-mould. The addition of sulphur hardened the iron in these tests, thereby causing the increased expansion and contraction shown in the diagram.
In Fig. 75, page 390, tests Nos. 9 and 10 illustrate another discovery made by this method of comparative tests, namely, that where free expansion is prevented, a greater contraction is effected in that part.
Test bar No.   9 was cast between iron ends, so ar-raction of 3 in 1536, or 0.195