574 C. H. Matheivson — Metallographic Description 



the recrystallized units are visible only in the cores of the 

 primary dendrites, which are themselves blackened by this 

 treatment. Since the original etchings were made in this 

 manner, it was not at first supposed that the specimen had been 

 reheated. The evidence of deformation was broadly apparent 

 in the threading out of the tin-rich network in a direction 

 perpendicular to the forces applied, particularly where the 

 greatest elongation occurred, viz., towards the apex of the blade. 

 By etching very lightly with an extremely dilute solution of 

 ferric chloride in hydrochloric acid (showing no color) the 

 detail of (2) became apparent within the central portions of 

 the original crystals in the regions a and c, according to the 

 diagrammatic sketch. The reason why the (2) structure can- 

 not be seen in the tin-rich network is plain when we consider 

 that the network is left comparatively bright (unattacked) after 

 etching, i. e., while it is true that a still more inert material 

 within this network, such as cuprous sulphide, can be shown 

 by selective etching of the tin-rich material, the finer detail 

 of the latter is difficult to develop and cannot be developed in 

 a manner continuous with that of the surrounding zones. 



That the recrystallized units (2) extend through the tin-rich 

 network constituting the boundaries of the original crystals is, 

 however, apparent by their incompleted character as seen up 

 to this point. The contrast usually shown between differently 

 orientated crystals and different parts of twin crystals cannot 

 be seen with distinctness in this case, since they are located 

 within the cores of the primary crystals which themselves etch 

 and darken selectively as opposed to the tin-rich boundaries. 

 In other words, the secondary (2) structure is more or less 

 masked by the primary cast structure and this fact alone 

 renders very close observation necessary in order to detect the 

 former. 



The lines of deformation within the secondary crystals which 

 we have designated as the third superimposed structure can be 

 seen only with the higher powers. Their uniform direction in 

 any given grain, or symmetrical part of a twinned grain, aids 

 in distinguishing the outlines of the grains making up this (2) 

 structure. Obviously, these lines can be seen only in the cold 

 worked portions, a and c, of the specimen. 



It may be urged that a description of structures seen under 

 the microscope is unjustifiable without the presentation of 

 photo-micrographs for comparison. Every metallographist is 

 aware that the successful preparation of photo-micrographs at 



