28 BULLETIN 184, UNITED STATES NATIONAL MUSEUM 



in the following manner: A4 (delta to gamma, 1,400°); A3 (gamma 

 to alpha, 910°); and As (magnetic transformation, 760°).* 



If the iron contains carbon there is another critical point at 723° 

 called Ai. The points Ai and A3 approach each other rapidly with 

 even a very slight admixture of carbon, and when the percentage 

 reaches 0.80 they merge — the point then being designated com- 

 monly as Ai. 



Hysteresis. — In pure iron the critical points mark practically 

 definite temperatures; but in alloys there is a more or less pronounced 

 lag (hysteresis) in the transformations, which thus take place some- 

 what above these temperatures on heating and somewhat below them 

 on cooling. The positions of the thermal critical points on heating 

 are designated as Aci, Aca, and AC4 (the c standing for the French 

 word chauffage), and on cooling as Aii, Arg, and Ar4 (French 

 refroidissement) . 



The admixture of certain metals lowers the Ajs point, and the 

 presence of nickel in particular not only greatly depresses the A13 

 point but also greatly increases the gap between the Aca and Ats 

 points. This fact, which is fm-ther discussed in Chapter VIII, has 

 a bearing upon the structure of meteoric irons. 



VI. ALLOYS 



To appreciate the complex nature of iron meteorites it is necessary 

 to consider them as having an origin somewhat similar to that of 

 artificial alloys. They are essentially natural nickel-iron alloys. Hence, 

 an outline of the general physical-chemical relationships of alloys is 

 here given as an introduction to the discussion of the physical chemis- 

 try of meteoric iron. 



Solid solutions. — A liquid solution is a complete and homogeneous 

 merger of its components, though not in definite proportions and 

 without any chemical combination — e. g., salt and water. An ex- 

 actly similar merger may exist in the solid state, forming what is 

 known as a solid solution. 



Most metals form solid solutions with one another, which are 

 termed alloys, and they also may hold in solid solution various non- 

 metallic substances. This is true of meteoric nickel-iron, which 

 forms solid solutions with phosphides, sulphides, and other substances. 



The term "solid solution" has supplanted in English and French 

 usage the earlier term "mixed crystals," although the latter (Misch- 

 krystall) is the term still used by German writers. 



♦ There is a difference of opinion as to whether another phase, the beta phase, should be recognized be- 

 tween 700° and 910°. Most authorities take the view that the thermal critical point designated as Aj at 

 760°, which is the magnetic transformation point, also marks the change between the beta and alpha phases. 

 The question is not here important, however; for according to the authorities that recognize the existence 

 of beta iron it is identical with alpha iron in that it is body-centered and cubic in crystallization, differing 

 from it only in electrical conductivity and in magnetic properties. The structural transformations for 

 pure iron therefore may be assumed to be from delta to gamma iron at A< and from gamma to alpha at Aj. 



