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52                          METALLURGY   OF   CAST   IRON.
(which changes the oxide of iron in the ore to metallic iron, and carbonizes it to form cast iron) has taken place and all the stock is liquefied, gases have escaped, and what passes to the point Y is some remaining fuel which replenishes the bed over the melted iron and slag. The total length of line at the different levels, B, C, D, and E, is the same. In cupola practice, foundrymen have the advantage over furnacemen in being able to observe the action of the stock until it has reached the u melting point." In observing stock settle at the last charge in a straight cupola, when all is working well, little or no change is noticed in the position of the material, and this is generally so true that the founder knows that whatever way stock is delivered into a cupola it will generally be found so situated when it reaches the " melting point." For this reason founders often have experience with " bunged-up " cupolas or iron dumped at "bottom-drop, '' which could not be melted owing to fuel or iron not having been charged evenly. Often stock reaches the melting point with fuel mostly on one side and iron on the other through carelessness in charging in that manner.
In the descent oi the stock, coke, limestone, and ore, all moisture is driven off, the thoroughly dry and heated ore now comes in the zone of reduction, where the oxygen is taken from it, and changed from oxide of iron to metallic iron, during which process the iron takes up carbon from the fuel, and, melting in the zone of fusion, finally arrives at the bottom in form to be tapped out. The non-metallic or earthy matter, in separating from the reduced iron, unites with the lime or flux and, being lighter than iron, floats on its surfacee reactiony they are often composed of about equal parts of silica and alumina. Bricks should contain silica or alumina in proportion to the amount of heat or friction they are                                                 if j