

EUTECTIC EESEAECH: THE ALLOYS OF LEAD AND TIN. 119 



constituent at the boundaries of these regions of similar orientation. To a very 

 small extent this is the case in fig. 33, but it is clearly illustrated in figs. 36 and 37, 

 which represent alloys containing a slight excess of tin (No. 36 represents an alloy 

 with 64 per cent, of tin, magnified 200 times, while No. 38 represents an alloy with 

 65 per cent, of tin, magnified 600 times). 



Figs. 34 and 35 show, under higher magnification (600 and 1200 diameters 

 respectively), other types of eutectic structure of the " laminated " variety, but iu 

 this case the lamination is not rectilinear, while still preserving a certain regularity 

 throughout well-defined regions. Study of these structures suggests certain con- 

 clusions as to the manner in which they have originated and as to their physical 

 nature. Important evidence as to this may be obtained by viewing a specimen of 

 pure eutectic, properly polished and etched, under oblique illumination with a low 

 magnification. Fig. 39 is a photo-micrograph of such a specimen taken with a 

 magnification of 12 diameters. A single photograph, however, gives a very feeble 

 idea of the true appearance of such a specimen. Viewed in this manner the various 

 regions of similar orientation already referred to are clearly differentiated in colour 

 and brightness, owing apparently to the manner in which the incident light is 

 reflected by the facets arising from the etching out of one constituent. The lustrous 

 appearance, with its brilliant diffraction colours, however, is strikingly different from 

 the "oriented lustre" observed when a similar section of pure metal is viewed in this 

 way. The conclusion which is to be drawn from this difference is probably that while 

 the eutectic has some form of crystalline structure, that form is different from that of 

 pure metals. When the specimen of eutectic is rotated under oblique light, this 

 difference becomes still more evident, and its character is revealed. In pure metals 

 the various regions or grains which are now well understood to lie simply holo- 

 morphic crystals characteristically exhibit a uniform brightness over their entire 

 area, and, when the specimen is rotated, increase and decrease in brilliance according 

 to the incidence of the light uniformly so far as the entire area of eacli crystal is 

 concerned. The regions or grains of the lead-tin eutectic behave differently ; in every 

 position of the specimen the incident light appears to pick out some radial line or 

 sector in each grain, and this line or sector appears bright ; as the specimen is rotated 

 the lighted sector appears to rotate in each crystal, thus giving rise to an appearance 

 strikingly similar to that which is observed when a transparent section of a spherulitic 

 mineral crystal is rotated under crossed Nicol prisms, except that the dark cross is 

 not seen as such in the eutectic " grains." Examination of a number of such 

 specimens has led the author to the view that the grains or regions into which 

 masses of pure eutectic are always found to be divided probably represent true 

 spherulitic crystals, i.e., crystals which possess a radiating structure, but which in 

 other respects are similar to the holomorphic crystals of pure metals. The effect of 

 the radial structure on the mechanical behaviour of the metal must, however, be very 

 marked. The detailed study of the phenomena accompanying plastic deformation 



