Summary: (1) The Widmanstätten structure belongs to a triad of secondary structures, the other two being the structure of large crystals and the network structure. (2) The fundamental conditions for the occurrence of Widmanstätten structure which an alloy has to comply with are: (a) to crystallize in the face-centred cubic lattice; (b) to be subject to secondary crystallization; (c) the transformation resulting by this secondary crystalization to occur in an already ‘granulated’ alloy; (d) the rate of cooling through the area of secondary crystallization to be such as to favour the deposition of the excess element, not at the boundaries only, but also inside tile granulae.

It follows that a large well-developed granulation will be favourable to the occurrence of Widmanstätten structure. It also follows that one and the same speed of cooling may, or may not, produce the Widmanstätten structure, since the speed has to be in a certain ratio to the mean diameter of the granulae. (3) The analogy between the equilibrium diagrams of the iron-carbon and iron-nickel alloys is very striking. Not only in both cases exist the areas of primary and secondary, crystallization, and of granulation, but the latest researches have shown that the δ-transformation also occurs in both cases. (4) From the character of the equilibrium diagram of the iron-carbon and iron-nickel alloys it is to be presumed that under suitable conditions the Widmanstätten structure may occur in both instances. As it happens, the Widmanstätten structure is of usual occurrence in iron-carbon alloys, and the Widmanstätten figures are a characteristic feature of meteoric irons. (5) As a rate of cooling which would cause the largest possible granulation and, afterwards, the quickest separation of constituents, will be also the more favourable to obtain the Widmanstätten structure, it may be inferred that such or like conditions were, apparently, realized in meteoric irons; viz. a very high temperature of the mass below the temperature of fusion, a resulting large granulation, and, then, a relatively rapid cooling, resulting in a quick separation of the kamacite and taenite.