176 Messrs. G. T. Heycock and F. H. Seville. 



area than in (1), nearly filling the field, and being only separated 

 from each other by an imperfect network ■ of less oxidised mother 

 substance. 



These two alloys are deeply etched in the process of polishing with 

 rouge, the softer primaries rich in copper being eaten, away. The 

 pattern is so large that it is best examined with a power of 10 or 

 20 diameters. 



In striking contrast to the above, alloys (3) and (3) A, chilled when 

 the alloy has been solid some time, show no pattern even with a power 

 of 300 or 400 diameters (photo. 5). 



Alloy (4), chilled at P, the next point of heat evolution on the cool- 

 ing curve, shows a pattern which is a close approximation to that of a 

 slowly cooled alloy, and alloy (5), chilled at a still lower temperature, 

 is an almost perfect reproduction of the slow-cooled pattern (photo. 6). 

 It will be noticed, however, that a little below the chilling point of 

 (5) there is another stage of heat evolution, and in harmony with this 

 we can find one point of difference between the pattern of (5) and that 

 of the slowly cooled alloys of the region CD. Both in these and in (5) 

 the surface is divided into large polygons bounded by .bands of a 

 smooth material, and the interior of each polygon is more or less 

 full of a broken fern or flower-like crystallisation of the same smooth 

 body as that of the bands. The ground in which the fern leaf lies is 

 more easily oxidised than the material of the fern leaf and bands, so 

 that the ground probably has more copper in it. In the slowly cooled 

 alloys near C there is very little of the fern leaf, but as we approach D 

 it increases in amount until at D it almost fills the whole area, not 

 absolutely, however, for a network of the darker ground can still be 

 traced here and there. A comparison of photos 3 and 6 illustrates 

 this growth of the fern leaf with the increase in the percentage of tin. 

 In the slow-cooled alloys the ground is granular — in fact, an immersion 

 lens defines it as a well-marked eutectic. In (5), on the contrary, the 

 ground appears to be uniform ; probably chilling at a temperature 

 below Q would convert it into the eutectic. 



All the alloys from a little to the left of C to beyond D exhibit 

 similar contrasts between the chilled and slow-cooled patterns, there 

 being for each alloy a region of temperature such that if it be chilled 

 in this region it shows no pattern. Alloys between D and E are still 

 more remarkable when chilled. 



If we apply Roozeboom's theory to these results, we see that in the 

 cooling curve the branch LM corresponds, as is obvious, to the cool- 

 ing of a liquid, and the short branch MN to the formation of mixed 

 crystals separating out of a liquid that is continually growing richer 

 in tin, so that the crystals are suffering transformation. The branch 

 NO, almost flat at first, and then only slightly sloping, corresponds 

 to an isothermal transformation of the mixed crystals followed by 



