between iron rollers (these we shall presently describe), by which it is shaped into a bar. The bars are afterwards cut into lengths, reheated without contact of fuel, again hammered or re-rolled; and this process is several times repeated when the best product is required. During the first treatment of the blooms, and also in the subsequent hammering or rolling, the oxygen of the atmosphere acts on the surface of the glowing metal, so as to cover it with thin scales of oxide, and these, carried into the interior of the mass, will give up their oxygen to any residual silicon, carbon, etc., producing a little more slag, carbonic oxide, phosphate of iron, etc., which by the pressure of the hammers or rolls are ultimately forced out of the metal. It will be observed that in producing the pig iron the chemical action is the separation of oxygen from the metal, while conversely an oxidizing action is set up in the finery and subsequent treatment, in order to burn off the foreign ingredients. But this cannot be done without at the same time re-oxidizing some of the iron itself, of which therefore there is always a considerable loss, by its formation into slag (silicate), cinder, foundry scale (oxide), etc. The quantity of iron lost depends of course on many conditions, such as the care exercised in the operations, but it occurs in all the processes that have been devised for the conversion in question, even in the most modern: its amount may be taken to range between 10 and 20 per cent. The reader is requested to bear in mind the nature of the chemical actions that have just been described, for in even the most recently invented processes the principle is the same in nature and effect. So completely can the foreign elements be eliminated by this, or some analogous process, such as we shall presently mention, that the finest Swedish bar iron contains more than 99½ per cent. of the metal, and in some cases only a very little carbon and a mere trace of phosphorus remain, amounting together to less than 1 part in 2000. Such metal is made from very pure ore, containing no sulphur and scarcely any phosphorus, while charcoal is the fuel used in all the operations. As already mentioned, the objection to the use of coke is the sulphur, phosphates, and siliceous matters it contains. Toward the close of the eighteenth century an invention came into use which obviated the disadvantages of the cheaper fuel for converting crude iron. This was the puddling furnace, brought into use after much experimenting by Henry Cort in 1784. In it the pig iron is fused in a _reverberatory furnace_, the form of which will be understood from Fig. 19, which is a diagram showing such a furnace in section, where _f_ is the fire, _a_ an aperture at which the fuel is introduced, _p_ the ash pit, _b_ is a low wall of refractory material called the “bridge,” over which the flame passes, and is by the low arched roof reflected or _reverberated_