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7&                           METALLURGY   OF   CAST   IRON.
caused by a furnace perfecting combustion of its own accord to such a point as to overreach the best temperature for driving well. It may be said that brick stoves have many advantages over iron stoves in permitting a furnaceman to regulate the temperature of his furnace so as to drive it well and increase or diminish the silicon or sulphur in the iron, and that a radical change is generally noticed in this direction when cooling down a 'l hot furnace," as by such procedure the silicon is often materially decreased and sulphur increased.
Humidity of blast. It is generally conceded by experienced furnacemen that a furnace will work better and produce more iron in cold than in hot weather. It is said that in June, July, and August a furnace never produces tonnage to equal other months in the year. The air is generally dryer in cool than in warm weather, and it is now an accepted fact that the extra humidity in the summer air over that in cold weather is the cause of the less tonnage in the summer months. Some will think the heat imparted to the blast would drive out all the moisture, but this is claimed to be simply transformed into a vapor which passes into the furnace as steam. It has been estimated that twenty tons of water are often transferred, by the blast, to the interior of a furnace per day by reason of the high humidity of air in summer months. Further comments on this subject can be found in Chapters IX. and XXXIX. using larger percentages than, ordinary, which may be called for by reason of having to use small, or what is thought to be inferior coke or fuel, and again in burdening a furnace with fuel in order to raise the silicon in the iron or guard against " scaffolding " or "slips" from the use of fine ores, etc. It may also beency                :|,