ON THE CRYSTALLINE STRUCTURE OF METALS. 357 



mentioned as Abiding additional evidence that the structure of metals in general 

 consists of crystalline grains built up in the manner which has been described. One 

 class of evidence has l>een obtained by an examination of specimens of iron which had 

 IKM-II deeply etched under very high magnification, i.e., 2000 to 3000 diameters. 

 Und.-i favourable circumstances it is {>ossihle to resolve the minute structure to which 

 the peeiiliar reflection of oblique light is due. The face of each grain is then seen to 

 l>r rnviTed with minute projections resembling scales, more or less square or oblong 

 in slm|>e and similar and similarly oriented over the entire face of one grain. Fig. 4 

 is .1 photograph of this appearance as seen in nearly pure wrought iron: it may be 

 rompared (as Mr. STEAD compares a like appearance occurring on a large scale in silicon 

 steel) to the arrangement of slates on the roof of a house. In other cases the action of 

 the add is different; the general surface of a grain cannot Ixa resolved under the highest 

 powers, but here and there the acid has etched out minute pits showing a distinct 

 geometric form. All these pits found over the face of one gr;iin are similar and 

 similarly situated figures, but the shaj>e and orientation of the pits changes as soon as 

 a iMHindary is crossed. This is shown in a very striking way where a comparatively 

 large |>it crosses the boundary so that a portion appeal's on each side. Each jiortion 

 preserves its proper shaj>e and orientation, and there is consequently a marked angle 

 in the sides of the pit where' the sides cross the boundary. The shape of all these 

 pits in iron is consistent with the assumption that they are plane sections of cul>es or 

 octahedra. 



Fig. 5 (Plate 16) is a photograph of such etched pits in Swedish iron. 



A good development of geometrically etched pits is not very readily obtained : in 

 some specimens of iron they occur with much greater readiness than in others, and 

 this occurrence is to some extent an accident of etching. Possibly the presence of a 

 minute quantity of impurity in the iron is an essential factor, but we have no 

 evidence on the subject. Geometrical etched pits are a well known phenomenon 

 in non-metallic mineral crystals. BAUMHAUEK* finds that they have a definite 

 relation to the crystal lographic nature of the crystal upon which they occur but the 

 facets develoj>ed by etching often lie in planes which are not parallel to the natural 

 faces of the crystal. He finds that these etched pits, though generally truly 

 geometrical, frequently show curved or irregular outlines which he attributes to local 

 concentration of the acid. We find that curved or irregular outlines often occur in 

 the larger etched pits in iron, and in view of BAUMHAITER'S ol)servations it is clear 

 that they cannot l>e taken to affect the evidence for the strictly crystalline nature of 

 the metal, since similar appearances are to be found in Ixxlies that are character- 

 ist ic illv crystalline. 



When metals are cast against glass or other smooth Ixxlies, to get a surface fit for 

 microscopic examination, evidences of crystalline structure appear apart from any- 



* ll.vi MiiAir.i:, ItamltAtc der Aetr.methode in der Krystallogrnphischen Forsohung.' WII.HKI.M 

 Kv.n MANN. l.i-i|i/ig, 1894. 



