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420                       METALLURGY   OF   CAST   IRON.
An experiment which, trie author conducted to demonstrate the fact just cited was to take a pattern 14 feet long by four inches by nine inches, and another exactly the same length but only one-half inch by two inches, and cast both together with the .same gates. Although the bars were of the same iron, a difference of seven-eighths of an inch existed in their contraction. The thin casting contracted one and three-quarters of an inch, whereas the thick contracted seven-eighths of an inch. Why is this? is a natural question, and in answer the author would offer the following hypothesis:
The carbon held in fluid iron, authorities claim exists in a combined form. How much of this will change to graphite when the castings or iron has solidified and become cold enough to handle, depends first upon the time of cooling, and second, the percentage of sulphur, silicon, manganese, and phosphorus, which exists in the iron.* The greater the silicon up to nearly four per cent., also the phosphorus up to one per cent., and the lower the sulphur and manganese, taking account also of the time consumed in cooling, the higher we will find the graphitic carbon. The greater the formation of graphite, the larger the molecules and grain of the iron ; and this is one secret of thin castings and hard iron contracting more than thick castings and soft iron, in cases where all conditions in moulding, cooling and freedom for contraction are substantially alike. For other qualities effecting this, see pages 394 to 396.
Two castings from one pattern, of the same iron, can,, by cooling one.more quickly than the other, be made to show considerable difference in their contraction, ow-
* The total carbon [is also to be included when thought to vary from any given standard.                                                          J  I