Any information connected with the manufacture ol steel is of great importance to a large class ot manufacturers and operatives in America. Chemistry had already been established upon a scientific basis by the adoption of the doctrine ot definite proportions at the time when attention was again directed to the compounds of iron with carbon. With regard to these substances, so impoortant in the arts, the law of definite combining proportions did not appear to hold good; but the per centage of carbon was greater in proportion as the carboniferous iron approximated more closely to steel, and from this to cast-iron. However, there still remained a possibility of reconciling this iact, with the law, by assuming the existence of a definite carburet of iron capable of combining with iron in definite or indefinite proportions, and determining its characters. Still the existence of such a carburet of iron has never yet been proved. In the course of a lormer investigation of this subject, 1 was of opinion that I had really obtained such a substance. But the evidence of subsequent experience is entirely the other way; and even if such a compound were discovered, the difficulty would not be removed, for it would still be necessary to admit that it combined in indefinite proportions with iron. It would appear as if the combination ot iron with carbon in indefinite proportions does not exceed a certain limit, and that the maximum per centage of carbon is about 593. The classification of the various kinds of carburetted iron, under the general name of cast-iron, steel, and bar iron, is entirely arbitrary, and based upon the physical characters. When entirely free from carbon, iron is so soft that it offers but little resistance to fric tion, and would be inapplicable to most of the purposes tor which iron with more or less of carbon is emplojed. By combination with carbon within certain limits, it acquires greater hardness; the elasticity and ductility are increased. The increased hardness is especially remarkable when the strongly-heated metal is-mtMetdy -cuultJ. Tfafcrrmuaifcr uf some carburetted iron has been made the distinction between bar iron and steel, inasmuch as all bar iron which becomes harder when suddenly cooled, is by universal consent termed steel. The analyses of a great number of varieties of iron has led to the result that the per centage of carbon may rise to 0.2, or 025, before the metal becomes considerably harder when suddenly cooled. The purer the iron is, the greater its freedom from adventitious substances, especially sulphur, silicon, and phosphorus, the larger may be the per cent-age of carbon requisite to determine its hardening when cooled suddenly. The best kinds of Swedish bar iron and that made in Germany from spathic iron and brown iron ores, do not become very hard even when containing as much as 0'35 per cent, of carbon, although the hardness is such as to justify the appellation of steel-like iron. The transition from this kind of iron to true steel is so imperceptible, that it is necessary to adopt some arbitrary means of deciding whether the metal is bar iron or steel. It the carburetted iron acquires on sudden cooling such a degree of hardness as to give sparks when struck upon flint, it may be regarded as steel; and this degree of hardness requires a per centage of carbon amounting for the less pure kinds of iron to 0.5, and tor the nearly pure iron to 0'65. However, steel containing such a small per centage of carbon is always but soft steel, which, to become capable of acquiring greater haidness, must be more highly carburetted. The hardness acquired upon sudden cooling increases as the per centage ot carbon increases, but not in the same proportion. For iron almost perfectly free from adventitious substances, a percentage of 1'4 or 1'5 carbon corresponds with the highest capability of acquiring hardness and tenacity. With a still higher per centage of carbon the steel acquires greater hardness; but its tenacity is lessened and the malleability decreases so rapidly with the increase of carbon, that with a per centage of 1*75 it can scarcely be welded at all.— When the per centage of carbon amounts to t 1'8, it is only with great difficulty that it can be forged, although with a very great degree )f hardness it may still possess considerable tenacity. Steel which contain 19 per cent, ind more of carbon can scarcely be forged at all, and with a per centage ot 2'0 the limit >etween steel and pig iron appears to be reached ; for such metal in the soft state, that is, before being hardened, cannot be beaten out while hot without splitting and breaking under the hammer. Steel, in virtue of the remarkable capability which it possesses, after cooling slowly from a high temperature, of being worked like soft iron, and then acquiring a considera-rable increase of hardness, without loss of tenacity on subsequent sudden cooling, has become a very valuable substance for various branches ot industry. However, it has not yet been possible to refer the altered conditions of hardness presented by the slowly and suddenly cooled metal to any altered state of combination of the carbon and iron in steel.— Such wide differences of hardness and softness as those presented by steel, which has been submitted to these two modes of treatment, can only be regarded as resulting from a total alteration of its molecular structure. The conjecture that the state of combination of the iron and carbon in hardened and sott steel respectively must be very different, is rendered in a high degree probable from the circumstance that such a difference in the state of combination of the iron and carbon in the carburets with a large per centage of carbon— the different kinds of pig iron—may be proved to exist with perfect certainty. A distinction has always been made between white and gray pig iron. These substances differ so obviously in their characters—color, hardness, tenacity, and brittleness—that the fact could scarcely have been overlooked. In addition to this, the difference in their conditions of fusion must not he overlooked, the gray kind requiring a much higher temperature than the white iron, and passing almost suddenly from a solid to a liquid state, while the white iron not only fuses at a lower temperature, but before liquefaction, becomes soft and then pasty. Before a trustworthy method of separating carbon from iron had_been discovered, it waa. supposed {Eai ~tEIs"3ifference in the behavior of white and gray kinds of iron was attributable to the per centage of carbon, for on dissolving gray iron in acids a much larger quantity ot carbon is left than when white iron is treated in the same manner. Now, however, it is known that this inference was erroneous! and that the characters of pig iron are dependent, not upon the greater or less per centage of carbon, but upon the state of combination of the carbon and iron. The gray iron, when suddenly cooled after having been melted, is converted into white iron; and white iron, when exposed to a high temperature, after melting, and gradually cooled, is converted into gray iron, without the per centage either of iron or caibon being in any degree altered. Every kind of gray iron corresponds to a white iron with precisely the same per cent-age of carbon; and the wholly different behavior and characters of these two kinds oi iron are no longer regarded as owing to the greater or less per centage of carbon, since it is known that the gray soft iran, malleable at the ordinary temperature, is a mixture of steel or steel-like iron with carbon, while the white, hard, and brittle iron is a true chemical compound ol iron with the entire quantity of carbon present. The analogy between the gray and white pig iron on the one hand, and soft and hardened steel on the other, is unmistakable; but no trace of uncombined carbon has ever been found in slowly cooled soft steel. E ven cast-steel, which contains from 19 to 2'0 per cent. of carbon, and which on account of this large per centage can no longer be torged, has never been found to contain uncombined carbon after the slowest possible cooling. It is only when the per centage of carbon amounts to 2 25 or 2 3. that carbon separates in the slowly cooled metal, and communicates to it the characters ot pure pig iron. If, therefore, a distinction is to be drawn between steel and pig iron, founded upon a character determined by the combining proportions, it would correspond with a per centage of caibon amounting to 2'25 or 2 3, because a part of the carbon is then separated on gradually cooling the mass. The more the per centage of carbon increases from this minimum to the maximum of 5'93, the lighter is the color of the metal and the greater the hardness of the white variety. In the gray iron, on the contrary, the quantity of carbon which separates, and which determines the darker color and greater softness of the metal, as well as the greater or less per centage of carbon remaining in a state of chemical combination with the iron, is dependent upon the more or less gradual solidification of the melted mass. It is therefore not sufficient to know the per centage of caibon in pig iron, as ascertained by analysis, in order to form an opinion as to the behavior of the iron in question; but it is at the same time necessary to determine how much of that cabon is chemically combined with the iron, and how much is present only as a mere mechanical admixture. With regard to the metallurgical processes, the object of which is to separate the carbon from pig iron tor the production of steel or bar iron, the state ol combination in which the carbon exists, is of far greater importance than the total per centage of this element. White iron requires for this purpose methods and processes different from those applicable to gray iron; and cases may occur in which the smelter would be obliged to convert gray into white iron, even although this has to be effected by an addition of carbon, notwithstanding that its separation is the real object of his operations.