CHAPTER XXXIV. EVILS OF EXCESSIVE IMPURITIES IN IRON. "• As a rule cast iron contains 92 to 96 per cent, of metallic iron, the balance being impurities such as carbon, silicon, sulphur, manganese, and phosphorus. While these latter five elements are essential in iron, an excess of their total percentages exceeding 6 per cent, of cast iron is generally injurious to the best strength. To illustrate how an excess of the above impurities can weaken iron, the following Table 37 is presented. The percentage of impurities and iron shown, also the strength tests, are obtained from the results seen in Tables 108 to 114, pages 536 and 537. By a study of the following Table 37, one should perceive that changes in the total percentages of the carbon, silicon, sulphur, manganese, and phosphorus can have quite an influence on the strength of castings. For example, the chilled roll mixture (Table 37) possessing only 4.803 impurities, as against 6.218 in the Bessemer mixture, with others between them showing a uniform decrease in strength, demonstrate that if the impurities exceed 6 per cent, of the total the iron generally decreases in strength according to the increase of impurities. One is not to be wholly guided by the results presented in Table 37, as any one can figure other tests, wherever found, and test the principles here set forth.they get low carbon in low silicon irons, which many prefer, they notice its heat effect in a very pronounced manner. Iron with less than i per cent, silicon may have carbon up to 4.50 per cent, while over 4.00 per cent, silicon iron may often not exceed 2.00 per cent, carbon.