Cooling of solids.
38. Some solid bodies, during cooling, show changes analogous to those observed in solutions, and are therefore termed "solid solutions." For instance, if a hot physically homogeneous solid obtained from the fusion of iron with carbon is cooled, there may result a separation in the solid of particles of either iron or cementite, the latter being a chemical compound of iron and carbon represented by the formula Fe3C; the particular substance separated depending on the percentage composition of the original solid. This separation continues, and the temperature falls, until the residual physically homogeneous material contains 0·9 per cent. of carbon and the temperature is 690°; the temperature then remains constant, although the body is surrounded by a cooling medium, until this residual physically homogeneous material has been wholly transformed into a fine-grained mixture of iron and cementite, containing 0·9 per cent. of carbon. This particular kind of mixture has been termed eutectic, though the transformation has taken place, not by solidification from fusion, but in a body which was already solid. Prof. Rinne has proposed for such cases the substitution of the term eutropic, thus avoiding the suggestion of fusion. The eutectic mixture of iron (or ferrite) and cementite is known as pearlite.
Overcooling.
39. Just as water may be cooled so quietly that it is still liquid at a temperature much below the normal freezing point, a mixture may be cooled in such a way as to pass much below the eutectic (or eutropic) point without the normal transformation taking place; it is then said to be overcooled. The equilibrium, however, is very unstable, and the transformation, once begun, takes place almost instantaneously throughout the whole mass.
Crystalline structure of artificial iron.
40. A structure analogous to that shown by the Widmanstätten figures, though on a finer scale, has been observed by Prof. J. O. Arnold and Mr. A. McWilliam[17] in cast steel containing 0·4 per cent. of carbon; the plates of iron (or ferrite) in the cast steel correspond to the plates of kamacite in meteorites. Further, it has been found that the plates in the cast steel disappear during the process of annealing; similarly, there are no Widmanstätten figures, and the structure of the material is granular, near the outer surface of an unweathered meteoric iron; presumably as a result of the high temperature to which the outer part of the mass has been raised during the passage of the meteorite through the earth's atmosphere.
Structure of meteoric irons.
41. At present it is generally imagined that kamacite and tænite are definite alloys, or perhaps solid solutions, of iron and nickel, the former being poor in nickel (6 or 7 per cent.) and the latter rich in that constituent (25 to 38 per cent.), that kamacite and tænite separate in succession from the molten mass or solid solution until the residual part is so rich in nickel that a eutectic (or eutropic) proportion is reached; the residual material then forms plessite, which, according to this view, is a eutectic (or eutropic) mixture of kamacite and tænite. But it is difficult to understand how the thin plates of tænite are deposited on the plates of kamacite, seeing that they contain more nickel than kamacite and plessite, and yet have an intermediate epoch of formation, prior to the epoch of formation of that tænite which is a constituent of the plessite; one suggestion is that the thin plates of tænite have been deposited on the plates of kamacite owing to the temperature having fallen well below the eutectic (or eutropic) point after the separation of the kamacite and before the eutectic transformation of the residual material has taken place. And Prof. Rinne[18] himself is of opinion that the Widmanstätten structure has been wholly developed in meteoric iron after the solidification of the mass; further, as the relations of the kamacite, tænite and plessite to the enclosed troilite indicate that the troilite was solid before the octahedral structure was developed, and as that mineral, under normal circumstances, solidifies at about 950°, he infers that the structure was developed below that temperature. In the case of the Jewell (Duel) Hill meteorite it was discovered by Dr. Brezina that, notwithstanding the pronounced octahedral structure, plates of troilite are embedded, not in accidental positions nor between successive octahedral layers, but parallel to the faces of the corresponding cube; whence Prof. Rinne suggests that this iron, now of octahedral structure, and possibly all others of a similar character, had a cubic structure at the epoch when they entered upon the solid condition. But, as both Prof. Rinne and Dr. Brezina[19] have pointed out, a fused mixture of nickel and iron, cooling undisturbedly in outer space, may have solidified at a temperature even below 950° and thus have been much overcooled.
Tænite possibly a eutectic mixture.
42. In the course of a recent elaborate investigation of the changes of the magnetic permeability of the Sacramento meteoric iron with changing temperature, Mr. S. W. J. Smith[20] has been led to infer that the magnetic behaviour can only be explained by imagining the meteorite to consist largely of plates of nickel-iron, containing about 7 per cent. of nickel (kamacite), separated from each other by thin plates of a nickel-iron constituent (tænite), containing about 27 per cent. of nickel and having different thermo-magnetic characters from those of kamacite; he suggests, however, that tænite is not a definite chemical compound, but is itself a eutectic (or eutropic) mixture, and consists of kamacite and a nickel-iron compound containing not less than 37 per cent. of nickel. And he points out that, while the tænite mechanically isolated from meteorites for analysis has approximately the lower percentage (27 per cent.), the tænite chemically isolated through the prolonged action of dilute acid (which would remove much of the admixed kamacite) has a higher percentage, which in several cases approximates to 40 per cent.