132 METALLURGY OF CAST IRON. or one-fifth to one-third per cent, of phosphorus to change the grade, similar as the alteration of one-quarter of one per cent, in silicon would do. The author believes that furnacemen will agree that it would be very rare to have such a variation as above in manganese and phosphorus, in irons made from ore that comes from one mine or locality. As there is a liability, on rare occasions, of manganese and phosphorus varying to an effective degree from similar ores, and then again a change in the total carbon to alter the grade of an iron, it may often pay those who are manufacturing castings, where such changes as above would seriously affect their iron, to always have an analysis of the total carbon, manganese, and phosphorus in connection with the silicon and sulphur. There is one thing to be remembered and that is, that a furnaceman has far less difficulty in obtaining a uniform grade when making low silicon irons, or that under 1.50, than above this percentage; and also that there is much more difficulty in obtaining a uniform grade in very hot weather, due to humidity of the air, | than when the thermometer is below 85 degrees F. j More on this point is seen in Chapters X. and XLV., pages 78 and 306. Furnacemen are finding that if they are not called upon to increase temperatures of j blast over 1,000 degrees F. (some find it best to keep \ between 850 to 900 degrees F.), and have a good uni- j form distribution of the blast, they can secure a j more uniform product than otherwise. Largely for ' these reasons furnacemen prefer to run on low silicon **i iron. j One is most impressed with the uncertainty of furnace workings when in urgent need of ten hundred jon of iron; and Sir........................ 2,720 "