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100                      METALLURGY   OF   CAST   IRON.
from 2 to 3 feet deep, and then filled up with a medium grade of bank sand, of a very open, sandy nature. The reasons for going down to such a depth to simply mold pigs that are not more than four inches deep, also for using such a coarse grade of sand having very little binding qualities about it, are found in the desirability of having conditions as favorable as possible for permitting the escape of steam from any excess of moisture or water, which the sand may contain, or for draining downward, and hence lessening the chances of a "boil. " The moulder must bear in mind that when once a stream of iron is started, the furnace-man cannot plug up a c' run-out'' or dampen the ardor of a little " kick," the same as when pouring a mould, and hence the precaution of not being dependent upon one's judgment to get sand just the right temper,'' etc. Where sand is as open as is generally used for pig beds, and as deep in the floor as above described, water, after having been absorbed to a certain point, will, to a large degree, filter through coarse sand towards the bottom of its depth, so that should an excess of water have been used, the chances are it will not cause the ft' boil'' it would certainly do if the sand was of such a character as that generally used for green sand molding in a foundry. Another point which makes it desirable to use such open-grained sand is that of saving labor in mixing sands. About all the mixing that furnace sand generally gets is what the force of water from a two-inch nozzle gives it. I have seen such a stream play steadily on one spot for two or three minutes and no attention paid to it. If moulding sand in a foundry received such abuse, the iron would mostly go to the roof the moment it struckat the regular notch.    It is often surprising how rapidly, as about 75 per cent of the heat generated from the solid fuel is utilized. This is attained where one ton of coke will produce one ton of iron; and Sir........................    2,720     "